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1984 Mine subsidence and strata control in the Newcastle district of the northern coalfield ewN South Wales William Arthur Kapp University of Wollongong
Recommended Citation Kapp, William Arthur, Mine subsidence and strata control in the Newcastle district of the northern coalfield New South Wales, Doctor of Philosophy thesis, Department of Civil and Mining Engineering, University of Wollongong, 1984. http://ro.uow.edu.au/theses/ 1250
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MINE SUBSIDENCE AND STRATA CONTROL IN THE NEWCASTLE DISTRICT OF THE NORTHERN COALFIELD NEW SOOTH WALES
APPENDIX A AND APPENDIX B
to accompany a thesis for the degree of
DOCTOR OF PHILOSOPHY from THE UNIVERSITY OF WOLLONGONG by WILLIAM ARTHUR KAPP, B.E. (Civil), M.E. (Mining) DEPARTMENT OF CIVIL AND MINING ENGINEERING 1984 APPENDIX A
BACKGROUND TO THE STUDY OF SUBSIDENCE IN THE NEWCASTLE DISTRICT APPENDIX A CONTENTS Page No
1 Geographical setting ..... A- 1 l.l Introduction A x 1.2 Physiography 1.3 Land use A A- 5 2 Geological setting " 2.1 History of coal formation A~ 5 2.2 Structure J _ 2.3 Stratigraphy J_ c 2.4 Conglomerate units A A- 9 3 Coal resources ** 3.1 Wallarah Seam A" 9 3.2 Great Northern Seam A j" 3.3 Seams of minor significance A io 3.4 Australasian Seam A io 3.5 Victoria Tunnel Seam A X1 3.6 Dudley Seam A \2 3.7 Yard Seam A-\I 3.8 Borehole Seam A " 3.9 Reserves and coal use 4 Mining and subsidence A_1 4.1 General historical review A~*-j 4.2 Early subsidence events a-17 4.3 Mining methods A _] 4.4 Current subsidence studies ^ iy 4.5 Application of subsidence studies A A-21 5 Survey procedures 5.1 Introduction A"^ 5.2 Layout of grids A 5.3 Levelling and distance measuring Zlil 5.4 Data processing Page A-ii
FIGURES
Fig. A. 1 Coalfields of the Main Coal Province Fig. A. 2 Coastal strip of the Newcastle Coal District Fig. A. 3 Locations of collieries, Newcastle District Fig. A. 4 Production graph of N.S.W. coal districts Fig. A. 5 Geology of Newcastle District Fig. A. 6 Upper Permian and Triassic stratigraphic units Fig. A. 7 Views of South Belmont and Redhead Beaches Fig. A. 8 Views of the Central Coast lakes and the Hawkesbury River Fig. A. 9 Environment of coal formation, Borehole Seam Fig. A. 10 Structures in the northern part of the coal measures Fig. A.11 Base of the Newcastle coal measures Fig. A.12 North east part of geological cross section Fig. A.13 South west part of geological cross section Fig. A.14 Areas affected by Belmont and Charlestown Conglomerates Fig. A.15 Isopachs of the Charlestown Conglomerate Fig. A.16 Seam sections and areas of investigation Fig. A.17 Seam sections and areas of investigation Fig. A.18 Seam sections and areas of investigation Fig. A.19 Isoash map showing combinations of the Dudley Seam Fig. A.20 Early coal mines in Newcastle Fig. A.21 Burwood Colliery workings, 1886 Fig. A.22 Newcastle collieries, 1887 Fig. A.23 Bord and pillar mining operations Fig. A.24 Examples of recent mining layouts Fig. A.25 Computer printout of calculated subsidence Fig. A.26 Computer printout of calculated strain Fig. A.27 Computer printout of calculated strain triangle results APPENDIX A BACKGROUND TO THE STUDY OF SUBSIDENCE IN THE NEWCASTLE DISTRICT
A.l GEOGRAPHICAL SETTING l.l Introduction There are about thirty large sedimentary basins in Australia ranging in age from Devonian through to Recent and coal has been located in most of these basins. The coals are from Carboniferous to Tertiary age. Most of the black coal production in Australia has come from the Permian coal measures of the Sydney Basin, which forms the major part of the Main Coal Province and occupies an area of 60,000 sq. km. The various coalfields which make up the Main Coal Province are shown in Fig. A.l (Stuntz, 1975). The Northern Coalfield is subdivided into five Coal Districts, with the Newcastle Coal District extending 90 km from the City of Newcastle to the Hawkesbury River, and inland for 3 0 km as shown in Fig. 1.2. The coal seams extend beyond the coast but knowledge of the seams under the Pacific Ocean is limited to the strip near the shore line which is shown in Fig. A.2. There are 26 operating collieries in the Newcastle District, all located to the north of Tuggerah Lake, around Lake Macquarie and north to Newcastle (Fig. A.3). The Newcastle District is a major producer of coal in the Sydney Basin. In the 1982-83 year 120 million tonnes of raw coal were produced in Australia. The N.S.W. contribution was 67 million tonnes. The Newcastle District has been a consistently high production area (Fig. A.4) with 18 million tonnes being mined in 1982-83 (Joint Coal Board, 1983). Both steaming coal for electricity generation and coking coal for use in the steel industry are mined, and both coals are exported. The large urban area of Newcastle is located in a scenically attractive area along the coast of the Pacific Ocean and around Lake Macquarie. The area has warm to hot summers and mild winters and the coastal areas are popular for residential and tourist development. A significant part of this area is closely associated with the exploitation of the local coal deposits and with the associated BHP steelworks in Newcastle. Many of the operating collieries are in or border areas of population and are Page A-2 near the coastline of the Pacific Ocean, or near the large tidal lakes of the Central Coast. The subsidence work has had a large influence in determining the maximum safe recovery of coal from beneath residential areas, other surface features, the Ocean and tidal lakes, and will continue to be of vital importance in the planning of longwall operations in areas where the effects of surface subsidence must be considered. 1.2 Physiography The landscape and natural environments are determined by such characteristics as climate, geology and topography which affect the drainage pattern, soil types and vegetation. The topography of the Newcastle District varies from the rugged mountain topography in the west where the elevations are as much as 500 m above sea level to the flat coastal plains to the east. In the areas around the operating collieries, and in particular where subsidence investigations have been and are being carried out to the east of Lake Macquarie the topography is flat to gently undulating. There are several creeks in the area which flow east into the tidal waters of Lake Macquarie, Lake Budgewoi and Tuggerah Lake, and south into the Hawkesbury River. The land surface features are to some degree conditioned by the nature of the rock formations. Although the geology will not be discussed at this stage, it is convenient to note that the base of the outcropping Newcastle Coal Measures defines the northern boundary of the Newcastle Coal District; the strata dipping generally to the south. The Newcastle Coal Measures are of Permian age and above these lie the Triassic Narrabeen and Hawkesbury Groups which influence the topography in the Northern Coalfield. A geological plan and section are given in Fig. A.5 (Crapp and Nolan, 1975). The stratigraphic units are set out in Fig. A.6. The topography and drainage patterns vary considerably in the Newcastle District, and are related to the different weathering characteristics of the main outcropping geological formations. The lowest part of the Newcastle Coal Measures at the northern boundary of the Newcastle District are generally soft and form an undulating landscape. The Lower Hunter River winds from Maitland to Newcastle through large open flood plains which are poorly drained. To the south of this flood plain the land rises to the belt of hills which enclose Lake Macquarie. Along the coastline south of Newcastle, the resistant sandstone and conglomerate of the Coal Measures form rocky headlands and sea cliffs up to 100 m above sea level. In the same area the wind blown sands along the coast surround the large attractive tidal lakes and Ocean beaches which extend further south to form the scenic resorts of the Central Coast. Page A-3
In the central and western part of the Newcastle District, the shales, claystones and sandstones of the Triassic Narrabeen group form the steep hillsides, broad valleys and coastal and estuarine flats. Drainage from the hills is into a series of coastal lakes and lagoons. The strata above the Newcastle Coal Measures, known as the Clifton Sub-Group of the Narrabeen Group consist of shale, sandstone and conglomerate and is exposed over much of the study area west and north of Tuggerah Lake. It is generally easily weathered and forms the broad gently sloping valleys of the coastal plain. The lakes and coastal lagoons are shallow and brackish. Although they are open to the Ocean they rely largely on the flow of flow of fresh water from the hills for flushing. The sandstone and shale of the upper part of the Narrabeen Group, known as the Gosford Formation compose the surface geology of the coastal hills south of Tuggerah Lake. This formation is heavily dissected and the weathered material forms the alluvium of the flat valley floors. The south and south western part of the Newcastle District are in the Hawkesbury Sandstone which is resistant to weathering and forms the rugged topography north of the Hawkesbury river with high, flat plateau areas. The strata consist of massive well bedded sandstone with occasional beds of shale. These weather to an infertile sandy soil. Where streams and rivers dissect the sandstone they create very steep valleys. The photographs in Fig. A.7 are looking north along the coastline at Belmont and at Redhead Beach. The first photograph in Fig. A.8 is looking towards the south east over Lakes Munmorah, Budgewoi and Tuggerah and shows Munmorah Power Station (Fig. A.2). The second photograph in Fig. A.8 shows the rugged topography of the Hawkesbury Sandstone in the south of the Newcastle District near the Hawkesbury River. 1.3 Land Use There is a high degree of diversity of vegetation and of land use in the Newcastle District. Except for the industrialised Lower Hunter, the patterns of land use reflect the area's natural features, land forms and soils. The City of Newcastle is in the north of the Newcastle District and with the surrounding metropolis has a population of 400,000. Newcastle was originally wholly dependent on coal mining. Until 1890 nearly three quarters of the State's coal output came from the Newcastle area. Many of the collieries were located between the Hunter River and the northern shores of Lake Macquarie. With the building of the steelworks in 1915, Newcastle increased in importance as a port and a centre of commerce and industry. The population growth to the south has been concentrated around the shores of Lake Macquarie and along the Pacific Highway which is the main road link to Sydney. The Lake Macquarie Shire Page A-4
is within the main subsidence study area and has a population of 200,000. Most of the land has been developed and the operating collieries affect some residential and light industrial areas. There have been instances where surface development has been allowed to take place after mining in current seams, but with later economic appraisals of other seams, areas of the surface which were originally thought to be free of mine subsidence considerations are being examined with regard to future mining prospects and in some instances mining has taken place. Associated with these residential and light industrial areas are roads of varying importance, railway lines, bridges and domestic water and gas reticulation systems. Lake Macquarie, the Pacific Ocean and the shorelines of these features require some degree of protection from the effects of underground mining especially where there are homes at low elevations. Further south along the coastline but on the western sides of Lake Munmorah and Budgewoi are located the power generating stations of the N.S.W. State Electricity Commission. These are powered by steaming coal from the collieries nearby (Fig. A.3). The seams are mined beneath the Lakes and low lying undeveloped land. Few structures are yet affected and mining is prohibited from beneath the power stations themselves. Mining has not yet extended to other parts of the Newcastle District and the need for an extensive exploratory programme has not arisen for the western, central and southern parts of the District. However limited drilling and other geological information have established that the coal seams which underly these areas will warrant further investigation as to their economic viability. Overlying the remainder of the Newcastle District are the townships and resorts of the Central Coast around Tuggerah Lake and further south to the City of Gosford with its commercial and industrial activities and urban areas. In addition the Sydney to Newcastle freeway is under construction with the associated bridge structures. The natural gas pipeline, the Main Northern railway line and the towers of the power lines are located to the west of the lakes. The western half of the Newcastle District is mainly natural vegetation most of which is classified as state and national parks, and state forest. The broken nature of the topography and the extensive areas of steep slopes mean that areas of good agricultural land are small and scattered. There are many small farms on the highland plateaux with citrus orchards being the main activity as they are suited to the better quality sandy soils of these upland plateaux. The farming areas are generally in the coastal hills in podzolitic soils derived from the Gosford Sub-Group to the west and south of Tuggerah Lake. These are either in the alluvial deposits along the creek systems or in the soils of the coastal plains, leaving the steeper topography with the natural vegetation. It will be many decades before mining Page A-5 will be considered in this area.
A.2 GEOLOGICAL SETTING 2.1 History of Coal Formation The Sydney Basin is part of the Main coal Province of New South Wales and was formed during the Permian and Triassic Periods from 300 to 200 million years ago. The Main Coal Province is divided into Coalfields which are subdivided into Coal Districts. The Newcastle District is part of the Northern Coalfield (Fig. A.l). It is generally agreed that in Lower Permian time Australia was close to the Lower Permian pole and was joined with Antarctica. There was extensive glaciation during this period. The waning of glaciation was the start of the first Permian coal-forming period when a series of glacial lakes formed and when large plants became established. Slow marine transgression occurred as the sea level rose with the melting of the ice caps. At this time there was an extensive land mass 'Gondwanaland' primarily made up of Australia, Antarctica, Africa and India and the effects of this large land mass on ocean currents and wind patterns were important to the periods of Permian coal formation. The main Carboniferous-Early Permian shore line was affected by a volcanic island arc, and upward movements along this line resulted in an off-shore land mass. The depression between this emerging land and the main continent received a thick sequence of shallow water marine and continental sediments of late Permian and Triassic age. This depression can be regarded as an elongate depositional basin with restricted access to the open sea (Shuntz, 1975). Widespread accumulation of material occurred in cycles related to transgression or regression and gave rise to the accumulation of transport material or to the formation of in situ material. The quality and quantity of coal material depended on the rate of accumulation and the relation to water level and resulted in large deposits of Permian Coal. The off shore land mass, now the New England District of NSW continued to rise and to move in a south westerly direction towards the main continent in the Late Permian and Triassic Periods. The intervening depression includes the present Permo-Triassic Sydney Basin. A study of the environmental conditions at the time of formation of the Newcastle Coal Measures lead to the plan in Fig. A.9 showing the depositional conditions pertaining to the Borehole Seam which is at the base of the coal measures (Branagan and Johnson, 1970). Page A-6
2.2 Structure During the formation of the Sydney Basin, the upward and south westerly movements of the New England block resulted in an overall tilting of the Sydney Basin to the south west. The final uplift of the New England block in the Triassic Period resulted in exposure by erosion in the north eastern part of the Basin which includes the northern parts of the Newcastle District. Further, the thin widespread marker beds which occur in the Permian and Triassic strata indicate that tilting occurred during deposition since these beds thicken in accordance with a progressive tilting to the south west. The Permian coal deposits of the Main Coal Province generally occur in fairly distinct structural basins with dips of less than 5 degrees. The northern part of the Newcastle Coal Measures occurs within a shallow synclinal structure known as the Macquarie Syncline. Its axis plunges gently southwards along Lake Macquarie. The structure contours of the Borehole Seam at the base of the Newcastle Coal Measures are shown in Fig. A.11 (Branagan and Johnson, 1970) . Thus the lower parts of the sequence, the Lambton and Adamstown Sub Groups, occur in the northern and western sections of the outcrop area, while the upper parts occur in the southern and central area of the Newcastle District and then swing to a south westerly direction further south (Fig. A.10). A cross section through the Coal Measures, shown in Figs A.12 and A.13 (McKenzie, 1962) is published in simplified form by Branagan and Johnson, 1970. The section shows that, as the Coal Measures strata thins westward towards the anticlinal flank, the seams tend to merge and coalesce. There are some shallow dome and basin structures within the main synclinal and anticlinal structures which have resulted from folding about east-west axes generally normal to the main fold axis. Joint planes frequently trend at about 320 degrees and 020 degrees. The north west direction is paralleled by several faults (Fig. A.10) as well as dolerite dykes. Faulting is usually of a normal type with sub- vertical fault planes. This displacement is usually small, less than 6 m. Where the dykes penetrate coal seams, the coal is converted to natural coke near the contacts. In the central-western part of the Newcastle District, geological investigations have established the presence of igneous sills in the Great Northern, Fassifern and Borehole Seams. 2.3 Stratigraphy The sequence of stratigraphic units in the Newcastle Coal District is correlated in Fig. A.6 with the stratigraphy of the Southern Coalfield. The Permian and Triassic strata are generally conformable and laterally continuous. The Triassic sequences are mainly terrestrial with only minor coal inclusions. The whole sequence has late Triassic or younger intrusions. The Hawkesbury Page A-7
Sandstone occurs in the south west portion of the Newcastle District (Fig. A.5) and does not overlie any areas of coal production. The Narrabeen Group is made up of two sub-groups. The Gosford Sub-Group consists of about 240 m of sandstone and siltstone in the Wyong area, to the west of Tuggerah Lake (Fig. A.2). The Clifton Sub-Group is made up of the Patonga Claystone (140 to 170 m), Tuggerah Formation (110 to 160 m sandstone and shale) and 120 to 200 m of the Munmorah Conglomerate, discussed later. The Munmorah Conglomerate has traditionally been placed at the top of the Permian Newcastle Coal Measures (Fig. A.5), although plant assemblages from earliest Triassic marine rocks in Western Australia and Pakistan correspond with those found within the Munmorah Conglomerate (Retallack, 1980). The late Permian rock sequence of the Main Coal Province is made up of shallow marine and coal measure sediments. The top part of the Permian coal measures unit shown in Fig. A.6 contains most of the coal resources of New South Wales. There are minor facies changes within this unit. A widespread sandstone marker bed occurs at the base of the Newcastle Coal Measures and overlies a mudstone of marine incursion. The Newcastle Coal Measures outcrop in the north of the Newcastle District. The sequence of rocks is shown in Fig. A.5. In the active mining areas the total thickness of sub-surface intersections varies from 410 m near Swansea to 100 m near Awaba (Crapp and Nolan, 1975). The measures thin towards the west and there is also some convergence of the seams in this direction. There are 14 main coal seams, the topmost mined seam is the Wallarah, and the lowest is the Borehole. Generally the upper seams are the steaming coals and the lower seams the higher rank coking coals although steaming and coking coals can be found interbedded. The complete sequence of the Coal Measures is well defined north of Swansea and around Lake Macquarie. Further south only widely scattered drillholes have penetrated to the full depth of the Coal Measures. Below the Newcastle Coal Measures are the Tomago Coal Measures, the Marine Maitland Group and the Greta Coal Measures. The Waratah Sandstone is a prominent marker bed, 10-30 m thick and separates the highly productive Newcastle Coal Measures from the lower Tomago Coal Measures which contain numerous and generally thin coal seams normally heavily interbedded with mudstone. There are workings of very limited extent north of the Newcastle District near outcrop and the coal was mainly for domestic use. These seams are generally too thin to be mined and contain numerous stone bands. The lower Greta Coal Measures are composed of thick seams of high volatile, low ash and high sulphur coals which are mined to the north west of the Newcastle District in the Maitland - Cessnock - Greta District (Fig. A.l). In the Newcastle District the Greta Coal Measures are deeper than 1000 m and will not be mined in the forseeable future. Page A-8
2.4 Conglomerate Units The natures of the various strata above the coal seams are important since their caving and deformation characteristics play a vital part in determining the resulting subsidence after mining. The most significant beds in the Newcastle District in thickness, extent and consistency are the conglomerate units which comprise almost 30 per cent of the strata (Diessel, 1980). In addition to the conglomerate, the strata above the coal seams are made up of coals, sandstones, shales, tuffs and various intermixtures of these sediments. The lower units are generally shaley, and those above the Montrose Seam are generally sandy (Fig. A.5). The three major conglomerate units are the Munmorah, Belmont and Charlestown Conglomerates. The Munmorah Conglomerate at the top of the Newcastle Coal Measures outcrops on the southern shores of Lake Macquarie and extends south. According to Uren, R.E. in Menzies (1974), it consists of 'grey lithic-quartz sandstone which is pebbly in part, conglomerate with pebbles up to 50 mm diameter, grey-green tuffaceous shale showing red-brown mottling in part, and dark grey shale. A shaly-sandy facies, in which pebbles are rare, can be recognised in the basal part of the formation'. The lower Belmont and Charlestown Conglomerates occur further north in the southern suburbs of Newcastle and around the northern part of Lake Macquarie. Individual conglomerate beds occur in lenses or irregular sheets up to 100 m thick and extend over areas of greater than 100 sq. km. Extensive drilling in the Lake Macquarie area has enabled the various conglomerate members to be mapped. The two major units, the Belmont and Charlestown Conglomerates, are shown in Fig. A.14. Mapping of each of the conglomerate units indicates that they formed as streams in localised depressions at the time of deposition (Branagan and Johnson, 1970) and had a source area to the north. The Charlestown Conglomerate predominates in the areas of the subsidence investigations discussed later. It is one of the thickest and coarsest rudites in the Newcastle Coal Measures. It is resistant to weathering and results in areas of high topographic relief to provide several popular residential sites in Newcastle. It is up to 100 m thick as shown in the BHP Coal Geology plan (Fig. A.15) but thins rapidly in a southwesterly direction. It has been traced in bores over a distance of 50 km to the south of its outcrop (Diessel, 1980). The following generalised description of the conglomerates is given by Branagan and Johnson (1970). 'The conglomerates are polymictic, the pebble consist including quartz, igneous and pyroclastic rocks with a predominance of andesite, toscanite, rhyolite and ignimbritic fragments, sandstone, coal and an abundance of chert and quartzite of various colours. The pebbles are sub-rounded and although the majority are less than 1 inch (25 mm) long, they range up to 6 inches (150 mm) and possibly morecrossbeddin. Sorting angd scour-filis poor.l structureGraded sbeddin are gwidesprea is uncommod in outcropn but. Page A-9
Usually the abundant matrix is sandy and the cement either siliceous or kaolinitic'.
A. 3 COAL RESOURCES The several seams presently mined or of economic importance are shown in the stratigraphic section in Fig. A.5. The seams can be conveniently divided into predominantly bright and dull coals. The durite rich Wallarah and Great Northern coals are used for fuel in the power stations around and to the south of Lake Macquarie. The coal in the seams in the lower part of the Coal Measures is used in the local BHP Steel Works for the production of metallurgical coke. The coke oven charge is a blend of coal from the Victoria Tunnel, Dudley, Young Wallsend, and Borehole Seams. In a westerly direction the interseam sediments thin and in some cases fade out so that the Nobbys and Dudley Seams coalesce to form the Young Wallsend Seam. This seam then merges with the Yard and Borehole Seams to form the West Borehole Seam. The various seams of economic importance in the Newcastle Coal Measures are discussed briefly to indicate their extent and thicknesses and other characteristics which affect subsidence patterns when these seams are mined. The chemical analyses of the coals as given by Crapp and Nolan, 1975, are not important to the subsidence work. The plans in Figs A.16, A.17 and A.18 (from Crapp and Nolan, 1975) which accompany the following discussion show the concentration of geological bores and the relevant seam section. This indicates the area for which knowledge of the seams is generally available. Since 1975 further drilling has taken place in areas of mining interest and details of particular locations are discussed with the aid of more recent localised geological information in Appendix B where they influence the subsidence investigations. 3.1 Wallarah Seam This is the top seam of the Coal Measures and is an important source of fuel coal. Maximum development of this seam occurs in the Wangi-Swansea-Vales Point area where the thickness is as much as 8 m. the seam is mainly dull coal with thin zones of bright coal. There are persistent mudstone laminae which vary up to 50 mm thick. The thickness of coal mined varies from 2 m to 3 m. The seam is generally overlain by conglomerates, sometimes separated from the coal roof by mudstone or shale which can have a thickness of up to some metres. The floor of the seam is generally mudstone or shale. In areas where the floor is claystone, floor heave can occur. Page A-10
3.2 Great Northern Seam This seam is the main source of supply to the thermal power stations on the western and southern shores of Lake Macquarie and around Lakes Budgewoi and Munmorah, and has been extensively mined in the Awaba and Vales Point - Munmorah areas. The seam is located generally from 20 to 45 m below the Wallarah Seam but towards the south-west this interval decreases to zero to result in a combined Wallarah - Great Northern Seam. In some areas there is some deterioration of the Great Northern Seam, and in other areas there is splitting of the combined seam which reduces the thickness of the main seam. The seam thickness generally varies from 6 m around Teralba to 3 m around Munmorah. The seam contains persistent mudstone bands from 10 mm to 50 mm thick. Where the conglomerate forms the immediate roof, little if any support is required. However caving can be difficult to initiate and when this has occurred the resulting collapse has resulted in complaints of noise and vibration from local residents. The floor is generally competent shales or mudstones. 3.3 Seams of Minor Significance Between the Great Northern and Australasian Seams, coal occurs in six main horizons. The most significant is the Fassifern Seam which lies at a distance of from 1 m to 10 m below the Great Northern and separated from it by mudstones and shales. Where the seam is split, the lower section is mined, and the interval from the Great Northern Seam can then be as much as 80 m. In that case conglomerates and sandstones form a major part of the interseam sequence. Where it is not split, the seam is around 6 m thick, contains several persistent mudstone and claystone bands from 10 mm to 100 mm thick and generally has a high ash content. In some areas the seam splits into two or three sections, the lowest of which were mined in areas from Teralba to Swansea at heights of up to 3 m. From the Fassifern to the Australasian Seam, the five coal horizons are normally less than 1.5 m thick. All the seams contain stone bands and generally have a high ash content. For these reasons none of these seams are mined. 3.4 Australasian Seam Although the Australasian Seam is not presently being mined, possibilities of mining are being examined in areas around the northern shores of Lake Macquarie. Extraction has taken place in this seam near Cardiff and Swansea. The total seam thickness is from 4 m to 20 m, including splits which in some locations are separated from the base of the seam by tuffaceous sediments or sandstone lenses. These splits converge to the west of an area of inferior coal quality with the underlying Montrose and Wave Hill Seams. In areas where the Australasian Seam has been mined, it is only the bottom 2.5 m which is of economic interest. The lower ash, high vitrinite content coal has been blended with other coals for metallurgical Page A-ll coke production. 3.5 Victoria Tunnel Seam The VT Seam is the top seam in the Lambton Sub-Group which contains the coking coal seams of the Newcastle Coal Measures. The VT Seam has been extensively mined east of Lake Macquarie. There the positions of the coal plies and bands divide the Seam into sections, the best range being from the floor to the '7 ft band' (Fig. A.17). The selection of the roof depends on the mining methods and the quality of the roof at a particular parting. The optimum development of the seam is along the northern part of its coastal outcrop. The seam thins from 3 m to 2 m in a westerly direction, and deteriorates to a zone of multiple seam splitting, shown hatched in Fig. A.17, which is uneconomic. Further west, the seam regains a coaly section of inferior quality, less than 1 m thick. Three distinct areas shown in Fig. A. 17 exist within the seam of mudstones and carbonaceous mudstones, known as 'want areas' with the change from coal being gradual. The floor of the seam is mudstone and the roof is coal and mudstone or claystone bands. Coal from the VT Seam has a high content of vitrinite and is blended with coals of low vitrinite content to form the coking coal blend for the Newcastle steelworks. 3.6 Dudley Seam The Dudley Seam lies beneath the VT Seam. Although it combines with the overlying Nobbys Seam and underlying Yard and Borehole Seams to form the various seam combinations shown in Fig. A.19 (Branagan and Johnson, 1970), the Dudley Seam still retains its identity. The seam thickness varies from 1.5 to 2.5 m. The seam is split by a mudstone horizon up to 500 mm thick. In the central area of the Dudley Seam, it becomes further split by shale and claystone bands and the coal quality deteriorates considerably, shown by the high ash contents in Fig. A.19. The combined Dudley-Yard Seam below the western shores of Lake Macquarie varies from 3.0 to 3.5 m thick and in the section each seam is identifiable. The Young Wallsend Seam is a combination of the Dudley and Nobbys Seams and is around 2.5 m thick. The Young Wallsend Seam continues to be mined extensively to the north west of Lake Macquarie. Towards the south the YW Seam coalesces with the Yard Seam and increases in thickness to about 3.5 m. Where the YW/Yard and Borehole Seams combine to form the West Borehole Seam, the total thickness is 6 m. The thickness and coal quality decrease towards the west. Page A-12
3.7 Yard Seam The Yard Seam was the first seam mined beneath the City of Newcastle but it is not presently mined as a singular unit. The various combinations of the Yard Seam with other seams shown in Figs A.18 and A.19 contain large reserves of low ash coking coal. The limits of the seam are approximate and are shown where the seam deteriorates in quality and thins to less than 1.3 m. In the north, the Yard Seam tends to merge with the underlying Borehole Seam. The main section shown in Fig. A.18 is located west of Swansea and is the area with prospects of future mining, having a thickness of 1.5 m. The shale roof generally extends to the Dudley Seam and the immediate floor is shale. To the north, in the hatched area, the seam is split by stone bands, and further north the seam thins considerably. To the west of the singular unit of economic section, the Yard Seam combines with the Dudley Seam, further west with the Nobbys to form the YW Seam, and then the Borehole Seam to form the West Borehole Seam. In these combinations, the seam retains its thickness of around 1.5m. 3.8 Borehole Seam This seam has been widely mined and has been extensively explored south to Swansea from its outcrop around Lake Macquarie. Coal from the Borehole Seam is the basis of the low ash coking coal blend for metallurgical coke. This coal has also been exported. South from Swansea where the depth of cover becomes greater than 500 m, few exploratory holes have been drilled to the Borehole Seam. Being at the bottom of the section, the outcrop of this seam defines the northern limits of the Newcastle Coal Measures. The base of the Borehole Seam, shown by the structure contours in Fig. A.11 delineates the Macquarie Syncline. The total thickness of the seam, including shale bands, varies from 8 m in the vicinity of the northern outcrop to several millimetres in the central area below Lake Macquarie. The working section along the coastline north of Swansea varies from 1.5 to 2.5 m high, the common working floor being the 'steel' band, and the roof the 'four inch' ply (Fig. A.18). Below this section, the coal is high ash and of inferior quality. The seam thins to the west, the l m isopach being defined by the hatched area in Fig. A.18. West of this area, the seam again thickens to 2.5 m and then decreases to about 1 m towards the western outcrop. Work by Branagan and Johnson (1970) has shown that as the seam thins, the ash content increases. Page A-13
3.9 Reserves and Coal Use The 26 collieries operating in the Newcastle District (Fig. A.3) produced a total of 18 million tonnes of raw coal in the year 1982-83. The District is the greatest producer of underground coal in the Sydney Basin. Both fuel coal and coking coal are produced. Fuel coal from the Wallarah, Great Northern and Fassifern Seams is used for electricity generation by the State Electricity Commission in the power plants constructed around Lakes Munmorah and Budgewoi and on the southern side of Lake Macquarie. The coking coals are blended with each other and with some coals from the Southern Coalfield to form the metallurgical coke used in the BHP Co. Ltd., steelworks in Newcastle. Both the fuel and coking coals have been and are being exported. The total in situ proven reserves of the three seams forming the main source of fuel coal are around 3000 million tonnes (Crapp and Nolan, 1975) for a seam thickness of greater than l m and a depth of cover less than 600 m. Recoverable reserves for working seam heights greater than 1.5m were estimated at 1500 million tonnes. The total in situ reserves of coking coal are around 1200 million tonnes for a seam thickness greater than 1.2 m, ash content less than 35 per cent, and depths from 100 m to 430 m. Recoverable reserves were estimated at 500 million tonnes. When calculating recoverable reserves of both fuel and coking coals, present and proposed residential areas were considered and restrictions placed on mining beneath the tidal lakes were taken into account (Crapp and Nolan, 1975). However recent and current subsidence investigations have resulted in a significant increase in the recoverable reserves beneath residential areas, beneath tidal lakes and beneath the Pacific AOcean. 4 MININ. G AND SUBSIDENCE 4.1 General Historical Review The early history of coal mining in Newcastle is covered in detail by Branagan (1972) and by Fryer (1980). The following general review makes reference to these publications, as well as to early reports and publications. The first recorded discovery of coal by white men in Australia was made by a party of escaping convicts on 30th March, 1791. The most likely location is near the entrance to Glenrock Lagoon 10 km south of Newcastle Harbour (Fig. A.2). Since news of their discovery did not reach Sydney for several years, later finds of more practical importance, such as at Port Stephens in June 1796 and at Bulli in June 1797, are often referred to as the first discoveries. Page A-14
In September 1797 Lieutenant John Shortland found coal in Newcastle Harbour which was probably the Dudley Seam. A mining expedition which left Sydney in June 1801 discovered other coal occurrences in Newcastle Harbour, and later that year coal was first mined. The miners were convicts and the operations were under the supervision of the military authorities. By November 1801 the tunnels, up to 30 metres underground, were producing 9 tons per day. By 1820, tunnels and shafts were up to three kilometres from the original diggings. Because of accessibility, the Dudley and Nobby's Seams would have been the first worked, and the better coal of the Yard Seam was worked when shaft sinking came into operation. The coal was first shipped to Sydney in 1801 and 100 tons of coal were freighted to the Cape of Good Hope. In 1802 coal was also exported to India (Branagan, 1972). The first authentic records of output were for the years 1826 (1834 tons), 1827 (4122 tons) and 1828 (4025 tons). From 1826 to 1831 the output was 19,719 tons (NSW State Government, 1908). The total output from 1800 to 1831 was estimated to be 100,000 tons and represents an area of 40 acres (16 ha.) in the Yard Seam on the basis of 60% extracted and 40% left in pillars (NSW State Government 1908). After some years of negotiations, the Australasian Agricultural Company sunk a 2.7 m diameter shaft, the A shaft, at the west end of Church Street, Newcastle and brought their mine into operation in 1831. In 1838 the B Pit and in 1842 the C Pit were sunk and the Yard Seam was extensively mined (NSW State Government 1908). These pits are shown in Fig. A.20a (Branagan, 1972). Coal was also mined from the Dudley Seam. Over 7,000 tons were produced in the mine's first year of operation. The A.A. Co., was given monopoly rights to the coal mining up to 1859 but because of its unpopularity, the Company gave up efforts to enforce it in 1845. After the breaking of the A.A. Company's monopoly, the first pits were established along the outcrop at Wallsend, Lambton and Waratah. These were followed by a second line of mines working the same seam through shafts. Among these, the F Pit, later the upcast shaft for the Sea Pit was sunk in 1854 to the Yard Seam and large areas were mined (Fig. A.20b - Fryer, 1980). In 1848, the Borehole Seam was discovered at a depth of 50 m at Newcastle and 44 m at Hamilton. The seam was 3 m thick and contained coal which was among the world's finest, located right at the port which accommodated the largest ships. The discovery of the Borehole Seam and the ready availability of this coal is given as one of the reasons for the late start of the mining industry of the Southern Coalfield (Sellers, 1976). In 1846 the breakwater was built between Nobbys Island and the mainland to improve the conditions in Newcastle Harbour (Fig. A.20), gas was introduced in many towns in the 1850's and the population increase at that time resulted in the development of local manufacturing industries. These events contributed to Page A-15 an expansion of the coal mining industry but the most important factor affecting the demand for coal was the opening of the Sydney to Parramatta railway line in 1855 and the Newcastle to Maitland line in 1857. In the 1850's the A.A. Co., still had several coal pits around Newcastle city but by 1914 the Sea Pit was its only producing mine. Other Companies to open collieries included the Scottish Australian Company (at Lambton) and the Wallsend Coal Company (at Wallsend). Several mines were in production by 1860 although more than 90 per cent of production came from four mines. Burwood Colliery commenced its operations at the site of the first coal discovery at Glenrock Lagoonin the early 1850's. The workings, in the VT Seam, for the year 1886 are shown in Fig. A.21 (Grothen, 1983). The mine at that stage was owned by the Newcastle Coal Mining Company. On 30th November 1854, the Coal Mines Registration and Inspection Act received Royal Assent, following the introduction of the Collieries Bill into the NSW Parliament (Whitmore, 1981). The position of 'Examiner of Coal Fields' was created in December 1854 for William Keone who played a considerable role in the development of coal mining in the Hunter Valley. He was appointed 'Examiner of Coal Fields' Hunter River District, and 'Keeper of Mining Records' in February 1863. The Department of Mines was inaugurated in September 1874 and the Collieries Regulation Act to regulate coal mining operations came into being in June 1876 (Sellers, 1976). The coal mining industry expanded in the 1880's when at least 40 collieries worked in the Newcastle area. This is shown by the plan of 1887 (Fig. A.22). The production in that year was just over 2 million tons. The Sea Pit was sunk to the Borehole Seam in 1888 and high production from that seam followed. Around this time mining progressed beneath the Hunter River and to beyond the high water mark of the Pacific Ocean. Bord and pillar mining methods were used, with 50 per cent recovery. A solid cover limit of 120 ft (36.5 m) was imposed (Atkinson, 1901, Humble, 1920). There were coal mining -disasters at Lithgow Valley, Ferndale and a near disaster at Maryville Colliery in 1886. Royal Commissions followed and The Coal Mines Act of 1896 gave way to the Coal Mines Regulation Act of 1912 which was amended as improvements to working conditions and safety standards occurred. In the 1930's, mechanisation was introduced into the collieries but it was not until the 1940's that mechanisation played any significant part, and even in 1955, 60% of the coal produced was still won by hand cutting methods. This figure was 10% five years later. With mechanisation in the mining industry came the necessity for the design of mine workings for maximum extraction, and safety and stability for both the underground workings and the surface. Page A-16
The Joint Coal Board (NSW) was established in 1946 by Acts of both the Commonwealth and New South Wales Parliaments to control the State coal industry. The need arose to increase output and to reduce industrial unrest. The JCB had wide powers and functions which enabled it to take any necessary action to ensure that coal was produced in sufficient quantities to meet requirements; to ensure coal resources were developed to the best advantages in the public interest; to ensure that the coal produced was distributed and sold in ways to secure economical use of the mineral and to promote the welfare of the men engaged in the industry (Anderson, 1977). The JCB encouraged the expansion of open cut mining to increase production and presently most of the North West Coal District (Fig. A. 4) is mined by open cut methods. 4.2 Early Subsidence Events From the early years of mining in Newcastle, there has been an awareness of mine subsidence and accompanying damage of buildings, which undoubtedly came from subsidence experiences in the United Kingdom. With the sinking of the A.A. Company's A Pit in Newcastle (Fig. A.20) in 1831, the opinion of John Busby (the first 'Mineral Surveyor and Civil Engineer ' appointed to manage the coal mines) was sought as to the depth at which coal could be worked without endangering the overlying buildings (Branagan, 1972) . In the 1880's more than 40 collieries worked in the Newcastle area and much of the residential area was undermined. Even today, some subsidence problems are made more difficult due to the lack of underground survey information or to the fact that when many mines closed, any plans they had were often lost or destroyed. Concern was expressed in 1890 of the undermining of part of Wickham Municipality, Newcastle, by the workings of Wickham and Linwood collieries. Some homes were damaged in Hannell Street, Wickham (Humble, 1920). A deputation to the Minister for Mines made special reference to the Church being in danger and that fast action was necessary for its protection. The Minister assured the deputation that the mine would be surveyed to ascertain the extent of undermining (Fryer, 1980). Other subsidence events at Stockton and Wickham caused damage to houses in 1894, 1896 and 1897. The subsidence as a result of pillar crushing is documented by Atkinson (1902). It was the practice to mine the maximum amount of coal by the pillar and stall system of mining, leaving pillars as small as possible to maximise on the recovery. The earliest cases of subsidence damage were due to failure of pillars which were left after mining. Between 1906 and 1908 damage occurred in a fashionable residential district of Newcastle known as the Hill area. Terraces in High Street (Fig. A.20) became cracked and fissures Page A-17 extended across roads. Dangerous cracking occurred to a cliff face near Newcastle Beach, a concrete reservoir in Tyrrell Street and Christ Church Anglican Cathedral. Sewer pipes, gas pipes, and numerous residences and commercial premises were damaged. Subsidence occurred in an area under which the Borehole Seam was worked and a survey at the time showed that the maximum subsidence was 83 0 mm. A Royal Commission was established to investigate the causes (NSW State Government, 1908) . The damages were attributed to the collapse of pillars in the Borehole Seam workings (5.5 m high at 106 m average cover) of the Sea Pit which were then affected by the much older pillars in the Yard Seam, 0.9 m high and 52 m above the Borehole Seam. Faulty construction of some buildings was given as a contributory factor. The Cathedral suffered damages more than any other building although there was only 25 mm subsidence recorded at the building. It was located over a pillar which was larger than usual and the Cathedral was thus damaged by high tensile strains. In 1916 and again in 1925 several areas of Merewether, 3 km south of Newcastle, subsided causing damage to houses, shops, kerb and guttering and water and gas mains (Wilson, 1973). These damages gave rise to the Mines Subsidence Compensation Act of 1928 and the formation of the Mines Subsidence Board. The Act was later replaced by the Mines Subsidence Compensation Act, 1961. A subsidence incident that occurred in 1943 enabled one of the first shafts in Newcastle to be located. This was the Asylum Pit located near Watt Street (Fig. A.20). The Coal Mines Regulation Act set the widths of roadways at 6 m and stipulated the minimum sizes of pillars in bord and pillar workings, the pillar dimensions increasing at certain increases in the ranges of depths of cover. The practice was to mine according to these regulations beneath residential areas and surface improvements such as roads and railways, leave a 2 chain (40 m) barrier, and then carry on with total extraction. This tended to increase the tensile strains in the area to be protected. With the increasing awareness of the benefits of subsidence investigations overseas, and the greater coal recovery with adequate strata control and subsequent surface subsidence, it appeared that with controlled mining it may be possible to extract pillars beneath structures to enable controlled lowering of the surface and to minimise subsidence effects. 4.3 Mining Methods The first hand mining operations in the early 1800's were tunnels or headings into the exposed seam whose distances were limited by restrictions due to ventilation and transport. The seam discovered in 1797 by Shortland was probably the Dudley Seam, and it was accessible from high ground which made it suitable for adit or drift mining (Branagan 1972). The ventilation problem was alleviated by using pairs of headings joined periodically to form pillars thus establishing a simple Page A-18
ventilation circuit to allow deeper mining. The pillars which were formed were robbed as mining retreated, but the integrity of the roof was maintained. With the improvement of hand mining techniques, pillars were extracted and the roof was allowed to cave behind. Typical mining layouts are shown in Fig. A.23 (Martin 1974) . Because of the distance restrictions to mining by the use of tunnels, timber lined shafts were sunk to the seams for access. The Market Shaft (Fig. A.20) was one of the earliest sunk in Newcastle, probably in 1817 (Branagan, 1972) from which the Yard Seam was mined. It was abandoned after being flooded several times. Then followed the Asylum Pit and the A.A. Company's first shaft, the A Pit, in 1831 (NSW State Government, 1908). The bord and pillar system, as used at the turn of the century, is described by Atkinson, 1902. The method as used resulted in a recovery of 50 per cent in first workings. Where two seams were mined, the pillars in the two seams were superimposed. Pillar sizes, minimum width 24 ft (7.3 m), were determined after experience of crushes with smaller pillars. The Borehole Seam, first mined in 1888 was worked in three stages (NSW State Government, 1908). The height of mining was 4.5m. 'The headings and bords were driven in the middle portion; followed by lifting the bottoms in the bords; and, finally, the tops were dropped. The bottoms were left unworked when removing the pillars'. This method was practised widely. As the thicker seams were mined to the limits of transport and ventilation, advancing hand worked longwall methods were developed for the mining of thinner seams. These are discussed by Brisbane, 1970, together with details of early examples. In 1850, the Government geologist recommended longwall mining in the VT Seam in lieu of pillar and stall in order to obtain larger coal (Branagan, 1972). In the 1860's the first practical mechanical coal cutter was marketed in the U.K. (Whitmore, 1981) and with further technological improvement, the pillar and stall pattern was developed into a geometric room and pillar arrangement for development work and also for pillar extraction on the retreat. However in New South Wales the coal was generally hand won but some mechanisation in ventilation and transport and the introduction of rail haulage resulted in parallelogram shaped pillars (Fig. A.23). Mechanisation was first introduced into the collieries in the 1930's but it was not until the late 1950's that more highly mechanised mining methods and equipment were developed. Trackless equipment for coal machinery and haulage enabled pillars to become a rectangular shape and led to the development of methods to minimise development and maximise on extraction of pillars, such as the lift and fender method shown in Fig. A.24 (Martin, 1974). Page A-19
With the increase in mechanisation and the mining at progressively greater depths, aspects of strata control become more important in the consideration of mining methods and mine layouts. In the early 1960's mechanised retreating longwall faces were introduced in the Southern Coalfield. The self advancing longwall supports were used in the Newcastle District in the early 1970's together with the standard equipment used in the room and pillar extraction methods in what became known as the shortwall method of extraction (Martin and Hargraves, 1972 and Kay and Mowbray, 1970). The longwall extraction method which is shown in Fig. A.24 (Martin, 1974) was introduced in the Newcastle District in the early 1980's. 4.4 Current Subsidence Studies Awareness within the mining industry increased regarding the unnecessary sterilisation of coal beneath surface features. It was decided to conduct subsidence investigations in order to investigate the relationship between surface subsidence and underground extraction. This would enable a guide to be established to determine the maximum safe recovery of coal from beneath the surface where structures or natural features require some degree of protection. Prior to the detailed subsidence investigations which commenced in 1970, various Colliery surveyors had conducted surveys over areas of pillar extraction. At that stage, there was no informed guidance or coordination of the subsidence work. In addition, the primary tasks of the Colliery surveyors relate to responsibilities underground and as such cannot devote the time necessary to carry out a series of surface surveys to the extent or with the frequency required for a systematic and thorough investigation of mine subsidence. The subsidence studies which formed the current programme of investigations were carried out in the residential areas to the south of the City of Newcastle (Fig. A.2). The studies were on the eastern side of Lake Macquarie, 10 km south of the City over the workings of Lambton and Burwood Collieries and a further 5 km south over the workings of John Darling Colliery. The Collieries further to the south, around Lake Munmorah and Lake Budgewoi (Fig. A.2) are mined for coal for the NSW Electricity Commission power stations nearby. The seams extend beneath these Lakes. The subsidence wqrk which was carried out there by surveyors from the NSW Department of Mineral Resources, and the local collieries is not discussed here. The first studies were in undeveloped rural land over pillar extraction at Burwood and Lambton Collieries. The results of these investigations were compared with calculations based on the U.K. Subsidence Engineers' Handbook. This information was correlated with the difference between the arenaceous geologies of the Newcastle area and the argillaceous coal measures strata in the U.K. which would cave more readily. The results confirmed that the subsidence is less in the Newcastle area than in the U.K. for the same mining geometry. Later subsidence Page A-20 studies over pillar extraction panels and shortwall panels showed that subsidence in the Newcastle District was significantly less than in the U.K. over narrow extractions with similar mining geometries (Kapp, 1978) . The work enabled the widths of panels and pillars in panel and pillar mining systems to be designed where it was required to extract coal beneath residential areas, and continuing subsidence work in these areas have further confirmed the design criteria originally adopted. Around the time of the Inquiry into Mining Beneath the Stored Waters of the dams of the Southern Coalfield, an overseas consultant was commissioned by the Department of Mineral Resources to make recommendations for mining the seams beneath Lakes Munmorah, Budgewoi, and the southern part of Lake Macquarie and the shore lines. The final report was used as a basis for two Annexures which were generally attached as conditions when approval was given to extract pillars or to mine beneath the lakes and the shore lines. The local subsidence work showed that these Annexures were too restrictive in areas where there were significant conglomerate beds. With longwall mining beneath Lake Macquarie, the Pacific Ocean and the shorelines, the results of the current subsidence work were used to successfully apply for more liberal mining layouts than would have been permitted under those Annexures. The Department of Mineral Resources has been appraised of the progress of the subsidence work at various stages of the programme of investigations as it is the Department's Inspectoral staff who approve particular applications for mining, taking into account all aspects, including subsidence. 4.5 Application of Subsidence Studies The monitoring of subsidence and strains over extraction of various dimensions at different depths of cover is discussed later. It has enabled an empirical method of subsidence prediction to be formulated. Thus colliery operators can plan for the maximum extraction of coal in areas where natural surface features or structures need some protection from the effects of subsidence. The Newcastle District has a significant amount of urban development and the coal seams extend-to beneath tidal lakes and the Pacific Ocean. There have already been substantial benefits from the subsidence work. South of Newcastle the suburbs of Charlestown, Whitebridge, Gateshead and Belmont overly early and current workings in the Victoria Tunnel, Dudley and Borehole Seams. Most of the investigations have been carried out in these areas and have resulted in the development of the particular panel and pillar layouts applicable to the Newcastle District. This has enabled pillar extraction and shortwall mining to take place beneath these residential areas, and beneath a light industrial area in Bennetts Green, located between Gateshead and Belmont. Page A-21
Between Belmont and Swansea, the V.T. Seam and the Borehole Seam contain vast reserves of coking coal. Longwall mining has now commenced in these seams. The first was beneath the sewerage treatment works and has extended beneath the shore line and the Pacific Ocean. It is anticipated that longwall methods will be introduced in these two seams and the intervening Yard Seam beneath Lake Macquarie, the shore line and the residential areas of South Belmont, some of which are at low elevations with respect to the Lake. The northern and north western shoreline of Lake Macquarie between Teralba and Toronto will be affected by longwall mining. The residential development of these suburbs, major road systems, the Sydney to Newcastle high pressure gas pipeline, and the Main Northern Railway overly the Young Wallsend and Dudley Seams. In these areas around and beneath Lake Macquarie, extraction will take place in one, two, and possibly three seams. The mining proposals include extensive series of longwall panels. The initial requirements of the subsidence work were to protect the surface and surface features. With the introduction of more extensive and planned mining in more than one seam, the effects of extraction in one seam on unmined second and third seams, or on pillars which have remained after mining in those seams, have become of vital importance. Thus information pertaining to caving, bed separation and pillar stability is significant. There is collabration with other organisations who concentrate on developing mathematical modelling techniques for subsidence using the physical characteristics of the strata and empirically derived subsidence data. One problem being give more attention is the application of the subsidence work to mining beneath the Ocean where the depths of cover decrease in an easterly direction away from the shoreline. It is required to determine the shallowest depth of cover for which particular mining layouts are applicable. The Wallarah and Great Northern Seams contain vast reserves of steaming coal used for electricity generation. These seams lie beneath Lake Munmorah and Budgewoi and the surrounding limited residential development. There have been subsidence investigations carried out in these areas by other mining companies and surveyors of the Department of Mineral Resources. A.5. SURVEY PROCEDURES The information obtained will enable a comparison to be made with 5.1the Introductionresults from the more detailed studies around Lake Macquarie to the north. the basic observations made in a subsidence study are those of level and distance measurement using standard survey techniques. The survey observations and the computer reductions of the observations are described by King and Green (1973). The precise survey work and some of the difficulties encountered with precise surveys are described by Milliken (1979). Page A-22
To obtain a subsidence contour plan a grid of stations is laid out and levelled at regular intervals. A subsidence contour plan enables profiles to be drawn along selected lines and shows the development of,subsidence by relating the surface movements to the extraction areas at the date on which the levelling was carried out. This leads to a fuller understanding of the subsidence process. However, obtaining a subsidence contour plan is time consuming and costly, especially with a high depth of cover in which case the subsidence grid would need to cover a large surface area. As an alternative to establishing a subsidence grid, levelling along a line of stations on the surface across an extraction area will give a series of subsidence profiles, one for each level run. Sufficient information is generally obtained from a line of stations established over the extraction and positioned to pass through the area where maximum subsidence is expected to occur. The stations are spaced equidistantly and are measured at regular time intervals to enable the displacements and strains to be calculated and to enable the relationships between subsidence, time and face advance to be investigated. The change in slope of the surface is calculated from the subsidence values and the distances between successive stations. Whether levelling a subsidence grid or line of stations, it is desirable to have two or preferably three widely separated bench marks located in ground unaffected by recent or planned mining activity. Each bench mark should be at a distance from the edge of the extraction which is at least equal to the depth of cover. 5.2 Layout of Grids The grids in the Newcastle district consisted of lines of stations covering each extraction area. The initial grids in undeveloped land were straight lines of stations set out for level observation for the basic calculation of subsidence, and distance measurements for the basic strain calculations. Grids were extended by either lengthening lines or establishing additional lines of stations as later panels were developed and extracted. Stations forming a number of approximately equilateral triangles were established at certain locations to enable the magnitudes and directions of the maximum principal surface strains to be determined. In residential areas where it was not possible to establish straight lines of stations, the survey lines followed roads in the area and subsidence was monitored as a check on movement of homes nearby. Observations of levels and measurements of distances were at regular time intervals, the time interval being more frequent at one to three months as the extraction face passed beneath the particular line of stations. With the finish of extraction, observations were less frequent at around once every six months, decreasing to once every three years as a check on residual subsidence and on the continuing stability of unmined pillars which were left to assist in the protection of the surface. Page A-23
Bench marks were established well outside the extraction area where it was considered that they would not be affected by mining. If it was suspected that an established bench mark could be subject to movement as mining progressed, another bench mark was established further from the workings. As the grid was extended, other bench marks were established. Bench marks were connected to each other by a levelling run to ensure their continued stability, especially in areas where other seams had been mined earlier. The stations consisted of Y section steel fencing posts 2 m long, driven by hand to refusal. The bars were then cut off at ground level and a fine mark was drilled in the top for measuring purposes where required for strain calculations. Each station was tagged for identification. Along streets in residential areas, roofing nails were knocked into the road pavement, circled in paint and numbered for identification and for ease of location. In a few instances, along the footpaths of straight roads, stations were established for levelling and for distance measurement. The spacing of stations recommended by the National Coal Board (197 5) for the calculation of strain is 0.05 of the depth of cover. In the areas of subsidence investigations in the Newcastle District, the depth of cover varied from 100 to 160 m. One study was at 22 0 m and another at 2 80 m. This would mean a station interval varying with the depth from 5 m to 8 m at the lower range of cover, or from 11 m to 14 m at the greater cover. To minimise the survey work an interval of 9 m (or 30 ft in the old British units in which the bulk of the work was carried out) was adopted generally in the Newcastle District for the spacing of stations along the lines where distances were to be measured for the calculation of strain. With the introduction of the more recent metric grids, a station interval of 10 m is adopted where distances are to be measured, and 20 m where levels only are required. 5.3 Levelling and Distance Measuring Levelling was carried out to second order accuracy with each run being closed to an accuracy of 6 mm, with minor exceptions where first order levelling was carried out. First order levelling was initially carried out in some highly sensitive areas where very small maximum subsidence values of the order of 20 mm were expected. The difficulties encountered with the precise work were discussed by Milliken (1979). Stations for distance measuring were set out at a distance close to that specified, and the distances were then measured with a steel band along the ground where possible on some grids, or suspended between tripods set up over the stations. Temperature, sag and tension corrections were made and an accuracy of 1 in 12,000 was achieved. The distances were measured on the slope and the magnitude of the slope was measured at the same time by means of a Wild TIA theodolite. To calculate heighthe horizontat of thl edistance sighter, thoen heighthe tnex otf statiothe ninstrumen were notet d anandd the Page A-24 vertical angle observed. The instrument was then moved along two stations and the procedure was continued. 5.4 Data Processing Initially, the usual surveyors' field and level books were used and the progressive and total subsidence values were calculated manually. The progressive subsidence at any one station is simply the difference in the reduced levels from successive surveys and the total subsidence is the difference between the reduced level at a particular date and the initial reduced level. The slope strain between any two stations is the difference between the final and initial slope distances divided by the initial slope distance. The same method of calculation was used to obtain the horizontal strain from the horizontal distances and is the method used by the National Coal Board (1975). The slope was calculate from the difference in total subsidence between adjacent stations and the horizontal distance between those stations. The extending of several grids and the establishing of subsidence investigations in other areas made these calculations very time consuming and several programmes were designed to handle the calculations, filing and data manipulation from the field booking sheets to the presentation of calculated results from the computer. Special field books were used for recording all observations for use in the computer programme. The new field sheets were similar to normal survey booking sheets and were acceptable for card punching. Two field reduction programmes were written to reduce and adjust the level observations and two for the distance observations. File maintenance programmes enabled the level and distance files to be updated by the addition of the latest survey information at each station. Storage of information was on magnetic discs which could be handled efficiently and expanded easily as the subsidence grid was extended. A separate file was maintained on disc for the levels and distances. The station names, observations and dates were filed in the same order as they appeared on the grid. The summary programmes print the dates and results calculated from the observations. Examples of each printout are given as Figs A.25, A.26 and A.27. They list the following information: 1. total subsidence at a particular station for each date of observation and the incremental subsidence since the previous observation, 2. total and incremental strains for both slope and horizontal distances for each distance measured, and 3. maximum and minimum principal strains in magnitude and direction for each strain triangle. Page A-25
All the earlier observations, reductions and plotting were in British units. The numerical values and scales on the various graphs were converted to their metric equivalents. •/ }l Wi- i. 1
itKXJO UOUC(S1ll ISOuGH
^•»l Maitland-Tomago •'MlwCfJUl Coal District Newcastle'' v Maitland - Cassnock Grata Coal District
SCALE
» *0 60 to 100 170
KIlOMf III t »•»-
FIG. A.l COALFIELDS OF THE MAIN COAL PROVINCE FIG. A.2 COASTAL STRIP OF THE NEWCASTLE COAL DISTRICT FIG. A.3 LOCATIONS OF COLLIERIES, NEWCASTLE DISTRICT Raw Coal Production Northern Districts of N.S.W. million tonnes r45 ///\ Singleton - North West
^ Newcastle
-i 1 1 1 1 1 1 1 1 ^T —i r 1970 '71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 "83 year ended June
Raw Coal Production Southern and Western Districts of N.S.W. million tonnes r45 Y/^{ South Coast
Burragorang Valley
West
1970 *71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 '83 year ended June
FIG. A.4 PRODUCTION GRAPH OF N.S.W. COAL DISTRICTS Formotlon Mtmb«r
Ve"*t Po.«t Cool 6^Q Munmorah Co^yo***™** Ktjfi."On CoAfl,lo Coirpor Tuff Wolloroh Coot MorvA.r.na Perh Tuff IfXjM., Cool iilff Point Cool Moon Itlond < Coth««-tn« Mid ftojr Ttfo'bo ConjIeffi.rotO Beach $& Bocooul Tuff Groot NortMfn Cool *-obo Tuff Chotn Vollty Cool Bo'ton Point COnfllOffWCt* ,Fc»a.if«rn Co S«lf*« Upp«r PHot Cool R*nj I Mtttah* S«On«mpton $ond*fO«t Boolaroo Lowtf Pilot Cool Worn*'. e©r Horfl«y Hi(l Coot Mount Hu'lon Auitratotion Coo' CKWHH(»» Co«flie 5to G'«b« Tififo'o Co*»f>o*»«'Ot« Eooo-oft^ Tuff Uooo* $ pi.* FVr, VollO J Cool L*«« Solif MtfOwOthOf Con«l0""*'0'0 VictOfki Tun*ol Cool Shophocdi Hill Nobby» Tuff Nobby f Cool Bor B.och S.o*>Oi Hilt Coiio>o<*»'0,o Lombton ., DudUy Coal CtKkto Cro-* Coiglo*"«r««* ftog#T Holo Yor-d Cool r.a+»ot Hilt I £ 0 I •» D F«"iOol» Conojomfoto 0»«U**«?< *•*»*/". So*.**. 9oo* S*n«*i« Bortholt Cool Worofoh Sondf.tO«o \ p.rm.on N««Mffff CM' M««-'«« —f- *«"«<•"• *•«• 0 KiL0MET»€3 K> Newcastle Coal District Newcastle Coal Measures FIG. A. 5 GEOLOGY OF NEWCASTLE DISTRICT Southern Area Northern Area (Illawarra) and Central Area (Lower Hunter) Wianamatta Group o Hawkesbury Sandstone Hawkesbury Sandstone •H U) W Gosford c Gosford fd Sub Group Sub Group •H CD a r-l xi o Clifton fd u Clifton Sub Group u u Sub Group u rd V z Sydney Newcastle Sub Group Coal Measures rd w fd Cumberland •H id 8 fS Tomago g 3 £ Sub Group Coal Measures Shoalhaven Maitland Group Group \ t FIG. A. 6. UPPER PERMIAN AND TRIASSIC STRATIGRAPHIC UNITS FIG. A.7 VIEWS OF SOUTH BELMONT AND REDHEAD BEACH FIG. A.8 VIEWS OF THE CENTRAL COAST LAKES AND THE HAWKESBURY RIVER SUGGESTED ENVIRONMENT OF COAl FORMATION Optimum conditions (in situ formation,subsidence 1£XV] keeps pace with plant growth,litll* introduced sediment. 0| | | | Shallow, in situ formotion, some inflow of fine -LLLi sediment. Scon of Milci 0 • ••••.. Variable, sinking fairly rapid, some inflow of fine • • • I 1 I 1 11 sediment and transported debris, closer to fluvial source . ///y/y Oeeper wafer, fairly rapid subsidence, introduced sediment FIG. A.9 ENVIRONMENT OF COAL FORMATION, BOREHOLE SEAM FIG. A. 10 STRUCTURES IN THE NORTHERN PART OF THE COAL MEASURES >4? NEWCASTLE .UJOOI.- I ~~SOs» V \ SCALE 6 Km. 8 3 i NEC FIG. A.11 BASE OF THE NEWCASTLE COAL MEASURES CI <>1 S3 O H H O U W co co co r/> O Pi u u H . ot> CN o o W w CN i—I -4 o M CM CM o CN to OJ u c/5 lambton Contour interval 50 ft (15.2 m) Belmont Conglomerate lambton Cardiff Contour interval 25 ft (7.6 m) Charlestown Conglomerate O I 2 3 4- 5 Kilometres I I I L_l 1 FIG. A. 14 AREAS AFFECTED BY BELMONT AND CHARLESTOWN CONGLOMERATES REHOLE IY .323 [/Me y^i Macquarie j ABZm 68 lm o'HZ 9CAU i'BOfiOO - ai'UiP 9O0 I.OOO i #z,.m ' 67-4m o?2r 1004 00 m FIG. A.15 ISOPACHS OF THE CHARLESTOWN CONGLOMERATE NtWCASTLC /. r# * r l> f / \ u c T R R 2 E ( S s I • EXPIO«*IO«V to«HOiss SEA* OUlCBOf w 0SOf*CM Wallarah Seam Great Northern Seam Fassifern Seam FIG. A.16 SEAM SECTIONS AND AREAS OF INVESTIGATION IfV J * 5> Typical S«ef>or-i 86 BQ*4 8 6" Pij -* 0 9an« * R Portiflej L R Port.«4 Uwoil ttudiioM MteUlf Son«J . I •*( .0"«10*1 »0«l Id* 0UTC*IO*> Cor-'" P«rlt*4 Coy ••'••J Australasian Seam Victoria Tunnel Seam Dudley Seam A. 17 SEAM SECTIONS AND AREAS OF INVESTIGATION .r.r? Yard Seam Borehole Seam FIG. A.18 SEAM SECTIONS AND AREAS OF INVESTIGATION Woe at ah YOUNG WAUSEND SEAM Wollund / NewcAsne/. Mtrewttkt lambton X\* S*' Cfiarltttomn A^-Z^) /s^5 / f^^^TXV"" Ufa4head V>' tlmon 11 s Seal* «f Mitof I i i • i i • i I FIG. A.19 ISOASH MAP SHOWING COMBINATIONS OF THE DUDLEY SEAM --• • • • • Maaey S«*TT - — — — UAOOS- Dt/d/ey 5a*m 8 O VTM ——-• — - laufrDvatty S**m TS^^^oo •—•• — -•• h»/-«r Stmin Appraism*/* Sa.-n Outcro&» Oykt FIG. A.20 EARLY COAL MINES IN NEWCASTLE 100 m UNDERGROUND WORKINGS OF BURWOOD COLLIERY AT GLENROCK LAGOON IN THE YEAR OF 1886. ''\t^;-:r.^ - ._ < / FIG. A.21 BURWOOD COLLIERY WORKINGS, 1886 PLAN sharing dw posditm of Ike CuUunes al *<»*• and- Opening aul in the Newcastle District ilLL • naaMrf Ul \mkr TO ,mi FIG. A.22 NEWCASTLE COLLIERIES, 1887 First workings First and second workings Hand Worked \ i—u-r\f*mr*&r&!*{—»OH—)r-\r==Si—1<—if First workings j First and second workings Machine Worked FIG. A.23: BORD AND PILLAR MINING OPERATIONS Lift and Fender Extraction Shortwall Mining Longwall Mining FIG. A. 24 EXAMPLES OF RECENT MINING LAYOUTS -SUBSZ:1E.NCE-_1NVE£IJCAT1CN-- -LEVELING -SUMMARY -LAMBTON COLLIERY _. STATION. REDUCED LEVEL DATE TOTAL SUBSIDENCE TO DATE . JNCREHENTAL SUBSIDENCE 5£.b70DD0 17- 6-71 -3.265000 -0.0260 00 98.622000 8-10-71 -3.313000 -0 .0 4,80 00 96.595000 3-12-71 -3.3i»0000 -0.027000 98.580000 22- 2-72 -3.355000 -0.015000 98.567000 25- 5-72 -3.368000 -0.013000 98.W5000 Zk- 5-73 -3. 88 102.216000 19- 5-70 102.280000 19- 7-70 .062000 .062000 102.300000 9- 9-70 .062000 .020000 102.282000 27-10-70 .064000 •o.oTeo oo 102.297000 l<.-12-70 .079000 .015000 102.295000 11- 1-7 .077000 •0.0 020 00 102.161000 ~17~-~2-~7 -O.05700fl_ -0.13<«000 101.202000 23- 3-7 -1.016000 -0.959000 100.070000 28- 1.-7 -2.ii.80oo -1.132000 - 99.389000 8- 6-7 -2.829000 -o.Tsiooo 99.302000 22- 7-7 -2.916000 •0.067000 99.275000 17- 6-7 -2.9i.3000 •0.027000 99.222000 8-10-7 -2.996000 -0.053000 99.201000 3-12-7 -3.017000 -0.021000 99.079000 22- 2-72 -3.139000 •0.122000 99.166000 ^17o520oT .0870 00 99.068000 2i»~5-73 -3.150000 •0.098000 69 100.791(000 19- 5-70 100.651000 19- 7-70 .057000 .057000 100.668000 9- 9-70 .071(000 .017000 100.651(000 27-10-70 .060100 -0.011.0 00 FIG. A.25 COMPUTER PRINTOUT OF CALCULATED SUBSIDENCE — • TRON TO —DATE UTSTANCE-"TOTAt—STRAIN— "INt.—STRAIN FEET U.K. PER METRE 2«<- 8-71 29.8100 -.03 1-10-71 29.80bO -<(. 5«( -0.13 9-11-71 29.6030 -!*.«.«. .10 22- 2-72 29.8110 -«..38 .07 23- 5-72 29.6060 -A.Sit -0.17 22- 5-73 29.8110 -1..36 .17 L 87 - L 88 2- 6-7 0 29.971,0 6- 9-70 29.9720 -0.07 -0.07 30-10-70 29.9750 .03 .10 11-12-70 29.9710 -0.10 -D.13 1*.- 1-71 29.9780 .13 .23 22- 2-71 29.9e 25- 3-71 29.9730 -0.03 -0.37 3- 5-71 29.8670 -3.57 —3.5*. i 10- 6-71 29.8620 -3.71. -0.17 7- 7-71 29.6590 -3.67 -0.13 2*.- 6-71 29.6630 -3.70 .17 1-10-71 29.6520 -1..07 -0.37 9-11-71 29.65<(0 -1(.00 .07 22- 2-72 29.6610 -3.77 .23 23- 5-72 29.6590 -3.8*. -0.07 22- 5-73 29.6i.S0 -1..20 -0.37 L 88 - L 69 2- 6-70 29.6610 6- 9-70 29.8550 -0.20 -0.20 30-10-70 29.8560 -0.17 .03 11-12-70 29.651.0 -0.23 -0.07 !«.- 1-71 29.6580 -0.10 .13 FIG. A.26i COMPUTER PRINTOUT OF CALCULATED STRAIN STRAIN TRIANGLE DATE MAX. PRINCIPAL STRAIN "IN. PRINCIPAL STRAIN ST AT. 2 - S»lrTr3- "ST ATTE TlAGNnuufc DIRECTION Mk&NlTUUt DIRECTION M.M./METRE DEGREES K.M./METRE DEGREES ZZ- 2-72 . 9066 3.2 -*.. Die*. 93.2 71 - L 72 - L 73 2- 6-70 8- 9-70 . 011.2 76.7 -0.2119 166.7 30-10-70 •0.1615 121.3 -0.211.1 211.3 l«t- 1-7 .931.2 35.1 .1253 125.1 22- 2-7 •1.6955 26.6 -2.7150 118.8 25- 3-7 0.9230 •13.9 -3.1,167 76.1 3- 5-7 .1,1.36 -6.3 -3.2918 81.7 10- 6-7 .*,789 -6.5 -3.171.2 61.5 7-7 .3910 -9.3 -3.0697 60.7 ?*.- 6-7 .3611. -11.«. -2.9359 78.6 1-1C-7 .3123 -10.6 -2.991.0 79.* 9-11-7 .1,1.89 -10.7 -2.9328 79.3 22- 2-•72 .5879 -10.6 -2. 961.0 79.** L 67 -- L 88 •- L 89 2- 6--70 6- 9--70 .0571 17.2 -0 2139 107.2 30- 10-•70 .031.7 63.6 -0 2579 153.6 11- 12--70 -0.C896 1.1. ,8 -0. 21.1.7 13*.. 6 1*- 1-•71 .1,022 1* .6 -0 1369 101.. a 22- 2--71 1.2166 20 .8 -1 .0266 110. 8 25- 3-71 1.3150 26.0 -3.1003 116.0 3- 5-71 -3. 3275 1.0.9 -5.6805 130.9 10- 6-71 -3.6055 *5.6 -5.621.3 135.6 7- 7-71 -3.7310 1.5.3 -5.9685 135.3 • 2*- 8-71 -3.6172 1.6.7 . -6.0131. 138.7 1- 10-71 -3.9805 *7.3 -5.9376 137.3 9- 11-71 -3.9i.62 1.9.9 -5.9271 139.9 FIG. A.27 COMPUTER PRINTOUT OF CALCULATED STRAIN TRIANGLE RESULTS APPENDIX B SUBSIDENCE INVESTIGATIONS IN THE NEWCASTLE DISTRICT APPENDIX B CONTENTS Page No. B.l Descriptions of areas studied B- l 1.1 Introduction and location plans B- l 1.2 Development of the programme of subsidence investigations B- 4 B.2 Details of study areas B- 7 Study 1 - Subsidence effects of static and travelling profiles over pillar extraction B- 7 1.1 Introduction B- 7 1.2 Geology and mining details B- 7 1.3 Subsidence over 2 N Panel B- 8 1.4 Subsidence related to mine geometry B- 9 1.5 Travelling and final subsidence and strain profiles . B- 9 1.6 Shape of the subsidence profile B-10 1.7 Strain triangles B-ll 1.8 Progressive changes in subsidence and strain .... B-12 1.9 Subsidence of the railway line B-14 Study 2 - Subsidence over two shortwalls B-15 2.1 Introduction B-15 2.2 Mining and geological aspects B-15 2.3 Elements of subsidence longitudinally over Shortwall 1 and pillar extraction B-17 2.4 Elements of subsidence in a lateral direction across shortwalls B-18 2.5 Features of subsidence profiles B-20 2.6 Subsidence related to time and face advance B-22 Study 3 - Subsidence over an extensive pillar extraction area B-24 3.1 Introduction B-24 3.2 Mining details B_24 3.3 Subsidence over lateral lines and pillar stability. . B-24 3.4 Development of subsidence over longitudinal lines . . B-25 3.5 Subsidence along Redhead Road and the railway line . B-26 3.6 Subsidence contours and strain triangles B-27 3.7 Subsidence profile characteristics B-28 3.8 Subsidence and damage at the Convent building .... B-31 3.9 Influence of bay length on calculated strains .... B-33 Page B-ii Study 4 - Panel and pillar layout using both pillar extraction and shortwalls to control subsidence. . . B 4.1 Introduction B 4.2 Geographical and geological setting B 4.3 Mining procedures B 4.4 Subsidence monitoring B 4.5 Features of subsidence profiles B 4.6 Subsidence over Q Panel and Gateshead Panel B 4.7 Increase in subsidence with time B 4.8 Subsidence related to time and face position .... B 4.9 Stability of pillars B Study 5 - Subsidence in Gateshead and related surface damages B 5.1 Introduction B 5.2 Subsidence over the first Belt Headings extraction . B 5.3 Subsidence over the Waratah and Gateshead Panels . . B 5.4 Excessive subsidence over the second Belt Headings extraction B 5.5 Evidence of pillar instability B 5.6 Damages to homes and services B Study 6 - Subsidence over various mine layouts in the Victoria Tunnel Seam B 6.1 Introduction B 6.2 Study 6A - L Panel B 6.3 Study 6B - Shortwall 9 B 6.4 Study 6C - Macquarie Panel B 6.5 Study 6D - Subsidence damages over F Panel B 6.6 Summary I B Study 7 - Subsidence along Bulls Garden Road over Dudley Seam extraction, Whitebridge B 7.1 Introduction B 7.2 Subsidence in Whitebridge over NW and X Panels ... B 7.3 Increase in subsidence in Whitebridge due to Y Panel extraction B 7.4 Subsidence along Bulls Garden Road over Y, Z and 0 Panels B 7.5 Subsidence in Green Valley Road, Charlestown .... B 7.6 Summary B Page B-iii Study 8 - Use of the panel and pillar system to control subsidence in residential and light industrial areas B 8.1 Introduction B 8.2 Mining details B 8.3 Subsidence over 3, 4 and 5 NW Panels B 8.4 Subsidence over 6 NW and 4 NW Left pillar extraction B 8.5 Summary B Study 9 - Subsidence over pillar extraction in the Victoria Tunnel Seam, over a longwall in the underlying Borehole Seam and its effect on a sewerage treatment works B 9.1 Introduction B 9.2 Geology and mining B 9.3 Subsidence due to V.T. Seam shortwalls B 9.4 Failure of pillars and pillar remnants B 9.5 Subsidence over the Borehole Seam longwall B 9.6 Effects of early V.T. Seam workings B 9.7 Rate of subsidence development B 9.8 Summary B Page B-iv FIGURES Pig. B. 1 Part of Newcastle District showing BHP Collieries Fig. B. 2 J Lieries Fig. B. 3 Surface topography. Studies 1, 2 and 3 " Fig. B. 4 Contoured bedrock surface. Studies 1, 2 and 3 Fig. B. 5 Coal geology plan. Studies 1, 2 and 3 Pig. B. 6 Early VT Seam workings. Studies 1, 2 and 3 Fig. B. 7 Pig. B. 8 Current Dudley Seam workings, Studies l. 2 and 3 Fig. B. 9 Early Borehole Seam Workings, Studies 1, 2 and Pig. B.10 Surface plan. Studies 4, 5 and 6 Fig. B.ll Coal geology plan. Studies 4, 5 and 6 Fig. B.12 Current VT Seam workings. Studies 4, 5 and 6 Fig. B.13 Fig. B.14 Surface topography, Study 7 Fig. B.15 Coal geology plan. Study 7 Fig. B.16 Early VT Seam workings, Study 7 Fig. B.17 Current Dudley Seam workings, Study 7 Fig. B.18 Fig. B.19 Early Borehole Seam workings, Study 7 Fig. B.20 Surface topography. Study 8 Contoured bedrock surface, Study 8 Coal geology plan. Study 8 Early VT Seam workings, Study 8 Current Dudley Seam workings, Study 8 There are 136 figures which apply to Studies 1 to 9. These are included with the relevant studies and are not t listed here. APPENDIX B SUBSIDENCE INVESTIGATIONS IN THE NEWCASTLE DISTRICT B.l DESCRIPTIONS OF AREAS STUDIED 1.1 Introduction and Location Plans The surveys which formed the basis of the subsidence ;"lStlfJlons "ere 9enerally in the residential areas to the h?£S»A NewJastle and were carried out in either undeveloped bushland with gently undulating topography or in residential areas with or without areas of open parkland. Where there was parkland in residential areas it was possible to establish straight lines of survey stations for strain measurements. Other grids were established in the sandy alluvium near the shoreline of the Pacific Ocean. Most of the earlier studies were carried out for applied research purposes to provide local subsidence information for application to future mining in residential and developed areas. Soye of the studies, and particularly some of the later work, were carried out either at the request of mine management, or as a requirement for mining, and not specifically for research purposes. However the results were used to supplement the other studies and to provide additional information on subsidence in the Newcastle District. The surveying work was planned by and was carried out according to the requirements of the author or was implemented by the author at the request of B.H.P. Collieries where subsidence work was a requirement of mining. Most of the surveying was done by the Survey Department of the B.H.P. Co. Ltd. located at the B.H.P. Steelworks, Newcastle, although some of the work was carried out by the colliery surveyors. The mining over which subsidence was monitored was in one seam either in virgin country or in areas where either overlying or underlying seam or seams had been mined. The mining methods in areas of subsidence investigations were first workings of bord and pillar with or without pillar extraction and shortwall and longwall operations. One or more of these various mining configurations are involved in each of the studies. After having obtained data over extaction areas in undeveloped bushland, it was decided to mine beneath residential areas by shortwall methods, using design criteria for mining Page B-2 geometries based on the local empirical information already obtained. With the continuing accumulation of this information, the same principles were later extended to longwall mining at greater depths of cover beneath the Pacific Ocean and Lake Macquarie. There are five B.H.P. Collieries in the Newcastle District. The two located west of Lake Macquarie are Stockton Borehole and Macquarie Collieries, with the Macquarie lease extending out under Lake Macquarie. The three Collieries located east of Lake Macquarie and south of the City of Newcastle are Burwood, Lambton and John Darling Collieries (shown in Fig. B.l). The leases extend east under the Pacific Ocean, and John Darling lease also extends west under Lake Macquarie. Burwood Colliery ceased production in October 1982. The boundaries shown are those in the Victoria Tunnel Seam. Other seams at Burwood Colliery have different boundaries. The lease for Lambton Colliery was recently extended south into what was previously the John Darling lease to enable access to more coal from Lambton Colliery. The new and old boundaries are shown on Fig. B.l and the old boundary is shown on the area plans. Three seams have been mined at Burwood, Lambton and John Darling Collieries; the Victoria Tunnel, Dudley and Borehole Seams. Some of the working in these seams took place as early as the late 1800's. Subsidence investigations commenced in 197 0 and some of the studies need to consider the effects of old mining in other seams. The subsidence investigations are divided into nine study areas, listed in Table B.l. The names of the collieries and their seams are represented by their initials. Plans showing the surface, the geology and the mine workings of Subsidence Studies 1 to 8 are given in Figs B.2 to B.20. The areas covered by these figures are indicated by the dashed lines in Fig. 5.1 and the circled number is the number of the first detailed figure of that area. Subsidence Study 9 at John Darling Colliery was at Belmont South to the east of Belmont Lagoon and detailed area plans are included in the description of that study. Page B-3 TABLE B.l Subsidence Study Areas in the Newcastle District Area Study Coll. Seam Main Panels Date of ~Surface~Features Plans Monitored First Survey B.2 1 L D 2N» 2NW "~5T7 0~~BushlandTRaIlway to 2 L D 2W 4.71 Bushland B.7 3 L D 0W-4W 4.71 Bushland;Road, Brick structure B.8 4 i VT W 8T7 0~Residential to VT SW1-4 7.72 Residential; Parkland B.10 5 B VT Belt Headings 2.78 Residential 6 B VT L Panel 2.73 Residential B VT SW9 10.76 Residential B VT Macquarie Panel 1.73 Bushland B VT F Panel 12.74 EK Avenue BTII 7 B D xTYTzToTianels 1177 3 Partly~res7 to and B.15 9.75 Partly res. B.16 8 "L D 4T5,6,~NW ~9.7 5 Residential;~ to and Light indus. B.20 6.80 Refer 9 JD VT SW 8-10 6.72 Sandy alluvium Study JD BH Longwall 1 5.82 Sewerage treat. 9 works; Pacific Ocean shore line For Studies 1 to 8, the plans in descending order are of l. the surface, showing topographic contours, 2. the contoured bedrock surface relative to AHD (if there is sufficient surface alluvium in which case these contours will differ from the topographic contours), 3. underground geological features (dykes and faults), 4. mining details in the VT Seam, 5. mining details in the Dudley Seam (where applicable), and Page B-4 6. mining details in the Borehole Seam (where applicable). Note that the plans of the surface topographic contours and the contoured bedrock surface together give the depth of surface alluvium at any particular location. Each of the plans covers a large enough area to include any mining which could have influenced the subsidence monitored by the grids shown. The seam in which current mining was monitored by the particular subsidence grid is indicated as such on the respective area plan. The Charlestown Conglomerate extends over the area and is a very important stratigraphic sequence affecting subsidence. Its extent and thickness was discussed in Appendix A. 1.2 Development of the Programme of Subsidence Investigations The subsidence work can be considered to have progressed in four stages as the knowledge of subsidence and its effects developed. These four stages can be described as follows. Stage 1 The subsidence work commenced with the monitoring of the pillar extraction in 2 NW Panel Lambton Colliery, and the continuing extraction into 2 N Panel in April 1970, described in Study 1. The investigations enabled maximum subsidence to be related to the seam height mined, and comparisons to be made with overseas experiences in different strata. This study also prpvided particular information on travelling profiles of subsidence and strain, on the calculation of maximum and minimum principal strains from the analysis of strain triangles, and on the effect of the distance between stations on measured strains. What was a privately owned railway line was located over 2 NW Panel and although it was usual for pillars to be left in place beneath such features, some limited pillar extraction occurred beneath the line while it was still in operation. Stage 2 Shortly after Stage 1, a bord and pillar district was developed in the V.T. Seam at Burwood Colliery beneath a residential area and, guided by the earlier results it was decided to extract pillars on a panel and pillar basis to maximise on the coal recovery while minimising the surface subsidence (Study 4). The w/h ratio was limited to 0.45. Following this work, two shortwall panels were mined in the Dudley Seam, Lambton Colliery, beneath undeveloped country, with the much greater w/h ratios of 0.66 and 0.62 (Study 2). The covering grid was also used to investigate any possible effects of bord and pillar first workings on the surface (Study 3). The surveyors continued to monitor the surface effects of total pillar extraction as mining continued into 3W, 1W and 0W Panels (Study 3). This extraction passed beneath Dudley Road, a significant road between the suburbs of Whitebridge and Redhead, Page B-5 and leading to the Pacific Highway north of Belmont. Although movement of a two storey brick building was reduced by leaving pillars unmined beneath and around it, the building suffered some damage. Studies on various aspects of subsidence due to total extraction which commenced over 2NW and 2N Panels (Study 1) were continued over the West Panels (Study 3). Stage 3 The subsidence information from Stages 1 and 2 of the investigations clearly indicated that subsidence in the Newcastle District was significantly less than that in the U.K. over extraction of similar mining geometries. This trend was further confirmed by a comparison of the regional geologies in the areas of subsidence work. It was therefore decided to mine four shortwall panels in the V.T. Seam, Burwood Colliery, just to the north of the West Panels, beneath a residential area and parkland (Study 4). The subsidence over the first and second shortwalls was examined before proceeding with the third and fourth shortwalls. These two would affect a steep slope rising to the Pacific Highway where it was known that the surface strata were argillaceous and considered to be of marginal stability. As the maximum subsidences observed over the first two panels were small and correlated well with previous experience, the later two panels were mined. The maximum subsidence has not increased in the ten years since this mining was carried out and there have been no reports of damage to homes in the area. The mining of these panels had established the panel and pillar layout as an effective method to maximise on the recovery of coal while ensuring that the resulting subsidence was contained to an acceptable degree. Generally, mining in residential and light industrial areas in the ensuing years has been according to layouts designed to minimise subsidence (Studies 7, 8 and 9). The subsidence continues to be monitored as required by the Chief Inspector of Collieries and provides additional information on subsidence and mine geometry and the various relationships between subsidence, slopes, curvatures and strains. The panel and pillar layout has been used for many years as a standard method to recover the maximum amount of coal in residential areas while at the same time ensuring that no damage occurs to houses. However there have been instances where houses were affected, in areas where mining layouts were not designed for surface protection (Studies 5 and 6D). The number of homes affected in this way is small when compared with the number that have been undermined and have been unaffected. Page B-6 Stage 4 The introduction of longwall mining methods in the Newcastle District in 1982 was to some degree dependent on the earlier successful application of the subsidence work to the design of extraction layouts in sensitive areas. The first longwalls were at John Darling Colliery beneath the shoreline of the Pacific Ocean, and at Macquarie Colliery beneath the shoreline of Lake Macquarie and some dwellings near the edge of the Lake. The first longwall was located in the Borehole Seam at John Darling Colliery between old workings and a geologically disturbed area (Study 9). The longwall extraction w/h ratio was 0.28, a very low value for the Newcastle area. This longwall also passed beneath the Hunter District Water Board's sewerage treatment works. Other longwalls are planned nearby and further to the south in the V.T. Seam and then in the underlying Borehole Seam, using the earlier subsidence experiences to estimate subsidence and expected strains. r CMUtCn^ 1 MtHMUNC MU\ Of*« •*/ -y, © Setvtt w W»fn«n I P«M rT XJARIl) -j* t>» THC moe -i >.• t \T&] LAMBTON IK *• * (•.;• •O i » '|0» • ^f 5 od Point k sfi LAKE JOHN DARLING MACQUARIE MAGWTTC NORTH MAGNETIC NORTH IS 11 Vt»ST Of TRW NORTH »! 19R0 »N0 MOWS USTfRlT RT 0 1* [VtRTTHRtE TWRS SCALE 170 000 1 3 a: ZE Q\ Figure number of first area plan FIG. B.l PART OF NEWCASTLE DISTRICT SHOWING BHP COLLIERIES \_ // \J (TK. C0..1 U/^M/y. ^Sj 7^.'-ZS, |vrv y ^h V« rt, Study 1 1/ S y/' II // //. \ '// K. tv '•/ i Scole I'10 000 OO 200 300 «O0 SOO mrtfw FIG. B.2 SURFACE TOPOGRAPHY, STUDIES 1, 2 AND 3 '•^..y ae« ^ o^° ! f Kedh FIG. B.3 CONTOURED BEDROCK SURFACE, STUDIES 1, 2 AND 3 / /f / > • * J^_ \^ \ „-VA\ -f-Owdfc» Swn Working* •'V^y'V - <^\ / FIG B.4 COAL GEOLOGY PLAN, STUDIES 1, 2 AND 3 4 vX Scale I '10 000 oo 200 300 400 500 m«tr«R FIG. B.5 EARLY VT SEAM WORKINGS, STUDIES 1, 2 AND 3 FIG. B.6: CURRENT DUDLEY SEAM WORKINGS, STUDIES 1, 2 AND 3 4BB3* Scale MO 000 00*00 300 «qo soo «•''• FIG. B 7 EARLY BOREHOLE SEAM WORKINGS, STUDIES 1, 2 AND 3 rfiBS5§aeRSaH m rass i • K^So Study .-->;• C9J Study 6B Study J i ,*—»« , CM«»i.t s?: •**ui *!*w \ i A BCR421 Bc%$i • Vis VA." fe>>" \ \ Study 6C >6 1S3 7 '--\ \ r/:-' ••••>- i4*N Study -4W_ ' ,'f' •V .#*& £••«*! :-r-' L^- ys; .1*0 J!.'. ''•>* ^iJ Study 5 ',5/5 •^ 5K'< '*.*.» *Mo *«K^^-T^r-.-^iv.' T Scale I > 10 000 OO 200 300 400 SCO \ 1 FIG. B.8 SURFACE PLAN, STUDIES 4, 5 AND 6 \ \ \ »Mm 1 >'«t#»f '-\\V J43 < ^•rTA V A i TT: ' m \* "Vol 69 X ii-r \ \ "» v: uoo \ - 1 L 1389 r-- "—-\- bee62PZ3* 7 L.^.J \ BcP^bif \ \ ^•30 )208 \ IT >23o ecp 69i 74-( \ JPS \\ \ ^ 82. IH 1,290 V\\v 9$ \\ \ \f Hoo 931 \ \\ \ \ 6^8 50/ \ 4-8; Burwood Colliery \ \ ^5o Victorio Tunnel Seam Workings < A, \\x \ v >I420 V\ Scale I' 10 000 OO 200 300 400 500 m«tr«a fe \ X FIG. B.9 COAL GEOLOGY PLAN, STUDIES 4, 5 AND 6 * \ \ "*§ W \ ^ o629 \ s\ \V\ Vs v ,L PANEL U3 «flu ,J UJ 'I U__ ™"^^^ I_JU:^ - ; i^ ^ ^-* v^ ^* .« •<&& .sZ/'C ' >'''•' " *" '/ ,-J\ ^ ! r-« /T T-^\'^-•v ^ r>/ ,.V5n\ tf* a# ^ ^.J. ... ^i^.^rv ?;^M )•:•K \.\^^^ v M- .:>^•vmrm V Z& ?4> w^ aa M\ W; fl i# ^ -^2 CO .Os _2- '^''W jd « l<-~5» "J - A9\ o W±%^i h :>. ivS1*-,' MB >v\ % i^?G OD •^a?'v. >. '•\U7 Sr" i> :W •>5 ^^A-'t ^ V ?mm T>,» i^s^ft T^:. r Jcr. > *—!— \ t^ fir n °noi ^^^i'^^' ' »•-— " «-. •rffclU Dri 0OQQc r^?^ •^LJL_ !-£?.-* ~jIDT' i J.'-'H^*v.i. , rv*?t?fe m*^' r^ffe? a ^ n crW ^ FIG B.17 CONTOURED BEDROCK SURFACE, STUDY 8 v1U).\> \X> Lombton Coll Ouaiay Seom Working. \ \ \ FIG. B.18 COAL. GEOLOGY PLAN, STUDY 8 \ \ ^v A \ \ \\ ^ c/ .. •- Jtl--Iw *%«dH^ AVE C r.. *1 Scale I '10 000 OO 200 300 4O0 500 m«tr« FIG. B.19 EARLY VT SEAM WORKINGS, STUDY 8 CURRENT DUDLEY SEAM WORKINGS, STUDY 8 FIG. B.2" Page B-7 B.2 DETAILS OF STUDY AREAS STUDY 1 SUBSIDENCE EFFECTS OF STATIC AND TRAVELLING PROFILES OVER PILLAR EXTRACTION l.l Introduction This was the first detailed study in the series of subsidence investigations in the Newcastle District and was over an area of pillar extraction in 2 North West and 2 North Panels, Dudley Seam, Lambton Colliery. The surface was undeveloped apart from a privately owned railway line which skirted the area along its north eastern and south eastern boundaries. The surface was sandy soil, flat to undulating topography and there was a light to medium cover of bush and trees. Plans showing the surface topography, geological information and seams worked are included as Figs B.2 to B.7. In this study, subsidence was related to the mining details and to slope changes and strains generated at the surface. Travelling subsidence and strain profiles over the advancing extraction were monitored and the effects of a small area of pillar extaction on the overlying railway line were noted. Stations forming a number of approximately equilateral triangles were established at certain locations to enable the magnitudes and directions of the maximum principal surface strains to be determined. Also, in some areas, stations were set out at intervals of 3 m from 3 m to 18 m to investigate the effect of the distance on measured strain. 1.2 Geology and Mining Details A plan of 2 North West and 2 North Panels, Dudley Seam, is shown in Fig. 1.1. Before the subsidence work commenced, some pillar extraction had already taken place in 2 North West Panel. Pillars had also been extracted in that part of 2 NW Panel which is to the north east of the railway line, from July to November 1968 and also from 1 NW Panel, to the north west of 2 North Panel. The extraction monitored in 2 NW Panel progressed in a south easterly direction, turned into 2 N Panel at the beginning of November 1970 and continued in a south westerly direction in 2 N Panel. To assist the subsidence work, the date of extraction of each pillar was recorded. Pillar extraction was completed on 30th April, 1971. A geological section. Bore 926, is shown in Fig. 1.2. The Victoria Tunnel Seam, 20 m above the Dudley Seam was mined before 1953. No pillars were extracted as shown in Fig. B.5. Beds of tuff, shale and sandstone lie between the Dudley and Victoria Tunnel Seams. Above the V.T. Seam the strata consist of shale, sandstone and (mainly) conglomerate. Over most of the area Page B-8 (Fig. B.3) there is a cover of sandy alluvium which is up to 26 m thick. The depth of cover over the panels varied from 110 to 130 m and the mean seam height mined was 2.0m. At locations of particular interest in the subsidence investigations, localised values were used for cover and mining height. 1.3 Subsidence Over 2 North Panel The subsidence grid is shown in Fig. l.l. One of the main purposes of line 200 to 461 over 2 NW Panel was to check on the advancing extraction and that its effects had not reached the main monitoring line over 2 North, line 35 to 119. The topographic section and various profiles over 2 N extraction are given in Fig. 1.3. The line of stations was established and first levelled and measured in May 1970, before extraction of pillars in 2 N Panel and before the effects of 2 NW extraction. The surface slopes gently away from a sandy hill which is above the start of extraction. The depth of cover from the top of the seam to the surface decreases from 12 8 m to a consistent 110 m. The seam section shows the positions of the line of extraction at progressive dates. The development of the subsidence and the associated profiles are related to the advances of the line of extraction to illustrate the various phenomena associated with the travelling and final subsidence profiles. The vertical dashed lines relate the elements of subsidence to each other at corresponding dates. The first subsidence profile is for 27th October, 1970. The corresponding face position and the mining layout at that date are shown. The maximum subsidence of 427 mm occurred at Station 39 with zero slope at the bottom of the subsidence trough where the compression had a maximum value C. of 2.9 mm/m. The maximum tension Tj of 1.8 mm/m occurred towards the edge of the subsidence trough at Station 54 where the slope was half the maximum. As the area of extraction increased with the advance of the face to 12th January, 1971, the maximum subsidence increased to 914 mm at Station 45. The profiles of the elements of subsidence continued to change with the advance of the line of extraction until subsidence was complete. The profiles selected to represent the final subsidence effects are at 22nd February, 1972. The profile at 28th April, 1971 is at the date when extraction was complete. Pinal subsidence occurred over the earlier extraction while subsidence was continuing over the more recently extracted area. After mining finished there was an additional subsidence of 16 8 mm to give a maximum of 1100 mm, this additional subsidence occurring at a decreasing rate. The maximum final tensile and compressive strains were 4.0 and 5.8 mm/m respectively. Page B-9 The humps in the bottom of the final subsidence profile are due to irregularities in the extraction of coal pillars, the first being the Christmas shutdown and the second being due to poorer pillar recovery. Associated with these humps are tensile strains, and compressive strains are associated with the troughs. The small humps of only 90 mm which occur in the bottom of the final subsidence trough cause significant changes in strain, where with a flat bottomed subsidence trough it would be expected that the strain would reduce to zero. At Station 93, typical characteristics of a critical subsidence trough occur. At the point of half maximum subsidence, the slope is a maximum and both the curvature and strain are zero and the displacement is a maximum. 1.4 Subsidence Related to Mine Geometry In the area of maximum subsidence at Station 85, the width of extraction w = 213 m, h 110 m, so that w/h = 1.94, a supercritical value. With the maximum subsidence, S , of 1100 mm and the mining height, m, of 2000 mm, the "modified" subsidence factor 'a' = S /m = 0.55. max The maximum subsidence is less than the full seam height mined due to - 1. not all the coal is recovered (about 85% recovery in 2 North). 2. the increase in volume of the broken roof strata; and 3. bed separation which could occur within the strata. The effects of roof caving and strata movements would be similar in the same strata throughout the Newcastle District. Thus, accounting for coal recovery, the subsidence factor should be obtained by using the effective seam height which is 0.85 x 2000 = 1700 mm. Thus 'a' = 1100/1700 = 0.65 1.5 Travelling and Final Subsidence and Strain Profiles The maximum slope of the travelling subsidence profile of 0.75% increased to 1.2% as the subsidence profile slowed to its final shape. Associated with the change in slope is the calculated curvature of the subsidence profile which is shown Plotted for two dates chosen to illustrate the change in the curvature. At the leading edge of each subsidence profile the corresponding maximum travelling tensile strains all approximate L8 mm/m (T., T2, T,). When final subsidence occurred, the maximum tensile strain increased to 4.0 mm/m (T.) which is equivalent to a maximum positive curvature of 2.6 x I0~*m'"1, or a Page B-10 minimum radius of curvature of 3800 m. Similarly compressive strains are associated with the trough of the travelling subsidence profile. The values of 2.8 mm/m (C1, C,) increased to 5.1 mm/m (C3) on 22nd February, 1971 due to the slight rise which developed at Station 57 in the subsidence profile rather than it remaining as a flat bottomed trough. The mean value of the final maximum compressive strain (C.) is 5.8 mm/m and corresponds to the maximum negative curvature of 2.8 x lO^m"1, or a minimum radius of curvature of 3600 m. 1.6 Shape of the Subsidence Profile. The shapes of the travelling and final subsidence profiles were defined by relating various features of the subsidence profile to the goaf edge. The points chosen are listed in Table 1.1. To examine the shapes of the profiles, points where subsidence is a given percentage of the maximum were chosen, and the locations of the maximum slope change and maximum tensile and compressive strains were listed. The non-dimensional profiles for travelling and final profiles are shown in Fig. 1.4 in relation to the ribside. The final profile is steeper than the travelling profile and for both profiles, the the subsidence over the ribside was from 25 to 30% of the maximum. The position of the point of 50% maximum subsidence for both profiles lagged behind the ribside (or was over the goaf) at a distance of 0.3 h from the rib. The relative positions of the zero strain are in Table l.l. For the travelling profile, the change from tension to compression occurs at a distance of 0.08 h ahead of the face, over solid coal, at a point where the subsidence is 0.23 of the maximum. For the final profile, the change from tension to compression occurs close to the transition point, at a distance of 0.24 h from the goaf edge, over the goaf. This is also shown in Fig. 1.3 where the locations of the points of zero strain are shown on each subsidence profile. In other words, for a travelling profile, the surface responds more rapidly to strain rather than subsidence. Page B-ll TABLE 1.1 Features of Subsidence over 2 North Di stance to d g oaf edge d (m) h Feature of 84-114 64-94 84-114 64-94 Profile 22.2.72 17 .2.71 (% S ) Final Travelling Final Travelling max 0 100 100 0.88 0.97 5 mm 82 88 5 46 55 0.41 0.49 10 + 27 + 38 + 0.24 + 0.34 20 + 9 + 15 + 0.08 + 0.13 30 - 6 - 3 -0.05 -0.03 40 - 20 - 18 -0.18 -0.16 50(tn) 30 34 0.27 0.30 60 38 46 0.34 0.41 70 49 58 0.43 0.51 80 58 68 0.51 0.60 90 70 82 0.62 0.73 100 106 113 0.94 1.00 G - 45 - 64 -0.40 -0.57 max Strain 22.2.71 +E 0 + 100 0 + 0.88 «max - 27 + 9 -0.24 + 0.08 -0E Strain - 95 - 85 -0.84 -0.75 max w = 213 m h = 113 m w/h = 1.88 1.7 Strain Triangles Triangles were established for the calculation of the magnitudes and directions of the maximum and minimum principal strains. The triangles were placed along the line of subsidence survey stations over 2 North and were made approximately equilateral. The series of plans in Fig. 1.5 shows the changes in the principal strains in relation to the extracted area of coal pillars as the line of extraction advanced. The principal strains are located at the triangles numbered 1 to 7. The plans selected show the pattern of ground movement for the dates on which the strain profiles in Fig. 1.3 were drawn. In each case the three dimensional nature of the subsidence trough can be visualised by associating the subsidence and strain profiles with the plans in Fig. 1.5. For example, on 30th October 1970, the principal strains of +0.5 and -0.2 mm/m at Triangle 3 changed to -2.5 and -1.8 mm/m on 14th January 1971, as the line of extraction passed beyond the Page B-12 triangle, causing it to come within the compression zone in the subsidence trough. With the further development of subsidence, the principal strains became zero and -4.6 mm/m on 22nd February, 1971 and +0.9 mm/m and -4.0 mm/m on 22nd February 1972. At the location of the triangle the hump on the subsidence profile is associated with a change from tensile to compressive strains in a longitudinal direction which is in the direction of mining. However the subsidence trough in a lateral direction resulted in the corresponding high compressive strains. The differences in the magnitudes of the two principal tensile strains at Triangles 1 and 6 of 0.5 and 3.1 mm/m respectively is due to the flatter ground slope at Triangle l. The maximum principal compressive strains are 6.1 and 3.8 mm/m respectively and occur near the point of maximum subsidence. 1.8 Progressive Changes in Subsidence and Strain As the face advances beneath a line of stations, subsidence at any one station will commence before the face reaches it. The subsidence will increase as the face passes beneath and will settle to a maximum subsidence value as the face moves away. This is illustrated for selected stations over 2 N Panel in Fig. 1.6. The times when the face passed beneath particular stations is also shown. This development of subsidence can be related to the progressive profiles in Fig. 1.3 at each of the stations 55, 67, 85 and 97 for which time-subsidence plots are given. The travelling strain profiles' in Fig. 1.3 show that the surface is first in tension, then compression as the face passes beneath. The surface strain will then assume its final value according to the shape of the subsidence profile. The time-strain plots in Fig. 1.6 correspond with the time-subsidence curves. Both the subsidence and strain at each point at each stage of their development can be correlated with the appropriate subsidence or strain curve in Fig. 1.3. Ideally, the central part of a bed of a supercritical subsidence trough should be flat with zero final strain. The final strains in Fig. 1.6 are related to humps and depressions in the final subsidence profile due mainly to irregular coal recovery during the pillar extraction procedure. The subsidence can now be related to the face position. The face continued to advance for the duration of the study apart from the mine shutdown for three weeks over Christmas. The profiles for 14th December 1970 (not shown in Fig. 1.3) and 12th January 1971 are near to each other and when examining the influence of the mean face position on the subsidence, the subsidence at 12th January 1971 was not included as it is anomalous and not representative of a continuously moving face. Points along the tail end of the profile, beyond station 85 were not used for the same reason since they are affected by the stationary face at the finish of the panel. Page B-13 Progressive subsidence values of selected stations are listed in Table 1.2 for vales of subsidence between zero and the final subsidence. These are shown as a proportion of the maximum subsidence at that point. The mean face position with respect of the station at the time of each subsidence survey was related to the depth of cover. The resulting dimensionless relationship between subsidence and face advance is shown in Fig. 1.7. It can be seen that 1. subsidence commenced when the face was at a distance of around 0.75 of the depth of cover before the surface point, 2. when the face was directly below the surface point, 20% of the maximum subsidence had occurred, 3. 50% of maximum subsidence occurred when the face had advanced to a distance equal to one quarter of the depth of cover from the surface point, and 4. subsidence was complete, apart from minimal residual effects, when the face had advanced to a distance of 1.5 of the depth of cover beyond the surface point. TI^BL E 1 .2 Subsidence and F,ac e Advance Face Face advance s s Station Date advance (mm) (m) Cover S 66 27.10.70 -113 -0.98 0 0 S=1000mm 14.12.70 - 26 -0.23 107 0.11 h=115m 17. 2.71 70 0.60 820 0.82 23. 3.71 155 1.35 960 0.96 76 14.12.70 - 92 -0.82 0 0 S=945mm 17. 2.71 23 0.21 400 0.42 h=112m 23. 3.71 58 0.52 790 0.84 28. 4.71 87 0.78 860 0.91 82 14.12.70 - 98 -0.89 0 0 S=1040mm 17. 2.71 0 0 205 0.20 h=H0m 23. 3.71 55 0.50 730 0.70 28. 4.71 99 0.90 920 0.88 Page B-14 1.9 Subsidence of the Railway Line The privately owned railway line shown in Figs B.2 and l.l has a maximum grade of 2% in the area under investigation. Empty trains travel downgrade and loaded trains travel upgrade. Also there is a cutting up to 9 m deep and an embankment 10.5 m high. At the time of mining, the line was used for the haulage of coal in 60 tonne rail cars from John Darling, Burwood and Lambton Collieries to the Steelworks in Newcastle. As two locomotives were used to haul laden trucks up the slope it was not anticipated that any difficulties would arise from the increase in grade and the small strains expected were not considered to adversely affect the line. There was also a rail motor passenger service which operated twice daily. The line was closed in 1973 due to poor patronage. The colliery was required by the New South Wales Department of Mines to leave a barrier of coal six chains wide beneath the line. Up to 50% recovery was allowed from the pillar by first working of bord and pillar mining. When pillar extraction was taking place near the barrier pillar the possibility of extracting pillars within the barrier from beneath the line was considered, and after preliminary calculations indicated that there would be no adverse affects to the line, permission was given to extract two rows of pillars beneath the line. The pillar extraction beneath the line was separated from the 2 NW extraction area by two rows of pillars which would be expected to yield and provide smooth profiles both along and across the railway line with low longitudinal and side slopes. Pillar extraction took place in August 1970, concurrent with the extraction in 2 NW Panel. The subsidence profiles in Fig. 1.8 show the effect of 2N extraction which caused the significant additional subsidence since August 1970. the maximum subsidence was 660 mm and the maximum cross slope was 0.3%. f /r rx r r T 7 - / y~/ ,' .^/\ ^ V~> ~ ' ^ i~> ^ r ] r •4t •4>7 tn i PPnrfenDnQL^QPEiBataLlQlar] ^RHHHOaa OD aa • a a Da n Q H a '*/'• ^ ONE NORTH PANEL Metres r . V 1° **> >• Feet t *• >•» •00 ~ op \ 1 i 1 1 1 l 1 U, ,',A i. ». J r— Chains Scale 2 chains to one inch - 1:1564 FIG. 1.1 TWO NORTH WEST AND TWO NORTH PANELS O-i Conglomerate Coal Tuff 0 e » a o o o i 0 o o o D O 0 i o o o o > o o ° * a B 9, t 50- . Sandstone Shale * ° A A A A A' 100- A.AAAA . A/v A A/- A.A A /^/\ Dudley seam Depth (metres.) FIG. 1.2 STRATIGRAPHIC SECTION «,S^^^~^^SCKK^s^ssa^^S^ps?SiSS2£S»S5SS^S^^ 360 - 360 . 350 u - 350 o t 340 - 340 in 330 -330 240 Hea" '«• positions ° ,240 230 - 230 • 220 J220 210 (a) Topographic section J 210 66 93 9* I 0 ^-tro ooooooooggaaoooooooooofi.ooooooooQQ.fOooooooooo- 0 1 200 O®zero o strain 9f .* 200 E 0 27 10 • 70 400 B 12 1 71 u 0 400 A 17 2 71 » 600 ®"rq 600 • 2ft 4 71 VI strain • XI BOO 22 2 72 3 °B, B0C 1000 **w- Ky y 1000 •••• (b) Subsidence from 19-May 1970 .10 -i 1-0 •0-5 0-5 o 27-10-70 B 12-1-71 A 17-2-71 • 28-4-71 a a. • 22-2 72 c - -0-5 0-5 -10'- (c) Change in slope from 19-May 1970 1-0 •3 45 58 66 1 » «2 i L. i 0 „ .1 > ._ a 12-1 -71 3 E 0 -\Aw • 22-2-72 :'° -1 : » »—- 1 A -3 - 1 v (d) Inverse curvature of subsidence profiles 1 W ' ' i i i i • i i i > * i 4 • . . . ^ i . . , . • . L .... , ...... , . , t . • • . . *7 i7T7 -1 5 'X • 27-10-70 B 12-1-71 c S ^—_^ s*s * 17-2-71 ••pfTf • 22-2 -72 l\ si Vc4 ' (e) Slope strains from 2-June 1970 •300 300 » 200 /T-* 200 • 100 100 0 D 12-1-71 N*+*•+*-»• 0 • 22-2 72 - 100 ¥• 100 - 200 200 " 300 300 (f) Displacement of surface points FIG. 1.3 ELEMENTS OF SUBSIDENCE OVER TWO NORTH X a i/)E O^1 X>|-C I O o o r>- w rH 00 u S5 H fd rd Q e g H W CD CQ H D •H Q 4-1 W OJ 0 H o •rH n H w 4H O tn Hw >H C QJZ5 ft •H H H O rd H 53 OJ a > •H rd CN CN o a H "DM &4 + X a E CM <7> 3 i_ jQ 4; U. CM CM cn (JO Jt & 3 i_ JQ w 4i H EH W i-q < PA H T o 1 ss ' 1 H 1 « c in TJ H , . L. •*— cn c w H <0 0» in 01 3 l/l 4* C l_ c CN a E o u J* in 0/ c £ (0 E* E O cn 8, c .1. CN Oi 3 in —1 CM • 1 1970 1971 1972 ISIOINIDU 1F1M1A1M1J 1J1A1S1O1N1DIJ IFIMIAIMI E E CD u C OJ t/> JD 3 1000L 1970 1971 1972 S,0,N ,D J i FiMi A|M i J i J A i S i 0 i N i D I J IFIMIAIM 4-0 c o 55-5T& 67-68 * 20 c OJ 20 c o in in Oi L. | 4-0 60 L FIG. 1.6 SUBSIDENCE AND STRAIN RELATED TO TIME 0 r 0-2 - 0-4 - 0-6 - 0-8 - 1 . Q | . i i i I . i t i I i i i i I i i i i I—i—i—i—i—1—i—i—i—i— 1-5 10 0-5 0 0-5 10 1-5 Face Advance / Depth of Cover O Station 66 0 Station 76 & Station 82 FIG. 1.7 SUBSIDENCE RELATED TO FACE ADVANCE R o> a H >H -4" -I I m Hi H o g En D U H Q S3 H I ' ' ' ' 1 i i ( l \ « in W CM ((U) axiapisqns Q S3 H fe (UJ) aDuapisqns I—i—i—i—i—j—i—i—i—i—) O LTI O O +- Page B-15 STUDY 2 SUBSIDENCE OVER TWO SHORTWALLS 2.1 Introduction The first two mechanised shortwalls in Australia were located in the Newcastle District at Lambton Colliery. They were used to mine in the Dudley Seam in an area where there was no surface development. The two shortwalls were planned to result in long extracted blocks of coal separated by two rows of pillars which would not be mined. In association with subsidence work already carried out in the same general area, the work over these two shortwalls would provide important data on subsidence characteristics related to subcritical panels. Information had already been obtained over nearby critical extraction (Study 1) and over pillar extraction from subcritical panels of w/h ratio 0.45 in a residential area resulting in a maximum subsidence of only 75 mm (part of Study 4) . The shortwalls proposed here would result in extractions 66 m wide, to give w/h ratios of 0.66 and 0.62 at their respective depths of cover. Subsidence information over these panels would enable optimum widths of subcritical panels to be determined for future planning in nearby residential areas. The two lines of survey stations established to monitor subsidence over these two shortwalls formed the initial stages of a more extensive subsidence grid. Additional lines of survey stations were later set out over adjacent areas where it was intended to develop and to extract pillars. Thus subsidence information would be obtained during first workings and subsequent extraction. These investigations were in an undeveloped area of light timber cover where the development of subsidence could be readily monitored. However two features in the area to be affected by later pillar extraction were a main trunk road and a two storey brick building. The two lines set out to monitor the shortwalls are designated 46-103-145-151 and 230-103-276 in Figs B.2 to B.7. The results of the monitoring of the later extensions to this grid are discussed as Study 3. 2.2 Mining and Geological Aspects On the plan of the Dudley Seam workings (Fig. B.6), the two shortwalls and the pillar extraction at the end of Shortwall 1 in this study make up the 2 West Panel. The detailed mining plans are given in Figs 2.1 and 2.2. Dates of mining and other details in the area of interest along the subsidence grid lines are given in Tables 2.1 and 2.2. The shortwall operations were described by Martin and Hargraves (1972). The coal recovery from the shortwall blocks was estimated at between 95 and 100%. Using a figure of 95% increases the S /mp value by 0.4%. Following the mining of Shortwall 2, the equipment was moved to Burwood Colliery. Page B-16 TABLE 2 .1 Mine Geometry, 2 West Panel Dates of extrn Extracted area (m) Cover w 1 Panel h Start Finish Width Length (m) h h (w) (1) SWl 27.4.71 3.9.71 67 430 102 0.66 4.22 SW2 17.10.71 24.3 .72 66 570 107 0.62 5.33 Pillar Extrn. 20.4.72 16.8.72 143 296 91 1.57 3.25 TABLE 2.2 Maximum Subsidence, 2 West Panel Seam height Extraction Effective S Panel m (mm) % seam height (mm; max (%) mE (mm) mE SWl 2100 100 2100 152* SWl + 2 2100 100 2100 226 10. 8 SW2 2100 100 2100 168* 8.0 Pillar Extrn. 2100 80 1680 1030 61.0 •Due to respective shortwalls The pillars that separate the two shortwalls were originally 28 m wide and were subsequently split by a 5.5 m wide heading to form two separate rows of pillars with 11 m of solid coal in each. Using an average depth of cover of 104 m and a pillar height of 2.10 m, the total pillar width to height ratio is 10.5 and the pillar width to depth of cover ratio is 0.21. During the mining of these shortwalls, development was taking place for future pillar extraction in 3 West Panel to the north east. Development then took place in 1 West and pillar extraction commenced in April, 1972. Above the Dudley Seam are other coal seams and beds of conglomerate, sandstone, tuffaceous sandstone and shale of different thicknesses. A geological section is shown in Page B-17 Fig. 2.3. Bord and pillar mining was carried out in the Victoria Tunnel Seam above the shortwall area from 1939 to 1941, and in 1946 and 1947. Some minor faulting occurred in the area, but of insufficient magnitude to affect the subsidence pattern. Bord and pillar mining was also carried out in the underlying Borehole Seam but at such a distance below the Dudley Seam that it would not be affected by the current extraction in that seam. 2.3 Elements of Subsidence Longitudinally over Shortwall l and Pillar Extraction The topographic and seam sections over the longitudinal centre line of Shortwall 1 are given in Fig. 2.4. The depth of cover varies from 128 m over the start of extraction to a localised value of 90 m at the intersection with the main cross line (Station 103). The depth of cover remains at about 90 m over the remainder of the Shortwall and the pillar extraction area at the end of the panel. The subsidence and strain profiles over the longitudinal centre line of Shortwall 1 are shown in Fig. 2.5. Some stations were disturbed by earthworks during the early stages of the work and were replaced by Stations 1 to 14. The subsidence due only to Shortwall l is shown by the profile of 1st September 1971. The additional subsidence around Station 70 is due to the extraction of pillars adjacent to the Shortwall as shown in Fig. 2.1. This increased the width of extraction at the start to 90 m. The next profile, 28th May, 1972 is due to the additional effect of Shortwall 2, and the initial subsidence over the pillar extraction at the end of Shortwall 1. The later two profiles show the additional subsidence due to the adjacent 1 West pillar extraction (October 1972) and 3 West pillar extraction (June 1973). It is interesting to note that the pillar remnants at the end of Shortwall 1, shown in Fig. 2.5 have remained stable, confined by the adjacent goaves, but having the vertical load relieved by the presence of the pillars which flank the Shortwall in a longitudinal direction. The profiles of strain are shown over Shortwall 1. The trend is that the maximum tensile strain over the start of Shortwall 1 was 1.5 mm/m and the maximum compression 2.0 mm/m. high tensile strains developed over the pillar remnants. These are shown in Fig. 2.6. The subsidence profile and associated slope change, inverse curvature and strain profiles are shown in Fig. 2.6 for one of the final subsidence checks, on 23rd September 1976 for that length of the subsidence profile where the bay lengths were 9 m (up to Station 145). The smoothed slope profile was used as a basis for the curvature calculations. The profiles can be Page B-18 TABLE 2.3 Elements of Subsidence for Longitudinal Profile Location Subs. Slope Strain Inverse Radius~of at mm change mm/m curvature curvature * x lO^m-1 km SWl 60-64 0.6 +1.5 +0.7 14.3 74 920 71-85 -2.0 -1.0 10.0 95-100 114 0.5 Pillar 125-129 +22.0 +10.9 0.9 Pillar extrn. 131-133 2.4 -12.0 -9.8 1.0 135 1120 divided into two sections, the first over Shortwall 1 and the second over the pillar extraction at the end of Shortwall 1. These sections of the profile are separated by the pillar support over which abnormally high slopes, curvatures and strains occurred. The maximum values are shown for the 23rd September 1976 profiles in Table 2.3. 2.4 Elements of Subsidence in a Lateral Direction Across Shortwalls A plan showing the extraction at 24th March, 1972, the date of completion of Shortwall 2 is shown in Fig. 2.7. The line of subsidence stations, 223 to 172, crossed the longitudinal line over Shortwall l at Station 103, at a distance of 122 m from the end of the shortwall. The maximum subsidence at this location is unaffected by earlier pillar extraction, and is not modified by the end effects . The topographic section is shown in Fig. 2.8. At the location of the section line the surface contours are approximately parallel to the longitudinal direction of the shortwall panels. The minimum and maximum depths of cover are 90 and 123 m. The maximum surface gradient is l in 2.5 to the northeast of Shortwall 1 (towards station 194). The cover over each shortwall panel at the point where maximum subsidence occurred is taken as the mean cover to the area of the surface that is within the sphere of significant subsidence influence from the panel at that point. For Shortwalls 1 and 2 the depths of cover were taken to be 102 and 107 m, respectively. Page B-19 The two subsidence profiles in Fig. 2.8 are at 11 October, 1971, due to the mining of Shortwall 1 and at 1 June. 1972, due to the mining of both Shortwalls l and 2. The profiles of the associated elements of subsidence are related by the vertical dashed lines at positions of special interest. The maximum values of subsidence, slope change, strain and curvature along the profiles in Fig. 2.8 are included in Table 2.4. The maximum subsidence due to Shortwall 1 was 152 mm and was associated with a maximum compressive strain of 4.3 mm/m. The maximum tensile strains of 1.1 mm/m and 1.5 mm/m are smaller in magnitude than the maximum compressive strain, which is characteristic of a sub-critical subsidence profile. The mining of Shortwall 2 caused both the subsidence and maximum compressive strain over Shortwall 1 to increase to 226 mm and 5.7 mm/m. The effect of the pillars between Shortwalls 1 and 2, which resulted in a hump in the subsidence profile, is to be avoided in the design of a panel and pillar mining layout. Towards the edge of the subsidence trough (station 223) small finite strain exist even beyond the area where the subsidence becomes zero. The movement of the sandy soil at the surface down the slope resulted in differential movement between the stations. TABLE 2.4 Elements of Subsidence for Lateral Profiles Location Date Subs Slope Strain Inverse Radius of at mm change mm/m curvature curvature stations % x lO-'m-1 km 185-186 11.10.71 + 1.1 0.9 11.1 1. 6.72 + 2.0 1.6 6.2 187-188 11.10.71 0.40 0 1. 6.72 0.56 0 103 11.10.71 152 0 -4 2.8 3.2 (SW 1) 1. 6.72 226 0 -5 3.2 3.1 191 11.10.71 0.37 0 192-193 11.10.71 + 1.5 0.9 11.1 194 1. 6.72 113 + 1.8 1.5 6.7 (pillars) 199 1. 6.72 168 0 -2.6 2.1 4.8 207 1. 6.72 0.28 (SW 2) 210-211 1. 6.72 + 1.7 0.7 14.3 Page B-20 2.5 Features of Subsidence Profiles. The shape of the subsidence profile over Shortwall 1 was defined by relating chosen features along the large scale plotted profile to the goaf edge. The points chosen are listed in Table 2.5. In addition to the points corresponding to subsidence as a percent of the maximum, the locations of maximum slope change, and maximum tensile and compressive strains were defined. The sign convention used is that points located over solid coal are in a positive direction from the goaf edge and those over the goaf are in a negative direction. The features listed in Table 2.5 are related firstly to the goaf edge for the w/h ratio of Shortwall 1 of 0.66 and secondly to the goaf edges of both Shortwalls 1 and 2 for the profile of 1st June 1972. The influence of the sloping surface and sandy alluvium can be seen in the greater subsidence to the side of Shortwall 2 for the profile dated 1st June 1972. This information will be correlated in Chapter 4 with similar information from other panels. Shortwall 2 was mined in ground that had already been disturbed by Shortwall 1. This is shown in Fig. 2.8 where the added subsidence over Shortwall 1 due to Shortwall 2 is greater than the initial contribution by Shortwall 1 to the total subsidence over Shortwall 2. Non dimensional profiles are plotted in Fig. 2.9 from the information given in Table 2.5. The fact that Shortwall 2 was mined in previously disturbed ground is reflected by the greater subsidence over Shortwall 2, left hand side. The shape of the subsidence profile is indicated by the inverse curvature. The profiles of curvature calculated from the subsidence profiles are compared with the profiles of measured strain in Fig. 2.8. The maximum positive and negative values in each profile correspond. The magnitudes of the calculated inverse curvatures and measured strains are compared in Table 2.4 and shown plotted in Fig. 2.10 for both the tension and compression zones. Included in Fig. 2.10 are the results from Study 4 as published by Kapp (1978). As the inverse curvature increases (or as the radius of curvature decreases) the absolute strain measured along that part of the subsidence profile increases. Page B-21 iTABL E 2.5 Features of Subsidence Profi le over Shortwall 1 SWl, LHS SWl . RHS 201 - 222 103 - 174 Feature 199 - 103 103 - 178 1.6 .72 1..6.7 2 of Profile d d d d l* of) _~ d(m)* h d(m) h d(m) h d(m) h 0 98 0.96 104 1.02 114 1.07 98 0.96 5mm - - - - 78 0.73 66 0.65 5 60 0.59 53 0.52 69 0.64 53 0.52 10 46 0.45 37 0.36 51 0.48 37 0.36 20 +23 + 0.23 + 18 + 0.18 33 0.31 16 0.16 30 +12 + 0.12 + 6 + 0.06 +20 + 0.19 + 8 + 0.0 8 40 + 2 + 0.02 - 2 -0.02 + 12 + 0.11 0 0 50 - 2 -0.02 - 7 -0.07 0 0 - 5 -0.05 60 - 7 -0.07 -13 -0.13 - 7 -0.07 -10 -0.10 70 11 0.11 16 0.16 12 0.11 14 0.14 80 15 0.15 21 0.21 18 0.17 18 0.18 90 18 0.18 26 0.25 25 0.23 23 0.23 100 29 0.28 37 0.36 36 0.34 33 0.32 G -10 -0.10 -15 -0.15 -11 -0.10 0 0 +eax + 8 + 0.08 - 6 -0.06 + 17 + 0.16 + 9 + 0.09 -35 -0.34 -32 -0.31 -33 -0.31 -33 -E -0.32 SmaW x 1 — 1 — 2 — 1 _ w 67 - 67 - 66 - 67 - h 102 - 102 - 107 - 102 - w/h 0.66 — 0.66 — 0.62 — 0.66 — d = distance to goaf edge It is a general principle in subsidence work that the maximum slope change and maximum strain increase with an increase in the maximum subsidence and decrease with an increase in the depth of cover, defined by the relationships G = k„. S G max + E = k,.S 1 max , and E - k-.S 2 max Page B-22 The values of kQ, k± and k2 can be defined for the 2 South West Shortwall exercise. There will be two values of kt and k, for a single subsidence profile, one for each side, as listed in Table 2.6. The values of k_, k1 and k2 will be compared with similar values obtained from otner studies, in Chapter 4. 2.6 Subsidence Related to Time and Face Advance The time-subsidence curves of Stations 103 and 199, located over Shortwalls 1 and 2 respectively, are shown in Fig. 2.11. The maximum observed rate of increase in subsidence was 2.7 mm/day. The subsidence at a particular time was related to the face position at the same time, as determined from Fig. 2.1. The ratio of the distance the face was behind or ahead of the surface point to the depth of cover was related to the ratio of the subsidence at that particular time to the maximum subsidence at that station due to the particular longwall (Table 2.7). The corresponding ratios are shown plotted in Fig. 2.11. Subsidence first commenced when the face was before the surface point by a distance equal to the depth of cover. By the time the face was beneath the surface point, 10 per cent of the maximum subsidence had occurred. A subsidence equal to 50 per cent of the maximum occurred when the face was beyond the surface point at a distance equal to the depth of cover, and subsidence was complete (apart from small additional residual subsidence) when the face had advanced toTABL threE 2.e 6time s the depth of cover. Slope and Strain Factors k -E k Shortwall w 8 h G kG +E l 2 (Wf (m) (%) (mm/m) (mm/m) h 1 0.66 152 102 0.40 2.68 1.1 0.74 4.3 2.89 0.38 2.55 1.5 1.01 2 0.62 168 107 0.26* 1.66 1.7 1.08 2.6 1.66 •reduced due to effect of natural surface slope Page B-23 TABLE 2.7 Subsidence and Face Advance Shortwalls 1 and 2 Station Date Face f S S Advance (M) SW % f (m) h S SW 103 6.5.71 -263 -2.6 0 0 S = 152 17.6.71 -103 -1.0 0 0 SW Cover=l02m 27.7.71 - 11 -0.1 10 4 25.8.71 88 + 0.9 80 35 11.10.71 285 + 2.8 152 100 199 11.10.71 -308 -3 .0 0 0 17.11.71 -181 -1.7 0 0 S = 160 17 .1.72 - 35 -0.3 2 1 SW Cover=107m 10.2.72 44 + 0.4 42 27 15.3.72 191 + 1.8 137 85 26.4.72 304 + 3 .0 160 100 -—* • LP 1 -T rJ w < H CO w CN 55 CO r-J 1 o « cn CN o H 1 r 8, N - •• >- ^ VL o ~A in H / \_z * O I; • s"l.t l.sW-—'i1——— \ O w •J r*>\ i!',on'.^ |; k, r 55 t. "•,, ,\ . —-v v^— •j^" __rl<-r- .'..• i < PM H CO w CN 55 o-* O w H U rJ -—, H PM CN CN CJ H :?/*/*Tit \&m^_ Sand 20- Conglomerate Sandstone 40- Q. O V.T. Seam 60- :->z->q Shale Tuffaceous sandstone 80- Dudley Seam FIG. 2.3 GEOLOGICAL SECTION cn i—i—i—r 3 i 10 0 -J c o u in a x OO 0 o 52 n O id H H CJ W SI co w CO (N O o M CD PH o o PH o in H o P 8 = 52 O in cn CN 1_ a in o oo H in o CO in o in ic IN IO N o o o ID IT) v* o o o in m n n C^ VD IT) CN (SI CN aDDjjns UUD3$ 8 UU S13A31 Q30nQ3cd 8 O CL a. rj in a j K u 1-1 rJ e> H PM Quifumi) v»»D^vg 46 50 55 60 65 70 75 80 85 95 100 105 HO 120 125 05 un 1« " IIMM.IMIIHIIUIIIIIIIIIII IIIUII ,,|| I . I , , I I I , M I I I 17, I , m , | f t 22 • / \ M (\ r N _ / ypv ^ \[J • \ I d ) Slop* Strain 10 L. \ : 8 :••••• ][ uu N o O- an ••••• D 0-" JLZO CM i 00 qo CO n I .« II » I jtzIO o una mannainn s 1DDJLZO D .v# .:••• HDD ^1 &%##? S3 O ^ CO From 2.A. 71 (b) Subsidence to 11.10-71 e and 1.6.72 + •0-5 • qo-5 ~° -0-5L - J0-5 (c) Change in slope •2 • -.2 •1 1 : u^S^y^s S^S^a^ 0 1 2 3 (d) Inverse curvature • n3 5L J5 (e) Slope strains a E in W W N to ID ID VD H H o •C rC JG P4 - 1 TJl-C ^ ^ \ 5 S £ w o 53 w o o o in Ul in in r- CN PQ rH rH rH CN aP H II II II II to X X X 5 55 fed fsd fed gfd O CN co to to to H to >H ^ * •• •> 2 Q CO. CO CO to O K UJ ffi ffi hq « J tf FH rH r-1 CN rH CO 53 H H H H O H rH rH H 53 rd rd fd fd £ S £ 3; +J 4J -P -P H u u u o o o o rC Xi A A CO to CO CO o a x < -ol-c + Strain mm/m FIG. 2.10 INVERSE CURVATURE RELATED TO STRAIN 7 1971 1972 J ,F|MiA|M | J i J | A iS i0|N J,D |F|M|A,M,J,J ,A|S,0,NlD i 50- E 1 100 u | 150i | 200t 250 300J - Face Advance/ Depth of Cover • 3-2-1 0 1 2 3 Eh—Q 103 A A 199 -5*- % sw 100 100 FIG. 2.11 SUBSIDENCE RELATED TO TIME AND FACE ADVANCE Page B-24 STUDY 3 SUBSIDENCE OVER AN EXTENSIVE PILLAR EXTRACTION AREA 3.1 Introduction The lines of survey stations set out monitor Shortwalls 1 and 2 and the pillar extraction at the end of those shortwalls (in Study 2) were extended to form a grid over the surrounding pillar extraction in the Dudley Seam. Pillars were extracted from 1 West, 3 West and then 0 West Panels, shown in Fig. 3.1. The only surface development affected by the West Panels were the Dudley - Redhead Road, a main trunk road and a two storey brick building adjacent to that road, used as convent at the time mining was taking place. One other feature of this study related to some first workings to the south east of 0 West Panel, located beneath a creek where the depth of cover decreased and where the depth of surface alluvium increased as shown in Fig. B.3. Since this study was in undeveloped bush land, there was opportunity to examine the applicability of the strain triangles around the convent building and to investigate the effect of variations in bay length on the magnitudes of the calculated strains. 3.2 Mining Details Following the mining of Shortwall 2, pillars at the ends of Shortwalls l and 2 were extracted. At the same time as the pillar extraction, pillars in 1 West and in 3 West Panels were developed prior to extraction. Adjacent panels were extracted in order by the lift and fender method from 1 West and a high recovery was achieved. Conventional pillar extraction methods were used in 3 West Panel, with pillars being extracted in successive rows. The plan shown as Fig. 3.2 is the extraction at 18th September 1972 and indicates the extraction procedures used in 2 West, 1 West and 3 West. The final plan in Fig. 3.3 shows the progress of the extraction and the survey lines which were established to monitor subsidence. For the whole of the West Panel area, the seam height mined varies around 2 m and the depth of cover is generally from 120 m over 3 West to 105 m over 0 West apart from local depressions along the course of two creeks. 3.3 Subsidence Over Lateral Lines and Pillar Stability The north east extension of the line set out to monitor the surface effects of the two shortwalls continued to be levelled and measured as pillars were extracted from 3 West Panel from 10th July 1972 to 5th October 1973. It was not possible to project the line beyond Station 230 because of the presence of Page B-25 Redhead Lagoon, as shown on Fig. B.2. Progressive levels and distance measurements enabled the profiles in Fig. 3.4 to be drawn. Extraction took place from beneath the line of survey stations (210 to 230) from 18th to 26th January, 1973, at which time the maximum subsidence was 470 mm. One month later, on 23rd February 1973, with two rows of pillars having been extracted, the subsidence had increased to 1150 mm. The maximum rate of subsidence increase was 27 mm/day for Station 220. The maximum slope change over 3 West was 1.8 per cent but decreased to a general value of 0.5 per cent before the trough bottomed. The south west extension of the line used to monitor the two shortwalls was over 1 West Panel. Extraction took place from 15th March, 1972 to 13th September, 1974. Subsidence along this part of the line (Stations 167 to 187) is shown in Fig. 3.4 with the profiles of strain. Extraction took place directly beneath this line from 23rd March to 5th June, 1973. The various features of the final profile over both 3 West and 1 West are shown in Fig. 3.5. Allowing for residual subsidence effects, the maximum subsidence along this line was 1030 mm. The time-subsidence curves are given in Fig. 3.6. The subsidence profile in Fig. 3.5 shows that the pillars between Shortwalls 1 and 2 and the pillars between Shortwall l and 1 West have not crushed out whereas the pillars between Shortwall 2, and 3 West which have been split have crushed out as indicated by the slight flattening out of the subsidence profile at Station 200. The geometric ratios of the two rows of pillars between Shortwalls 1 and 2 were discussed in Study 2. For the single row of pillars between Shortwall 1 and 1 West, the depth of cover was 105 m and the pillar height 2.0m. With the row of pillars being 18 m wide, the pillar width to height ratio was 9.0 and the pillar width to depth of cover ratio was 0.17. This row of pillars was flanked on one side by a supercritical area of total extraction and as such cannot be compared for stability considerations along with those pillars in a panel and pillar layout. A further extension of the same line (230-195-167) continued over the remainder of 1 West Panel and then over 0 West Panel. The subsidence and associated strain profiles are given in Figs 3.7 and 3.8. The slope and inverse curvature profiles over the edge of extraction of 1 West (21st September, 1973) and the final profiles over 0 West (13th August 1975) are given in Fig. 3.9. Discounting residual subsidence effects, the maximum subsidence over 0 West was 94 5 mm. 3.4 Development of Subsidence over Longitudinal Lines The longitudinal survey line over Shortwall l was continued to be monitored as extraction took place in the adjacent 3 West and 1 West Panels. The final monitored subsidence and strain profiles in Study 2 (Fig. 2.3) show the additional effects of 3 West and 1 West extraction on the subsidence caused by 2 West. The calculated slope and curvature profiles can be correlated with the measured strains. Page B-26 The line of survey stations (600 to 678) set out in a longitudinal direction over 1 West passed behind the two storey brick structure and was extended to monitor the effects of first workings where geological information showed that the depth of solid cover decreased significantly at the location of a creek. Mining from 1 West was by the lift and fender system, the direction of mining being at right angles to the subsidence line 600-678 (Fig. 3.2). Sub- panels at 56 m centres were formed, adjacent to one row of pillars at 27 m centres. The sub panels were stripped and then pillars extracted, to maintain a working face giving an extraction width of 113 m. Each sub panel took two months to mine. Progressive subsidence profiles and strain profiles are given in Figs 3.10 and 3.11. The calculated slope and curvature profiles for final subsidence are shown in Fig. 3.12. The humps in the subsidence profile correspond to the pillars which were extracted adjacent to and at the same time as the lift and fender sub-panels, the recovery of coal from the pillars not being as high as from the sub-panels. The maximum subsidence over 1 West is shown to be 1000 mm, discounting residual subsidence, as shown on the time-subsidence curve of Station 606 in Fig. 3.6. The maximum associated slope changes and strains are shown in Figs 3.11 and 3.12. The longitudinal centre line 600 to 67 8 was extended into an area over Freshwater Creek (Fig. B.2) at the request of colliery management, to examine if there were any surface effects of pillar development and pillar splitting in North East Heading. At that location the depth of cover decreased and the depth of surface alluvium increased, resulting in a significant decrease in the depth of solid strata down to a localised minimum of 20 m over the Dudley Seam. The thickness of alluvium is up to 40 m. The workings are shown in Fig. B.6. The mining height was 2.0m. First workings with pillars generally at 25 metres was completed on 7th April 1977 and pillar splitting to form pillars as small as 15 m centres was complete on 24th January 197 8 although not at the minimum solid cover. Surveys from 5th November 1974 to 4th November 1978 showed no subsidence at the surface. 3.5 Subsidence along Redhead Road and the Railway Line Since approval had been given for the total extraction of coal from beneath the Redhead Road, which cut diagonally across 1 West Panel (shown in Fig. 3.1), a requirement of mining was that subsidence be monitored. Stations were established and the initial levels were on 25th August, 1972, some time after Shortwalls 1 and 2 were mined, after the mining of the first sub panel and after the start of the second sub panel in l West Panel. thus subsidence was lost at the start of the line, up to Station 270. Subsidence profiles along Redhead Road are shown in Fig. 3.13. The maximum subsidence of 1050 mm at Station 270 was where the depth of cover was 118 m and the seam height mined was !950 mm. Page B-27 The road consisted of a gravel base coarse with a bitumen and aggregate surface. Cracking was observed as the mining progressed and in the compression zones, small compression humps and associated cracking were noted. Cracks were observed on 14th May 1973 visit, between Stations 265 and 267, and were up to 5 mm wide and in a direction parallel to the extraction line and to the main cross line 103-167. Slight compression humps, in the same direction, were observed at stations 267 and 274. The extraction line of 1 West was at that stage under Stations 604 and 2 80 (on the road) . Photographs are included as Fig. 3.14. Most of the cracking occurred between Stations 303 and 304 as shown in Fig. 3.15. This is at the top of the subsidence hump where the tensile strains would have been high. Slope changes on either side of the hump were 1.4 % (around Station 2 96) and 2.2% (around Station 306) . This hump is caused by the block of coal left beneath the Convent from which no pillars were extracted. Normal road maintenance gangs repaired the cracks and traffic was not disrupted. Subsidence was required to be monitored along part of the now disused railway line, discussed in Study 1, which was adjacent to 0 West extraction. The plan and subsidence profiles are shown in Fig. 3.16. Maximum subsidence from 3rd December 1973 to 7th June 1978 was 82 mm. 3.6 Subsidence Contours and Strain Triangles The final subsidence profiles, including results around the Convent building discussed later, were used to draw the plan showing the contours of final subsidence (Fig. 3.17). The contours were interpolated between the final subsidence profiles using the extraction outline as a guide. The effects of the pillars which were left unmined around Shortwall 1 become evident in relation to the subsidence contour plan. The remnants of the pillars to the north east of Shortwall 2 which were extensively split, have crushed out as evident from their minimal effect on the subsidence profile (Fig. 3.5) and the subsidence contours. Strain triangles were established at various locations and the maximum and minimum principal strains and their directions were calculated at various stages of the extraction according to the procedures outlined in Chapter 2. The plan of final principal strains shown in Fig. 3.18 corresponds to the final subsidence contour plan (Fig. 3.17). In comparing these two Plans it is noted that the subsidence contours have been interpolated between lines of subsidence observations and could be modified in accordance with maximum strains and their directions. Also, any lack of correlation could be caused by odd irregular strains due to the knocking of an observation station. This would distort the arrangement of principal strains. It is noted that on Fig. 3.18 where both maximum principal strains are less than 0.5 mm/m, they are not shown (designated by °)- Where there is a gross anomaly it is obvious that a station has been knocked, this is designated by E. Page B-28 Some general observations can be made from a comparison of Figs 3.17 and 3.18, keeping in mind the qualifying statements made earlier. The maximum principal tensile strain is usually at right angles to the direction of the subsidence contour lines. The maximum principal compression strains are located in a subsidence trough, whether it be general or localised, or at a location where the subsidence contours form a concave shape. A comparison can also be made between the maximum strains and their directions (Fig. 3.18) and the final strain profiles along the lines of observations, as shown in Figs 3.4, 3.9 and 3.12. The maximum tensile strain at a particular location is not necessarily indicated by the strain measured along a single line of stations and where any important structures are to be monitored for strain changes, strain triangles should be established in order to determine the magnitudes and directions of the maximum principal strains. For the same reason, when examining any relationship involving maximum strains, it is important that the direction is at right angles to a long goaf edge. 3.7 Subsidence Profile Characteristics The subsidence work carried out over the West Panels, Dudley Seam enabled information to be obtained concerning 1. the shape of the subsidence profile over a supercritical area, 2. maximum subsidence related to the seam height mined, f 3. k factors which relate maximum slope changes and strains to maximum subsidence values, and 4. the correlation between maximum calculated curvature and maximum measured strain. To obtain an accurate assessment of the shape of the subsidence profile, the subsidence profile should cross a long straight goaf edge at right angles with solid coal on one side and total extraction on the other. This requirement was not met in the final layout plan because of the irregularity of the goaf edge but during the progress of extraction of 1 West Panel, two instances were provided where the subsidence had stabilised and extraction was at such a stage where the aforementioned condition was met. For the supercritical extraction case, as for 1 West Panel, various points along the profile, as a percentage of maximum subsidence, were examined in relation to horizontal distance from the goaf edge. The location of the maximum slope and strains were also defined. These points are listed in Table 3-l. Non dimensional subsidence profiles are plotted in Pig. 3.19 and discussed in association with the profiles from other studies in Chapter 4. Page B-29 From Table 3.1, the limit angle to zero subsidence is 31.7 degrees, and to 5 mm subsidence, the angle becomes 22.4 degrees. The mine geometries and maximum subsidence values are summarised in Tables 3.2 and 3.3. The seam height was reduced in accordance with the recovery of coal to give an effective mining height. The maximum subsidence varied from 0.50 to 0.69 of the effective mining height. The low value of 0.5 0 may be due to insufficient information on the maximum subsidence because of its location near the start of the line monitored for subsidence. The high value of 0.73 located over 3 West Panel represents 100 mm more subsidence than what would have been expected from a S/m value of say 0.65. Mining in the expansive 3 West Panel took place up to the dyke-fault system shown in Fig. B.4. Subsidence investigations on the Southern Coalfield (Kapp, 1980) have shown that when extraction takes place up to a geological feature, this can result in additional subsidence when the feature disrupts the lateral continuity of the superincumbent strata. It would not be unreasonable to assume that there has been a slight but noticeable increase in the subsidence over 3 West due to the dyke-fault system. Unfortunately the swampy nature of the surface at this location did TABLnotE allo3.1 w the line to be extended out over the goaf edge. Features of Subsidence over 1 West Feature of Distance to d profile goaf edge (« max d (m) h 450-470 600-620 450-470 600-620 18-4-74 20-10-73 0 + 73 + 60 + 0.68 + 0.56 5mm + 46 + 42 5 + 26 + 15 + 0.24 + 0.14 10 + 15 + 3 + 0.14 + 0.03 20 - 2 - 9 - 0.02 - 0.08 30 - 9 - 18 - 0.08 - 0.17 40 16 26 0.15 0.24 50 (tn) 26 31 0.24 0.29 60 33 36 0.31 0.34 70 40 43 0.37 0.40 80 48 50 0.45 0.47 90 58 59 0.54 0.55 100* LOO L20 0.93 1.12 Gmax _ 27 __ 36 _ 0.25 — 0.34 +Emax 0 - 9 0 - 0.08 -Emax — 60 — 55 - 0.56 "" 0.51 w - 320 m h - 107 m w/h - 3.00 * Distance to define due to irregular bottom of profile. Page B-30 TABLE 3.2 Mine Geometry for 3, 1 and 0 West Panels Extracted area (m) Cover w 1 Panel Stations h Width Length (m) h h (w) (1) TIT" 22 0 67 0 215 to~22 5 12 0 ITi? 57?8 1 W 320 >600 170 to 175 112 2.86 >5.36 320 >600 450 to 460 107 3.00 >5.36 320 410 600 to 610 106 3.02 3.87 320 >600 269 to 272 118 2.70 >5.08 0 W >250 730 478 to 480 107 >2.34 6.82 TABLE 3.3 Maximum Subsidence for 3, 1 and 0 West Panels Effective Panel Seam height Extraction seam height ?w max m (mm) % m. (mm) mE 3 W 1980 80 1580 1300 "oT82 1 W 1980 80 1580 1030 0.65 2020 75 1520 855* 0.56 2060 80 165 0 1000 0.61 1950 80 1560 1050 0.67 0 W 1980 75 1480 945 0.64 * May not be maximum subsidence - at end of profile Maximum slopes and strains can be related to the maximum subsidence by the relationships given in Table 3.4. Knowing the maximum subsidence, slopes and strains for a given mine layout, it is thus possible to obtain the particular k factors. These are listed in Table 3.4 for the West Panels, Dudley Seam. Curvature profiles have been calculated from measured various subsidence profiles. These are related to profiles of measured strain. The corresponding maximum curvatures and strains are listed in Tables 3.5 and plotted in Fig. 3.20. together with the results from Table 2.3 in Study 2, obtained from the initial profiles over Shortwalls 1 and 2 in 2 West Panel. There is a clear trend in that the strain, whether tensile or compressive, increases as the inverse curvature increases. Page B-31 TABLE 3.4 Slope and Strain Factors w S h G +E -E Panel _ (mm* (m) % (mm/m) (mm/m) h 3W 1.83 1300 120 1.8 1.66 *1 -5.0 0.46 1W 2.86 1030 112 1.2 1.30 *1 -4 0 49 3.00 855 107 1.2 1.50 2 0 0.25 -0 0 08 800 106 1.3 1.70 2 2 0.29 -4.0 0 53 3.02 1000 106 1.6 1.70 2 4 0.25 -6.0 0 64 0 W 2.34 945 107 *2 2.5 0.28 -2.8 0.32 G = kr,.STnav ; +E = k-.S^,, 2 max G max l max *1 Overlapping profiles, +E exaggerated *2 Influenced by uneven goaf edge (extraction to side) 3.8 Subsidence and Damage at the Convent Building An old two storey brie k building was located next to Redhead Road over 1 West Panel. The original lease agreement was that coal pillars should be left in place beneath the building and to a distance of one chain (20.1 m) around the boundary of the property. Since the property was occupied by a closed order of the Roman Catholic Church, i t was decided to monitor around the boundary of the property ra ther than through it. A series of subsidence survey station s and strain triangles was established as shown in Fig. 3.21. Sub sidence was first monitored on 2 0th December, 197 3 and the distances were first measured on 3rd January, 1974. Severe damage was caused to the convent building due to mine subsidence. Records were kept by the Mines Subsidence Board. Subsidence events occurred from September 1974 to February 1975. Cracks began appearing on 10th September 1974 as extraction from 1 West was nearing completion, up to the property boundary (Fig. 3.3). Seven days later there were fine hair line cracks in the building and on the ground. The maximum subsidence along the north west fenceline was 450 mm. On 2 8th October cracks on the main building were up to 5 mm wide and cracks in a concrete path were as much as 10 mm wide. The fence gates were adjusted to allow them to swing freely. On 30th January 1975 a noise was heard and upon investigation, ground cracks up to 10 mm wide and 300 mm deep were observed outside. The principles described in Chapter 2 were used to calculate the maximum and minimum principal strains and their directions. These are shown in Fig. 3.21 and 3.22 with the corresponding Page B-3 2 stages of extraction for 10th June, 1974, 8th October, 1974, llth August, 1919775 5 anandd 3r3rdd NovemberNovember,, 19751975.. ThThee maximum tensile strain on 8th October 1974 was 3 mm/m at an oblique angle to the northern boundary. Station 300 on Redhead Road (Fig. 3.3) is near the convent land and is located where there is significant curvature along Redhead road (Fig. 3.14) when damage occurred to the building. TABLE 3.5 Inverse Curvature Related to Strain Strain Inverse Radius of Panel Date Station (mm/m) Curvature Curvature (xlO-'m-1) (km) FIG. 3.5 3 W 20.8.74 223 -4.8 -1.0 10.0 201 -6.5 -3.5 2.9 2 W 20.8.74 196 +7 .0 +4.5 2.2 190 -6.0 -5.0 2.0 1 W 20.8.74 186 + 8.5 + 4.5 2.2 177 -4.5 -2.2 4.5 FIG. 3.9 1 W 21.9.73 452 -3.0 -4.0 2.5 459 + 2.2 + 2.7 3.7 1 W 13.8.75 452 -3.0 -2.0 5.0 p W 13.8.75 477 -3.0 -2.0 5.0 504 +2.2 + 1.0 10.0 FIG. 2.6 (Study 2) FIG. 3.12 1 W 24.10.73i 607 -7.0 -6.0 1.7 618 + 3 .0 + 4.0 2.5 1 W 21.9.76 655 -5.0 -3.0 3.3 660 + 4.0 + 5.0 2.0 Page B-3 3 3.9 Influence of Bay Length on Calculated Strains At four locations survey stations were set out at distances of 3 m to 18 m (10 ft to 60 ft) in 3m (10 ft) intervals as shown in the top diagram in Fig. 3.23. Each time a distance measuring check was carried out the 3, 6, 9, 12, 15 and 18 m distances were measured as shown and the strains were calculated based on those distances. The distance over which the measurement is made is termed the bay length. The confidence in the strains is dependant on the effects of movements other than those due to mine subsidence such as knocking of the stations, seasonal and weather related movements and accuracies related to distance measuring techniques. Various calculated strains at a particular set of stations were selected and grouped into various phases of movement. Mean values were calculated and these are shown in Table 3.6. For example, at the set of stations from 614 to 620 the strains at the time of initial settlement in September and October 1972 were averaged, and the strains due to final settlement after December 1974 were averaged. Stages in the development of subsidence for this particular example are illustrated by the progressive subsidence of nearby station 606 in Fig. 3.6. It is clear from the relationships shown in Fig. 3.23 that the magnitude of the measured strain decreases as the bay length increases. For strains above 5 mm/m the trend of the relationship is that the measured strain decreased by 2 mm/m as the bay length increased from 3 m to 18 m. f If the same relationships as shown in Fig. 3.24 would apply at different depths of cover it is possible to examine the effect of changing the bay length on the resulting measured strain. The recommendation of the National Coal Board (U.K.) is that bay length should be equal to 0.05 times the depth of cover. That is for h = 100 m, bay length = 5 m and for h = 300 m, bay length = 15 m. An average bay length of 10 m applied at the depth of cover of 100 m would underestimate the actual strain based on the 0.05 h criterion by l mm/m where the strain is of the order of 5 mm/m. The same average 10 m bay length used at a depth of cover of 3 00 m would overestimate the actual strain by 1 mm/m where the strain is of the order of 5 mm/m. For magnitudes of strain smaller than 5 mm, the variations shown in Table 3.7 are obtained from the trends shown in Fig. 3.24. Page B-34 TABLE 3.6 Variation of Strain with Bay Length Initial Dates No. Tension Station interval of of or date meas. obsns. comp. 89- 89- 89- 89- 89- 89- 90 91 92 93 94 95 6.4.71 15.12.72 8 C 1.47 0.98 0.81 0.76 0.69 0.55 to 13.12.73 116- 116- 116- 116- 116- 116- 117 118 119 120 121 122 6.4.71 2.6.72 14 T 172 3 IToO o7?3 (J764 0~74 8 o75 9 to 18.7.73 201- 201- 201- 201- 201- 201- 202 203 204 205 206 207 2.4.71 15.3.72 7 C 0.67 1.68 1.57 1.28 1.01 0.85 to 8.11.72 2.4.71 19.12.72 1 C 2.38 3.13 3.11 2.68 2.49 2.24 2.4.71 25.1.73 1 C 3.57 4.87 4.48 3.94 3.76 3.32 2.4.71 28.2.73 5 C 5.73 6.35 6.03 5.23 4.85 4.36 to 6.6.73 2.4.71 24.2.75 3 C 35.98 9.94 6.19 5.51 5.02 4.41 " to 17.8.76 614- 614- 614- 614- 614- 614- 615 616 617 618 619 620 27.11.72 13.9.73 2 T l7?l 176 8 174 9 I7?8 172 9 Not to meas 20.10.73 27.11.72 6.12.74 2 C 1.01 0.08 0.82 0.73 1.23 Not to meas 11.4.75 Page B-3 5 TABLE 3.7 Effect on Measured Strain Due to Change in Bay Length Increase or decrease in strain Actual strain for change in bay length of 5 m (mm/m) (mm/m) ______. 1.5 to 3 0.5 3 to 7 1.0 i £S@g_3@&s MSOT&V»isgp§»£# ^. Scole I-10 000 •OOIOO 300 4O0 900 "»t~ FIG. 3.1 WEST PANELS AND SUBSIDENCE GRID a « • j _ J—i I IOO aw 300 400 too m FIG. 3.2 MINE LAYOUT AND SURFACE FEATURES JI IL ' l 11 i l—ll—l I 11 | • i jj. _[____ E__j i__ ___] UH CZ] CZJ D • • r ^—L£ JL i ll 3iZlli iII iar—ii—int- IT" li-- I- - ~^ • I ^ Wis* 1 ! ! If. * ;l . I banc FIG. 3.3 MINING SEQUENCE, WEST PANELS SW- N.E. T - _ Q * a "» - •" 2oO 400 6O0 leap 110(3 I—* > •! I I 1 I 1 1 1 1 i t 1 I I I 1 till I I I ' *- ..I.JAII. IIIJI 1 • ' ' ' • I IT) Si _ 0» if? _ S £ 3 * % ' ... I . I . . . I • I . I . . . I . . . I . . I . I ... I .... I ... I (oo 10 .0 «-a-74 5.0 - S £Q s.o a a Z o 45 ^o i i i i i i 10-0 L IO f ' • I I ' •—• « ' I • < ' ' ' I ' ' ' < ' ' I FIG. 3.4 PROGRESSIVE SUBSIDENCE AND STRAIN PROFILES, 1-3 WEST S £ u 0 z LI g in 00 D (fl UJ 0 z < I u _ Q. 0 _i (/) LI or D h < _ I O y r- (/) ^ (r Ll s FIG. 3.5 FINAL SUBSIDENCE, SLOPE AND CURVATURE PROFILES o o 0 o o o o o o 0 o o o o o o o o o o 0 r- CN n ^ in to r- co 01 I- S9 I S9 I 091? - 091? EH 0L> - o^-t? to W <0 O Q > — O 08* - - 08t> - w - u Sw3 - Q CHO PQ — £> 06f - Otot? CO co CO w OOS - oos « - o — o !• r- • t — [ • J - O IS oig rO •tf ^ in l\l lO r^ r^ h- ^J r^ H r r* CJ! •t cfl r* ra <7l CO (—" 00 IO 00 m ff) ^ CN) ,- r- CD r- N _ S i J 8 M -J- J_ _L _] o 0 O O 0 o o o o o o o o o o o r- CN fO V m o o o o o 10 f^ 00 CD o (LUUU) 3oi\3Qisans o O cn co r* ID m n CN o <- CN n i m co N co oi T T ~T i | | | | j i— , S9 I <5 9 I OSt> OSt? CJ 10 O o t- o _ CD 0) co 00 CN sf - CM I 9f I 9 P C9t> o e9t7 m - EH o CO ZLP ZLP PLt> > o CO H *3P fr8t> s 98t> 98t? EH CO w > H CO CO S6t> - S6t? L6P _ _6t> w o o«o p^• O H £05 ^os PH 60S 60? 8lS 8 IS OZS 025 cze L SZS L J_ _L J_ _L _L _L J L _L J_ _J I o cn oo co m to CN CN tO s m CD I oo oi o (UU/UJU-0 NlViJlS 0 i— h CM oo to oi in O «> O CD (NO m s (- 0 0 01 01 oo oo h I Is I 1 I I I I I I I I I ' i t i i i i i i t i i | | I I | I I I I I I I I I I I I I I ' I ' ' II'' l~T——I I CD O CO CN O in CN oo rO 01 in o ID CO in in m o o 01 01 00 00 h m FIG. 3.9 FINAL SUBSIDENCE, SLOPE AND CURVATURE PROFI 8^9 8^9 0^9 0^9 099 099 059 099 Of 9- 0*9 0C9 OC9 029- - 029 019- - O 19 009-J 009 (u-iu.;) 30N3cnsans O r- 01 oo r- <£ in O CM fl o CM o CO r- X) 0L9 - m o o OZ.9 CM ° n i n i - E 099 - - 099 o -J 0S9 - OS9 Of 9 - Of 9 0T9 0C9 0£9 - 029 019 - O 19 fc *8i _ 009 J E 009 -1 1 I I J 1_ J L J I I L J J _1 L r r- O 01 "0 u if) ^n M--0'-cNn IT) CD h 00 01 O •" (UJ/LUUJ) Nivyis OinOOr)*-WNr*riK)*oi K oO'-cNNnnttinincflu) K KUUlilUlLPlJJCOUJlilUOLflu) U) • •• I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I ,!, 1 I ,, , »QQDQDOQO oooooooooooy0 1 00 i o 8 6 4 2 ft^oa^oooe 0 2 4 6 8 1 O >' i i i i i i i i i i i I I 1 I I 1 I I I i i i i t i t i i i t i t i i i i i i i 0 ID UJ n 0) n o in o (fl N N n n * in UJ oo- CD li) ID 10 UJ U) CO U> u> U) u) FIG. 3.12 FINAL SUBSIDENCE, SLOPE AND CURVATURE PROFILES O o n 0 0 0 0 o o o o O o o n 0 0 o o 0 o o o O o r- CM r- CN m N CN CN f) in r~ h IN fl r^ •« 10 h o (N Kl 1^ CN N i^ OO IN n 00 T- >- cn r- CN DO h In t 01 iC 00 0 - M 10 10 CM CM (N in - CN 01 •- •- r- 02c IIIII11 02C OiC 0 IC Q o « Q ooc -OOC W « Q CD O 062 - 06Z CO w H o 082 -082 w u S3 H Q 0^2 -0^_2 H CO P nCO H n t 092 - -092 CD H fc 052 -1 -0 5 2 n 0 0 0 0 0 0 O o o o o n o 0 o 0 0 o 0 u CN fl If m CO !*> 00 0) o o o o N (UUCAJ) 30N30iSQnS Cracking at Station 266, June 19 73 FIG. 3.14 DUDLEY-REDHEAD ROAD / APPROXIMATE CLNLBAL LINE. OF MAIN 70NE OF CCACfcS - FROM FAINT TO 20mm WIDE. - ADLA MAN CATUOUC CMUfcC APPROXIMATELY 3-0 WIDE. &WAY TO lt-n& IcLDWLAD OT To SCALE FIG. 3.15 CRACKS ALONG REDHEAD ROAD / f) o IT. FIG. 3.16 SUBSIDENCE ALONG RAILWAY LINE 100 zoo 300m _l FIG. 3.17 SUBSIDENCE CONTOURS OVER WEST PANELS vwvvvvvwvwvwvwvxvr-cv-^vv^ N XWWW W STRAIN (mm/m) FIG. 3.18 MAXIMUM AND MINIMUM PRINCIPAL STRAINS X a m£ o CN O o • • n II •ol-c JC £ \ \ co £ w H fc LO O in o § (DO o W II II o 13 3 (3 H ro co cn Q H >* CO a o o m p i I pS3 E-t o O co CO o in o H "31 +J 4-1 CO S3 CD CD W g 5 S3 O o o S3 -o|x: H CD H fc X a i/)e 0s > 5 c eg c Q) £> c {£ 3 c G i/) c E J_ *— «-» E I— «-» 2 O Q ""0 0 C7N o CO CVJ o o CNJ^ ^ 0 ^ -I*1 _ *0 0-D ^0 o OJ oo o o o I L vo m ( UJ 01 x) 9Jn40AJn3 asj3AU| FIG. 3.21 PRINCIPAL STRAINS AROUND CONVENT, 1974 DiSTAUCE (m) FIG. 3.22 PRINCIPAL STRAINS AROUND CONVENT, 1975 89 90 91 92 93 94 95 201 202 203 204 205 206 207 116 117 118 119 120 121 122 614 615 616 617 618 619 620 G G G G G G G 3m—i 6m .<- 9m 4 .<- 12 m .*- 15 m 4 18m 2T 0---0 89-95 n—a 116-117 1- a—& 614-615 (a) *—x 614-615 (b) 89 90 91 92 93 94 95 etc etc etc. etc. etc. etc. etc 10 T 201 - 207 5 - Survey 5 Survey 4 Survey 3 >o*—Ek. "O-- -GU- Uf -a Survey 2 Survey 1 f *— 89 90 91 92 93 94 95 etc etc. etc etc. etc. etc etc FIG. 3.23 STRAIN RELATED TO DISTANCE MEASURED Page B-36 STUDY 4 PANEL AND PILLAR LAYOUT USING BOTH PILLAR EXTRACTION AND SHORTWALLS TO CONTROL SUBSIDENCE 4.1 Introduction Guided by the results of the subsidence work discussed in the first three studies, application was made to the Chief Inspector of Coal Mines, NSW, to extract pillars from the West Panel of the Gateshead Panel in the V.T. Seam, Burwood Colliery beneath part of a residential area of Gateshead. Pillars were extracted on a panel and pillar basis, extracting two rows and leaving one. The panel w/h ratio was 0.4 5 and the maximum subsidence associated with this extraction was 7 5 mm. Plans were then made to extract four shortwall panels to the north beneath the same residential area and parkland. Pillars were left unmined between these panels and subsidence investigations were carried out to carefully monitor the surface effects as each shortwall panel was mined. The ground sloped up to the Highway which was located next to a cutting in the unstable ground and which was influenced by the outcropping Australasian Seam. The subsidence investigations over the earlier panels allayed any concern for the stability of the area. The maximum observed subsidence was 113 mm. The pillars that were left between the panels were stable and the result was a very flat subsidence profile with maximum slopes of 0.22 per cent. The maximum strains were small and there were no reports of damage to homes or services in the area. Following the success of this exercise, the panel and pillar system was used more extensively in Newcastle when mining in areas where the surface or structures on the surface required some form of protection. 4.2 Geographical and Geological Setting The surface is generally flat and horizontal over most of that part of the residential area of Gateshead which is above the West Panels and Shortwalls l to 4 in the V.T. Seam. The topography rises in the north and to the east in the vicinity of the Pacific Highway where there has been a history of instability due primarily to the Australasian Seam which outcrops in this area. The depth of cover varies from 107 m in the south to 130 m in the north. Although the area is residential, it includes the parkland shown in Fig. 4.1 which was ideal for the location of survey stations. The houses have timber frames with a flexible external cladding and are constructed on reinforced concrete footings, as specified by the Mine Subsidence Board in areas proclaimed as Mine Subsidence Districts. The photographs in Fig. 4.2 show the type of house construction. Page B-37 The V.T. Seam is the only seam mined in the area. Above the seam are beds of conglomerate, sandstone and shale of varying thicknesses shown in the section in Fig. 4.3 (Bores 964 and 931). The immediate roof consisted of laminated lutite, mudstone and siltstone higher above the seam resulting in good caving conditions. 4.3 Mining Procedures First workings of bord and pillar mining took place in the West Panels with pillars 2.0 m high at 32 by 22 m centres. Pillar extraction commenced in August 1970 in the south and was completed from the four panels in March, 1971. Two rows were extracted and one was left unmined as shown in Fig. 4.3. The extracted area was 49 m wide and the pillar support 15 m wide. Development then commenced for the shortwall panel to the north. Mining commenced in Shortwall 1 on 2 7th September 197 2 and shortwall 4 was completed on 14th May, 1974. As one panel was being mined, the next panel was being developed. The panel geometries are given in Table 4.1. The maximum and minimum mining heights in each panel vary up to 150 mm from the mean value shown. The panels were separated by two rows of pillars to provide support for the superincumbent strata. Details of the pillars are given in Table 4.2. The average longitudinal centre to centre spacing of the cutthroughs in each row of pillar support was 3 6 m. With 85 per cent recovery of coal from the pillars extracted in the West Panels and slightly less than 100 per cent recovery of coal from the shortwall blocks, the overall recovery of coal from the two areas shown in Fig. 4.3 was calculated to be 70 per cent. After the Shortwalls 1 to 4 were mined, Q. Panel was developed to the west of Shortwall 1 and pillars were extracted from March to August 1975, on a panel and pillar basis (Figs 4.1 and 4.3). Following the later Shortwalls 5 to 9 some pillars were extracted from the headings which surrounded the Shortwalls 1 to 4 as mining retreated from the area. Pillars were extracted from the West Headings to the north of Shortwalls l to 4 and from the Gateshead Panel to the east of both Shortwall 4 and the West Panels, shown in Fig. 4.1. Existing subsidence stations monitored the subsidence over the West Headings and the Gateshead Panels as listed in Table 4.1. Page B-3 8 TABLE 4.1 Details of Extraction Dates of mining Extracted area Depth of cover Mining (m) (m) height Panel (mm) Start Finish Width Length Min. Max. 0 West 8.70 2040 50* 104*) 1 West 2040 50 195 ) 2 West 2040 50 195 ) 107 110 3 West 2.71 2040 50 195 ) SW 1 27.9.72 20.2.73 2190 66 380 110 131 SW 2 28.5.73 16.7.73 2220 64 360 113 116 SW 3 2.10.73 31.1.74 2160 62 660 107 122 SW 4 4. 2.74 14.5.74 2190 71 625 104 131 Q 3.75 2190 44 91 2190 50 137 113 127 15.8.75 2190 52* 195 W Hdgs 21.9.77 28.10.77 2160 50 370 130 140 G/Head 27.1.78 29.5.78 2290 50 715 100 125 * Irregular dimension, mean value given Page B-3 9 TABLE 4.2 Geometries of Pillars Cover* Rows Width Heights Width Width Pillar (m) of of of Pillars Pillars Pillars Height Depth (m) (m) 0W - 1W 107 15 2.04 7.3 0.14 1W - 2W 107 15 2.04 7.3 0.14 2W - 3W 107 15 2.04 7.3 0.14 3W - 5W 110 15+23+20 2.10-2.30 26.4 0.53 SW 1-2 111 13+13 2.23 11.7 0.23 SW 2-3 116 11+13 2.20 10.9 0.21 SW 3-4 122 11 + 13 2.20 10.9 0.20 SW 4-G 105 17+15 2.25 14.2 0.31 110 2.25 14.2 0.29 SW - Q 114 12+12+9 2.20 15.0 0.29 Q Panel ill 21 2.20 9.5 0.19 117 18 2.20 8.2 0.15 •Depth of cover at locations of survey lines 4.4 Subsidence Monitoring A grid was laid out by the mine surveyors along the streets over the West Panels as shown in Fig. 4.4. The stations consisted of nails in the road surface at 15 m intervals. The bench mark was established at the corner of the Pacific Highway and Sydney Street, well outside the extraction area and unaffected by mining. The initial level run was on 18th August, 1970. Subsidence profiles along Goundry, Jamieson and Jennings Streets are shown in Fig. 4.5. No subsidence results are available along Casey Street because the gravel road surface was sealed after the initial level run. The maximum observed subsidence over the West Panel of 70 mm occurred near the intersection of Goundry and Jennings Streets. By interpolating from the subsidence contours and the final subsidence profiles, the maximum subsidence over the West Panel was 7 5 mm. The second grid was established over Shortwalls 1 to 4 by the BHP Survey Department. The two straight lines of stations for subsidence and strain monitoring were established in parkland (Pigs 4.1 and 4.3), one across Shortwalls l to 4 (82 to 114) and one along Shortwall 2 (236 to 297). An additional line was later set out along Heshbon Street across the southern part of Shortwalls 3 and 4 (3 50 to 400) where the depth of cover was less than in the north and where further subsidence checks were Page B-40 required in the residential area. Levels were at monthly intervals while mining was taking place. When final subsidence occurred observations were then once every two years to check the continuing stability of the area. In addition to the level observations, distances were measured regularly to calculate the surface strains. The surface topography and the underground workings along the line of subsidence stations 82 to 114 over the four shortwalls and along the line 23 6 and 297 along Shortwall 2 are shown in Figs 4.6 and 4.7. These lines were first levelled in June and July 1972 before the start of extraction of Shortwall 1 in September, 1972. Along the section line 82 and 114, the depth of cover varied from 110 m over Shortwall 1 to 127 m over Shortwall 4. The profiles in Fig. 4.6 show the stages of subsidence after Shortwalls 1,2,3 and 4 were completed. The maximum subsidence of 113 mm occurred over Shortwall 1. Values of maximum subsidence over Shortwalls 2, 3 and 4 were 85 mm, 73 mm and 61 mm. Along Shortwall 2 the depth of cover varied slightly around 114 m. The subsidence profile shown in Fig. 4.7 for 15 October 1973, is three months after Shortwall 2 finished and before any effects occurred from Shortwall 3 which had just commenced. The final subsidence profile on 14 May 1974 is a flat bottomed trough with a maximum subsidence of 85 mm. The topographic section and subsidence profiles over the south end of Shortwalls 3 and 4 are given in Fig. 4.8. The initial survey was in July, 1973, before extraction commenced for Shortwall 3 in October, 1973. The maximum subsidence over Shortwalls 3 and 4 at that location are 7 0 mm and 67 mm. The pillars between the shortwalls caused the slight rise in the subsidence profile. The subsidence contour plan in Fig. 4.3 was constructed by interpolating between the final subsidence profiles. The maximum subsidence was related to the mine geometry. The main factors are the mining height m, the width of extracted opening, w, and the depth of cover, h as listed in Table 4.3. It can be seen that for panels where the width is up to 0.6 h, the subsidence is only 5 per cent of the full seam height mined. These results are summarised in Chapter 4 where account is made of percentage coal recovery. Page B-41 TABLE 4.3 Subsidence and Panel Geometry Extra cted area Mining Max S height subs max Width Panel Depth w ra m w h (mm) (%) (m) (m) h West 49 107 0.45 2040 75 3.7 SWl 66 110 0.60 2200 113 5.1 SW2 64 113 0.57 2230 85 3 . 8 SW3 61 122 0.50 2200 73 3.3 SW4 71 127 0.56 2200 61 2.8 Q 52 120 0.43 2400 52 2.2 TABLE 4.4 Values of Elements of Subsidence Location Subsidence Slope Strain Inverse Radius at (mm) Change (mm/m) Curvature of Station (%) (xlO-'m-1) Curvature (km) 86 - 87 + 0.8 1.0 10.2 87 - 88 0.22 0 92 113 0 -3.0 1.6 6.1 94 - 95 0.18 0 Pillars between + 0.6 0.6 17 .0 shortwalls 0.5 20.3 2, 3, 4 85,73,61 - 0.5 4.5 Features of Subsidence Profiles Associated with the maximum subsidence of 113 mm over Shortwall 1 was the maximum compressive strain of 3.0 mm/m (Pig. 4.6). The maximum change in the slope of the surface was were in the zones of 0.2 per cent and there the strains transition'from tensiorTto compression. The panel and pillar iayout produced a flat overall subsidence profile with maximum tensile strains over the pillars between Shortwalls 2, 3 and 4 of Page B-42 0.6 mm/m and maximum compressive strains over those shortwalls of 0.5 mm/m. The maximum slope at this end was less than 0 i per cent. The profile of inverse curvature calculated fvL ft subsidence profiles is of similar shape to the strain profile6 Corresponding values along the profiles of the elements of subsidence are shown in Table 4.4 for the final profile on 9 July, 1974. The maximum change in slope in a longitudinal direction over Shortwall 2 was 0.1 per cent. -Longitudinal Over the southern end of Shortwalls 3 and 4 the maximnn, slope change was 0 12 per cent (Fig. 4.8) because of the smaUer subsidence values than those across the northern end of the shortwalls (Fig. 4.6) and the resulting decreased curvature of the subsidence profile. Subsidence at various points along the subsidence profiles were tabulated (Table 4.5) and plotted to show non dimensional subsidence profiles in Fig. 4.9. The locations of the transition points are given in Table 4.6. The profiles are discussed in Chapter 4 together with corresponding profiles from other studies. 4.6 Subsidence over Q Panel and Gateshead Panel After the completion of Shortwall 4, pillars were mined from Q Panel to the west of Shortwall 1 on a panel and pillar basis, shown in Fig. 4.4. Following that extraction, and as mining was retreating from the area some of the pillars were extracted from the development which surrounded the four shortwalls. The arrangement of pillar extraction was designed to result in minimum subsidence. The subsidence was monitored over the pillar extraction from the West Headings to the north and the Gateshead Panel to the west of the four shortwalls. A line of survey stations was established as part of the original grid in July 1972 from BS50 to BS82 from the bench mark to the main part of the grid. Regular level checks were made along this line and the maximum measured subsidence of 40 mm occurred from Stations 77 to 79. The maximum subsidence over the panel was interpolated to be 5 0 mm. Two rows of pillars were extracted from the Gateshead Panel (Fig. 4.1) and the resulting subsidence was monitored by levelling from Stations BS 3 85 to 400 and Stations BS 455 to 450. The maximum subsidence values were 40 mm and 34 mm respectively, as summarised in Chapter 4. Page B-43 TABLE 4.5 Features of Subsidence Profiles SWl SW4 SW2~7~Start SwITTinish 82 - 93 129 - 144 264 - 282 236 - 248 Feature 9.7.74* 7.6.77 29.11.73 29.11.73 of Profile d d d d ( % ) ** (Smax) d(m) h d(m) h d(m) h d 0 + 80 + 0.73 + 72 + 0.57 + 85 + 0.76 _ _ 5 mm 62 0.56 56 0.44 50 0.45 - - 5 62 0.56 65 0.51 66 0.59 - - 10 45 0.41 56 0.44 +47 +0.42 *** - 20 31 0.28 46 0.36 + 12 + 0.11 -27 -0.23 30 23 0.21 36 0.28 - 3 -0.03 -36 -0.31 40 15 0.14 27 0.21 -11 -0.10 45 0.38 50 + 9 + 0.08 + 18 + 0.14 20 0.18 54 0.46 60 + 3 + 0.03 + 8 + 0.06 27 0.24 60 0.51 70 - 3 -0.03 + 2 + 0.02 36 0.32 69 0.58 80 - 9 -0.08 - 6 -0.05 48 0.43 78 0.66 90 -16 -0.15 -13 -0.10 -55 -0.49 -90 -0.76 100 -33 -0.30 -36 -0.28 -78 -0.70 -117 -0.99 Gmax + 3 + 0.03 Too small and - 9 -0.08 - - +Emax + 15 + 0.11 irregu!la r to TOO small and irregiala r -Emax -33 -0.30 detect • to detect. SW 1 4 2 W(m) 66 71 64 h(m) 110 127 112 w/h 0.60 0.56 0.57 * Same features as 7.6.7 7 ** d = distance to goaf edge *** Oblique line to station 236 Page B-44 TABLE 4.6 Location of Transition Point tn w Date t w h _ of Distance JL SW (m) (m) h Profile Station (m) h 1 66 110 0.60 9. 3.73 87 +9 +0.08 95 - 9 -0.08 12.12 .75 87 +9 +0.08 2 64 113 0.57 12.10.73 109 0 0 3 61 122 0.50 19. 3.74 122 + 9 +0.07 4 71 127 0.56 9. 7.74 134 0 0 7. 6.77 136 +18 +0.14 3 61 115 0.53 3. 6.77 366 +18 +0.16 4 71 108 0.66 3. 6.77 387 + 9 +0.08 t = location of 50% Smax (transition) point in relation to goaf edge. + over solid over goaf 4.7 Increase in Subsidence with Time Curves showing the increase in subsidence with time are plotted for stations over each of the shortwalls in Fig. 4.10. The dates of the start and finish of each shortwall, in Table 4.1, are also shown plotted. Each station was affected by the underlying shortwall, and the adjacent panels. By examining the time-subsidence curves in relation to the dates of mining of each shortwall it was possible to obtain the effects of each shortwall on the subsidence, as shown in Table 4.7. Page B-45 TABLE 4.7 Separate Effects of Shortwalls on Overlying Stations Subsidence at Station Due to Shortwall Station Previous" UndirTyini'Toll^winr" Subsidenc< B.S. Shortwall Shortwall Shortwall (mm) (nun) (mm) (mm) 1 33 88 " 2J 113 2 105 21 52 12 85 3 116 9 52 12 73 372 6 52 12 70 129 9 52 61 382 9 58 67 258 12 61 85 For example, the time-subsidence curve of station BS116 over Shortwall 3 (Fig. 4.10) shows that when Shortwall 2 finished (2F) on 16 July 1973, 9 mm subsidence had occurred. There was no further subsidence until Shortwall 3 commenced (3C) and by 31 January, 1974, when Shortwall 3 finished (3F), 61 mm subsidence had occurred, resulting in 52 mm subsidence due to Shortwall 3 only. The effect of Shortwall 4 was to increase the subsidence at Station BS116 by a further 12 mm. In addition the maximum rate of increase of subsidence estimated from the curves in Fig. 4.10 are shown in Table 4.8. Most of the subsidence occurred when the face was in the vicinity of the station on the surface. Although there were insufficient points on the curve to give an accurate value of this maximum rate of subsidence, close approximations to the maximum rate are given. The maximum rate was 1.5 mm/day over Shortwall 1, with the maximum subsidence of 88 mm occurring in 74 days. The maximum rates of subsidence over the following shortwalls were less, and for Shortwalls 3 and 4, subsidence occurred at a maximum of 1.0 mm/day. Page B-46 TABLE 4.8 Time Rates of Subsidence over Shortwalls Approximate* Maximum Rate"of"subsidence due to Shortwall Station B.S. Previous Underlying Following Shortwall Shortwall Shortwall mm/day mm/day mm/day 1 93 177 oTI 2 105 0.1 0.7 0 3 116 0 1.0 0.1 372 0 0.5 0.1 129 0 1.0 0 382 0 1.0 0 •Actual rate could be slightly greater than estimated value 4.8 Subsidence Related to Time and Face Position The subsidence recorded at particular dates (s ) are listed in Table 4.9 for selected stations over the Shortwalls. Table 4.10 shows the distance the face had advanced beyond the surface point related to the depth of cover. The subsidence due to the previous shortwall (Table 4.6) was subtracted from the observed subsidence to give the actual subsidence at the particular date due to the underlying shortwall (s-^) which was then shown as a proportion of the maximum s /s ) subsidence due to the underlying shortwall ( sw sw • For each date when a value of subsidence was obtained, the face position at that date was determined from a plan of the mine workings which showed the face position every two weeks. The graphs of s /S for stations over each shortwall are shown in Fig. 4.11swwh!c7h shows the relationship between the development of subsidence at a station due only to the underlying shortwall and the time taken for the face of the shortwall to pass beyond the station. Page B-47 TABLE 4.9 Progressive Subsidence Related to Maximum Subsidence s Maximum s Shortwall Date Recorded SW subsidence SW (Station) s (mm) due to 0 shortwall S~~ (mm) S SW sw (per cent) (mm) 1 7.12.72 0 0 8 8 0 (93) 15. 1.73 61 61 6 9.0 19. 2.73 88 88 100.0 3 12.10.73 9 0 52 0 (116) 26.11.73 55 46 88.2 4 19. 3.74 12 3 52 5.9 (129) 26. 4.74 49 40 76.9 TABLE 4.10 Face Advance Related to Depth of Cover Cover Days Face Advance h (m) since in relation Shortwall Date and under extraction to surface f (Station) lying date point — of f h extraction (m) 1 7.12.72 110 2 - 12 0.11 (93) 15. 1.73 9.12.72 + 37 + 44 0.40 19. 2.73 + 72 + 134 1.22 3 12.10.73 125 18 -125 1 00 (116) 26.11.73 30.10.73 27 + 198 1 59 4 19. 3.74 128 13 - 95 0 74 (129) 26. 4.74 2. 4.74 24 + 76 0 6 Page B-48 Subsidence commenced around 10 days before the face passed beneath a particular station, with 50 per cent of maximum subsidence due only to the particular shortwall occurring at from 15 to 20 days after the face passed. Subsidence was complete after 7 0 days. The subsidence is shown in relation to the face advance in Fig. 4.11. The curve shows that subsidence commences at a point on the surface when the face is around half the depth of cover before the surface point. When the face is beneath the surface point, 30 per cent of the maximum subsidence has already occurred. 4.9 Stability of Pillars The rows of pillars separating the extracted panels in the West Panels and the pillars separating Shortwalls 1 through to 4 each are rectangular in overall plan with regular geometries. They are thus amenable to examination using the method of Wilson and Ashwin (1972). The much wider blocks of coal separating the various panel groups were not examined for stability. Pillar stabilities were calculated below the lines of surface survey stations to enable a comparison with the observed subsidence, using the geometries in Tables 4.1 and 4.2 and summarised in Table 4.11. The calculated factors of safety for pillars in the West Panels and between the Shortwalls using tan B = 3 were from 0.74 to 0.87, and using tan B = 4, were from l.ll to 1.30. The subsidence investigations have shown that the pillars have long term stability as there has been no increase in subsidence since the pillars were formed in the years from 1970 to 1974. Thus the values of the calculated factors of safety in this first example of a panel and pillar layout in a residential area were taken as unity for a basis of comparison with values calculated for various panel and pillar layouts at other collieries in the Newcastle area. In making this adjustment consideration was made of the fact that the factors in method of calculation were developed in the U.K. for U.K. conditions and conditions in the Northern Coalfield would be likely to result in different factors. The best method of comparison is to examine the application of the method in areas where pillars are known to be stable. Page B-49 TABLE 4.11 Pillar Geometries Cover*l Adjacent Rows Pillar*3 Pillar Pillar (m) Panel of Width Heights Widths Pillars (m) (m) (m) West *2 1 50,50 107 15 2.04 Shortwall 1 - 2 2 66,64 111 13+13 2.23 2 - 3 2 64,62 116 11 + 13 2.20 3 - 4 2 62,71 122 11+13 2.20 2 62,71 110 11+13 2.16 SW4-G 2 71,50 105 17+15 2.25 2 71,50 110 17 + 15 2.25 * 1 Depth of cover at locations of survey lines * 2 Same pillar dimensions for each of three rows separating panels * 3 Mean cut-through spacing varies from 30 to 35 m TABLE 4.12 Calculated Pillar Stabilities Pillar Calculated StabTlity Width Width F of S Number tan B=3 tan B=4 Height Depth West 0.74 1.11 0.93 7.3 0.14 Shortwall l - 2 0.87 1.30 1.09 11.7 0.23 2 - 3 0.78 1.20 0.98 10.9 0.21 3 - 4 0.76 1.17 0.95 10.9 0.20 0.82 1.25 1.03 11.0 0.22 SW4-G 1.06 1.33 14.2 0.31 1.03 1.29 14.2 0.29 Page B-50 A better terminology would be 'stability number' rather than 'factor of safety' since at value of F of S above those in Table 4.12 pillars will have a stability equal to or greater than those which were formed at Burwood Colliery. A stability number of unity can be given to the pillars in the case where subsidence surveys have proved their continuing stability. In other words, values of calculated factors of safety for other areas will show the margin over unity for which rows of pillars in an existing panel and pillar system are known to have long term stability. The values at which pillars are of marginal or doubtful stability is considerably less than unity as shown by similar calculations elsewhere in the Newcastle area. Thus the basis of comparison for pillar stability is conservative. Generally a stability number of unity could be assumed for any of the pillars in Table 4.12 which are all stable in the long term, but to add a safety margin, a Wilson and Ashwin calculated F of S of 0.80 was assumed for the stability number of unity. The principal geometrical factors which affect pillar stability of supercritical length are its width and height and the depth of cover. The width of a pillar which is required to be stable will need to be increased to cater for a greater seam height and will need to be increased at a greater depth of cover. Other factors which need to be considered are the spacing of cut throughs and the widths of panels which the pillars are designed to separate. The width/height and width/depth ratios are shown in Table 4.12 for pillars at the locations where stability calculations were carried out. The most slender pillars are those separating the extractions in the West Panel. These pillars are the smallest in the mining district under consideration as shown in Table 4.2 where the geometries of all pillars are listed. The calculated factor of safety for those pillars was the smallest (Table 4.12). FIG. 4.1 SURFACE PLAN AND MINE LAYOUT View east from Station 62 View west towards Station 230 FIG. 4.2 HOUSE CONSTRUCTION ABOVE SHORTWALLS JLJlUL_\UUUJtt_Jl 20- 40- 60- 80 __hano: 100 MM. 120- SQOCT KO HDDCI 160 QQC: www Seal*- I metres I Strata section FIG. 4.3 PANEL AND PILLAR LAYOUT WITH GEOLOGICAL SECTION 0 VJ a. 1:4,000 ICC 2C0 3C0 Itl..!....!'.'''.''!!^**1 m 'FIG. 4.4 SUBSIDENCE GRID OVER WEST PANEL 0 100 200 300 400 500 600 700 800 900 1000 I I I I I I 1 i i II 0 r I.D. 18.8.70 «—©3.1 1.72 A—a 8.7.71 100 (a) GOUNDRY STREET SUBSIDENCE 0 100 200 300 400 500 600 700 800 900 10001100 50 50 100 L J 100 jwsr ___: (b) JAMIESON STREET SUBSIDENCE 400 30C 200 100 0 i i i i 1 0 : 1 0 u O z 50 a E % E D 100 L- 100 (c) JENNINGS STREET SUBSIDENCE FIG. 4.5 SUBSIDENCE OVER THE WEST PANEL in CD CM cn -4" oo O CN Wes1 S5«oaj«oC5x«n «K»«I9IOIOOOO oogo^-j:^;*? c £2 2 ££$!££ C<^BS2{30Q8__33 East < • i > i i i • • T'J"1" • i • i • • i • « • • . i » • • i • i i*7T l *7~T*7 . . . TTTTT S.W.I (a) Topographic section »J< (b) Subsidence from 17.7.72 to 9.7.74 -0-3L (c) Change in slope ,^A— -2L (d) Inverse curvature *^yv^^w*/*V^ (e) Strains from 1.8.72 to 16.5.74 100 - - 1 00 50 50 con o o O o o O 0) in 00 uO . QM CM . n CM CD |x CM CM CNM CM i i i i i i i i i i i i i i i n i CM CM I I I I I I I I i 0 111 I I I I I I I I I I 0 I I I I I I I I I I -50 -50 -C CO n C CD CO 00 W CN 00 Topographic Section CM -100 L 100 am o o o o o o cn CtM ro \f m CO N oo oo CMM CM CM CM CM CM CM CN I I I I I I I I I I I I I I I | I I I I I I I I I I I I I I I I I i I I I I I I I I I I I I I I—I .I I I I I I I I 0 50 I.D. 18.7.72 X 2S.6 73 100 o 7.S.73 1 50 * 14.5.74 Subsidence Profile FIG. 4.7 TOPOGRAPHY AND SUBSIDENCE ALONG SHORTWALL 2 0 50 Oti 50 «*eee _ _ < -50 n u (/I _ U - D 0 SW3 SW4 (a) TOPOGRAPHIC SECTIO N -100 L 350 360 370 380 390 400 i _ i 0 oooooooooooo o ^eSSSS--? 0 o-o 11.7.74 _ 150 L c/i 150 (b) SUBSIDENCE FROM 6,7.73 0.3 0.3 K° 0.2 0.2 0. I UJ 0. i Q. 0.0 ?«W^ ^9©€ 0.0 O 0,1 0. I _i 0 2 J) 0.2 0,3 0.3 (c) CHANGE IN SLOPE FROM 6 7.73 FIG. 4.8 PROFILES ACROSS SHORTWALLS 3 AND 4 X a mE $ o _ r> r^> _ LD in ir> • • • • o O o o II II II II x X! X! X \ \ \ \ ^ J5 3 JS CO W m •ol-c H o 1 e Pn i E I I o CN CN o oCM rH r- r~ 00 H w II II II II u X X X X Q —J (0 rd rd H g g g g CO CO CO CO CO CQ D ft* x CO rr 03 u •H 3Z o >H zr>d C O Q -P •H H D CO MH CO EH •— —' 2 CO « K W rH rr tN CM H H rH H rH D rH r-i H rH rO rt rd fd 2 3 S 2: 5 O -P -P +> 4-) 2 m 5-1 n H 0 0 0 0 X X X X cn CO CO CO CO • O • X 0 IT) -ol-c _ H 6 + PH X a E in 01 01 H EH E EH U z w Q H CO PQ D CO J3 H w CO < « m N u cn 53 H 01 ( OIUJ ) eauepisqns o 1 T - ", i • i ^1 Z o (D 01 1- r- CM n — < •— • / m if) 1- 00 / c J > 2 _l < o _ O( i ^ (— n •H- tj_ rr • O o I T 1r _— o IT LU j Q o X ru Id _ Q 0 a <1 W O V U ^ •J) o s^^ Z< 53 // > Q Q < in ' y _ Id H o y U i o / < < b_ • < fe o o * . i i i . 1 • 1 ! _ EH - to- Q i o o O O O O •0 o w EH fe (%)' S/ s « W U W Q H 00 i ' i • i r CO c PQ O o CO CO z q H o i- fe " < - 0 O < • rr < p- o - CM x Ld rr Ld o p- s^ i_ c < >- < ' (%) "s/"s Page B-51 STUDY 5 SUBSIDENCE IN GATESHEAD AND RELATED SURFACE DAMAGES 5.1 Introduction Subsidence has occurred in Gateshead due to extraction in the V.T. Seam. The subsidence effects of the first panel and pillar layouts, those of the West Panels and of Shortwalls 1 to 4 were discussed in Study 4. Following the success of this work, extraction then took place of some of the pillars that were formed by the headings surrounding the area, with subsidence no greater than 50 mm in the residential area of Gateshead. Extraction continued into the pillared areas to the east, west and south of the West Panels, known as the Gateshead and Waratah Panels and the Belt Headings shown in Fig. 5.1. Some of the original subsidence survey stations used in the investigations discussed in Study 4 were used. New survey lines were also established. There was some additional mining by means of pillar splitting in the Waratah Panel which resulted in failure of the pillar remnants and excessive subsidence. No other seams have been mined in the area. The subsidence caused damage to several homes overlying the extraction. Damages were repaired by funds of the Mines Subsidence Board. 5.2 Subsidence over the First Belt Headings Extraction A line of subsidence stations was established along Oxford Street to replace the earlier stations which were removed by road works. The line, Stations 450 to 468, 477 to 500 was part of the overall extension to the original subsidence grid. The grid extension was first levelled on 27th June 1977, well before any pillar extraction from the Gateshead, Waratah or Belt Headings Panels as shown by the dates of extraction on Fig. 5.1. The plan of the streets in Gateshead lying above the area of pillar extraction is given in Fig. 5.2. The line of subsidence stations 500 to 482 along Oxford Street monitored extraction in the first area to be depillared in the Belt Headings. The contours of equal subsidence are shown in Fig. 5.2 and on the extraction plan in Fig. 5.3. Pillars were extracted from 3rd August to 30th September, 1977. The dimensions of the extracted area are width w of 76 m, length 1 of 180 m. For the depth of cover h of HO m, the ratios w/h and 1/h are 0.69 and 1.64. The subsidence profile along Oxford Street and the increase in subsidence with time are given in Fig. 5.4. The maximum subsidence over the first Belt Heading extraction occurred at Station 491. There was an initial subsidence of 320 mm, this increased to 350 mm in November 1978 an<* then gradually increased to a final maximum of 410 mm and remained at that value at the final survey run on 7th October 1982. por a seam height mined of 2300 mm and a recovery of 85%, Page B-52 the effective mining height m_ becomes 1960 mm. Thus the S /m ratio is 0.19, using the maximum subsidence of 350 mm,mwrHcB excludes residual subsidence effects. Subsidence contours were obtained by using the measured values along Oxford Street to sketch the contours accordinq to the extraction layout. A section across the subsidence trough was selected where the slopes would be a maximum. The slope changes and curvatures were calculated, resulting in the profiles shown in Fig. 5.5. The maximum slope change was 0.8%. The maximum inverse curvatures were +1.5 and -7.0 x l0-«m_1 and corresponded to radii of curvature of +6.6 and -1.4 km. The high compressive and low tensile strains are typical of a subcritical subsidence trough. The bottom of the subsidence trough is in Oxford Street where the greatest curvatures and compression strains occurred. The magnitudes of the compressive strains decreased rapidly from the peak value, so that even at the 3 00 mm contour, the inverse curvature was -2.5 x 10_4m~1. With possibly one or two exceptions, affected homes are in the zone of tension. Information from Lambton Shortwalls in Study 2 already published (Kapp, 197 8) would indicate that the maximum tensile strain associated with curvatures of +1.5 x lO-'m-1 would be +2 mm/m. The Subsidence Engineers' Handbook (NCB, 1975) shows that slight damage could be associated with strains of this order. There were no reports of damage to homes or services in this area. 5.3 Subsidence over the Waratah and Gateshead Panels Following extraction of pillars in the first part of the Belt Headings, pillars were then mined at various stages in the Waratah Panels from 13th October 1977 to 6th July 1978. The maximum recorded subsidence due to that mining was 50 mm. At the same time the Waratah Panels were being mined, extraction of pillars in the Gateshead panel to the west of the Shortwalls and the West Panel (Fig. 5.1), was taking place. Extraction passed beneath the Oxford Street line at the end of April 1978 and was completed on 19th May 1978. The maximum subsidence was 34 mm at Station 453 (Fig. 5.4). The Gateshead Panel, of supercritical length 720 m, had an extracted width w of 55 m and at a cover h of 115 m at Oxford Street, the w/h ratio was 0.48. With a seam height of 2400 mm, a recovery of 85%, the ratio S_ /m„ is 0.02. max & 5.4 Excessive Subsidence over the Second Belt Headings Extraction To avoid extending the area of maximum subsidence over the first Belt Heading extraction, pillars were left unmined in the Belt Headings before proceeding with the later pillar extraction from 4th to 23rd August 1978, as shown in the plan of Fig. 5.3. Page B-53 The first pillars extracted at a width of 78 m were close to the last of the Waratah Panel extraction. The depth of cover was 112 m and the seam height mined was 2300 mm. The extraction in the second Belt Headings panel continued at a reduced width of 6 8 m and was completed on 2 3rd August 197 8 apart from some additional splitting of pillars at the end of the panel which took place in September and October 1978. The subsidence profiles in Fig. 5.6 are along the BHP Survey lines 529 to 501 along Goundry Street, 536 to 550 in the open grassed reserve and the Colliery survey line 1300 to 200 along Church Street, all located on the surface plan in Fig. 5.2. The progressive increase in subsidence with time is shown in Fig. 5.7 for the points of maximum subsidence along the three profiles in Fig. 5.6. Subsidence surveys in March and October 19 81 and October 1982 showed that no additional subsidence had occurred. Roadworks along Church Street after the November 197 9 survey resulted in the loss of those stations. Subsidence was first recorded on 1st August 1978 at Station 541 in the reserve area over the July extraction in the Belt Headings (Fig. 5.1). Subsidence in Goundry and Church Streets was first recorded immediately after the August extraction in the Belt Headings. There was an initial maximum subsidence of 560 mm in Church Street. The initial subsidence drop of 310 mm in the grassed reserve (Station 541) indicated a rate of 150 mm per month for August 1978. Thereafter the subsidence increased at an average rate of 14 mm per month to a maximum of 670 mm in July, 1980. Plans showing the subsidence contours at 100 mm intervals interpolated from the results for November 197 8 are shown in Figs 5.2 and 5.3, and the contours for November 197 9 are shown in Fig. 5.8. Subsidence contours are not shown over Shortwalls 3 and 4 and over the West Panels since the subsidence there is less than 100 mm. Only two subsidence lines cross the zone of major subsidence and it is difficult to obtain accurate subsidence contours under the circumstances. The contours in Fig. 5.8 are guided by the outline of the extraction and give an indication of the surface areas worst affected. Slope changes and curvatures were calculated for the measured subsidence profile along Church Street. A bay length of one-twentieth of the depth of cover or 10 m was used. The profile was modified to account for the apparently anomalous subsidence at Station 750, and smoothed (Fig. 5.9). The maximum slope change along Church Street was 2.6%. The maximum inverse curvatures were +10 and -12 x lO-'m'-1 and corresponded to radii of curvature of +1.0 and -0.8 km. 5.5 Evidence of Pillar Instability It is evident in Fig. 5.3 that the first pillars from the second Belt Headings extraction were close to the last of the Waratah panel extraction, and separated by pillar remnants which would not have provided the stability required to separate the Page B-54 two extracted panels. The consideration that pillar remnant failure was the major cause of the excessive subsidence is further evidenced by the initial rapid, then delayed subsidence of significant magnitude. (Fig. 5.7). This reflects a gradual deterioration of stooks as the overburden load continued to be transferred with the gradual failure of narrow or split pillars. The progressive increase in subsidence of Station 480, over the Waratah Panel is shown in Fig. 5.4. This indicates the effect of the movement of the pillars after the extraction of pillars in the second Belt Headings panel which took place in August 1978 (Fig. 5.1). The maximum subsidence of 50 mm on 2nd November 197 8 increased to 150 mm in October 1980 as progressive pillar failure occurred north along the Waratah Panel. The maximum subsidence slowly increased to 160 mm on 7th October 1982. It can also be seen from Fig. 5.3 that splitting of some pillars in the Belt Headings would have extended the caving as evidenced by an extension of the subsidence trough to the south. This process of pillar failure gradually increased the caved area, or effective goaf area, with a widening of the extraction and a resulting increase in subsidence. 5.6 Damages to Homes and Services Damages to homes were first reported to the Mine Subsidence Board by residents of Church Street Gateshead on 12th August 1978. some cracking occurred to paths, foundation brickwork and interior walls, especially at internal corners and cornices. Some doors would tend to swing open or closed due to the slope changes. An inpection was made of the affected homes by the author together with an Inspector from the Mines Subsidence Board, on 1st April, 1980, after movement was considered to have finished, and again on 19th August 198 0 when the accompanying photographs were taken. The following details are a compilation of notes made by the author, and information subsequently provided by the Mines Subsidence Board. The plans in Fig. 5.10 locate the damaged homes with respect to the mine workings and the November 1979 subsidence contours. The first plan shows the lot numbers of each dwelling and indicate the directions of the changes in slope of the homes which were out of level as observed in the field by inspectors of the Mine Subsidence Board. The second plan indicates the blocks on which homes needed repairs and includes those where the cost of repairs exceeded $10,000 as shown on Tables 5.1 and 5.2. Page B-55 TABLE 5.1 Damage and Repairs to Weatherboard Homes in Church Street ChurcNatureh St. of DatRepairs**e of Level Cost Claim Change (mm) Lot House or strain damage 26 51 14. 8.78 Strain Paths, int. 1744 27 53 14. 8.78 150 Rel, Br, paths, Int 19833 14. 1.82 Ponding of water 952 28 55 14. 8.78 150 Rel, Br, paths, Int 16008 29 57 23 . 8.78 200 Rel, patio, paths, Int 14350 31 61 24. 7.80 Strain Br, paths, Int 3098 34 67 27. 4.79 150 Rel, Br, chimney, Int 15081 35 69 13. 8.79 100 Rel, Br, paths, Int 13361 36 58 6. 10.78 Strain Minor interior repairs 285 25 32 14. 8.78 Strain Rel, Br, paths, Int 3659 19. 9.80 Strain Cone, path replaced 300 24 34 14. 8.78 Strain Rel. Br, paths, Int 5968 23 36 23. 8.7 8 325 Rel, Br. paths, Int 11562 13. 3 .81 Renewed brick cladding 5600 22 38 14. 8.78 175 Rel, Br, paths, Int 15144 21 40 14. 8.78 100 Rel, Br, paths, Int 12409 20* 42* 28. 8.78 Strain Minor repairs to Br, 1330 Int 291 19 44 24. 7.80 Strain Br 7370 18 46 14. 3.80 35 Rel, Br, paths, Int 11208 15 52 4. 3 .80 175 Rel, Br, patio, Int 21650 14 54 27. 2.80 250 Rel, Br, patio, Int * Brick veneer flats ** Rel - Relevelled; Br - brickwork; Int - interior Page B-56 TABLE 5.2 Damage and Repairs to Weatherboard Homes in Goundry Street Goundry St. Date of Level Nature of Repairs*** Cost Claim Change (mm) Lot House or strain damage 3 43 8. 9.78 20 Rel, Br, paths, Int 3800 4 45 14. 9.78 125 Rel, Br, patio, Int 12097 5 47 30. 8.78 350 Rel, Br, paths, Int 15528 6 49 19. 7.79 125 Rel, Br, paths, Int 18982 8 53 17.11.81 125 Rel, patio, paths, Int 13185 10* 57* 18. 9.78 150 Rel, Br, patio, Int. 17544 11* 59* 7. 8.81 155 Rel, patio, paths, Int 10360 12 61 13. 9.79 90 Rel, Br, paths, Int 13108 13 63 1.12.78 100 Rel, Br, patio, Int 19147 6** 27** 27.11.79 Strain Br, patio, path 4494 * Fibro dwelling ** One house in Sydney Street (corner Goundry) *** Rel-relevelled; Br-brickwork; Int-interior Tables 5.1 and 5.2 list the date each claim for subsidence damage was received by the Mine Subsidence Board, the disturbance to the building and the nature and cost of repairs. This information was provided by the Board. There were 3 8 claims for compensation accepted, made up of damage to 2 9 timber framed and clad houses, one block of flats of brick veneer construction, four damage claims to sewer mains and drains and four claims for damage to paths and driveways. Page B-57 TABLE 5.3 Strain Damage to Sewerage and Drainage Pipes Street Lot House Date of Renewed Cost Claim or Repaired Church — — 2. 3.79 Repaired 283 Church - - 6. 6.79 Repaired 470 Goundry 5 47 27.10.80 Renewed 257 Goundry 6 49 25. 6.81 Renewed 1545 In some cases the change in level along the length of a house is less than the maximum slope change from subsidence observations. The direction of the level change related to house damage in Tables 5.2 and 5.3 may not be in the direction of maximum slope change as indicated by the subsidence contour plan. Also the subsidence contour plan was interpolated between the lines of survey observations and can be used only as a guide to indicate areas most severely affected. Repairs were carried out on a contract basis under the supervision of officers from the Mine Subsidence Board. Total final costs for restoration works was $315,722. The average cost of repair to 29 timber framed dwellings was $10,741. One problem experienced by several property owners was that of altered surface drainage patterns for which little could be done. The in-ground vitreous clay sewer pipes in house drains were reported to have performed well in the subsidence area. Those which were replaced were due to reverse gradients. It is interesting to note the timing of major claims. Those received in August and September 1978, were from Lots 26 to 29 and 25 to 20 Oxford Street and Lots 3, 4 and 5 Goundry Street; all in the zone of major initial subsidence. The remainder of the claims were made at various times, as late as November 1981. Although some residents may not have been as prompt as others in submitting claims, this still indicates the slow increase in subsidence over a long period as discussed earlier in relation to the mining. There were two locations in Goundry Street where wires between the power poles and houses of Lots 4 and 10 tightened oyer the distance of 20 m. The insulators pulled away from their fixtures. Repairs were effected. Water pipes to houses on Lots 27 and 2 8 Church Street failed, and there was one water mains failure in front of Lot 28. There was also a sewer mains compression failure in front or Lot 21 Church Street. These are the first two claims which are Page B-58 listed in Table 5.3. Two later incidents occurred in Goundry Street. Photographs showing the types of dwellings and typical damages are included as Figs 5.11, 5.12 and 5.13. The first photograph of Fig. 5.11 is looking south down Church Street and the second photograph shows the homes on the opposite side of the street, the three shown (lots 23, 22 and 21) having incurred damage in excess of $10,000. The front of house number 36 on Lot 23 was raised by four courses of bricks and two additional steps were added at the front. The view of the house on Lot 21 Church Street (Fig. 5.12) shows that the footing on the right hand side is out of vertical. The patio is sheared at the mortar joint and is out of vertical. The view of the house on Lot 5 Goundry Street (Fig. 5.13) shows that the piers towards the rear of the house are noticeably out of vertical. Repairs were being carried out when the photograph was taken. Maximum slope changes and inverse curvatures along Church Street are shown in Fig. 5.9. The maximum measured slope change was 2.6%, and in a 10 1 length of dwelling would result in a level change of 260 mm in general agreement with observed values in Table 5.1. The maximum peak curvatures drop away sharply to half their peak values, and then decrease more gradually. Maximum values of +10 and -12 x 10-4m-1 would be associated with strains of the order of 15 mm/m, using an extension of the curvature - strain relationship shown by Kapp (1978). This would cause a change of 150 mm in the length of a 10 m long dwelling, resulting in "severe" damage according to the NCB (1975) damage classification included as Fig. 5.14. These maximum strains would occur in a narrow zone. Half the peak value, say 7.5 mm/m, would cover a wider zone and result in "appreciable" damage. > ^•nnUDDDDDDLIb JDDDQO rooDtipaniiDoo D ODD jnaDOHarjannDD"DDDttDIIDn: L naDDaanDDDr aan a ID ODGP \MMJ&lM B i i rH i—11—- t*H ^J ^ mrinnp 5aD£B " £ ^r o — o o o 0 0 0 or- CoM ^ OSfr - n o o o o ee-t7 0 o1— o o o CN to •* Q £> ? 09> - Q < w K w w S9* EH E _ H &H <56f .. oog - o o o o o o o o o o o o o o o o o o CN tO ^ Dt u M in!:< b WQ:C SO NE sw FIG. 5.5 PROFILES ACROSS BELT HEADINGS 1 529 525 520 515 510 505 501 • >., i «—>—i—t J > i » > t i—i—i—> i '»»»'•'' 17-?-7ft ( a) Goundry Street 300 J 536 540 545 550 I • I I 1 1 1 1 1 » L- 17-2-76 ( b ) Reserve 1300 1000 800 600 400 200 • • « . i • ' i • i i i 18-6-70 0- 100 200 c~ 300H (c) Church Street «£ 400 __ w 500-1 600 700- 600- .5.6 SUBSIDENCE PROFILES, GATESHEAD FIG w g a EH H w u w Q H W ra D w < u 2 in H 2 u. -r- —i— o 8 8 8 O§ 1~ CT) O § § o oiui - eouepisqns 00 o in *t Q e E y c / <- <0 _ o ? c ^ CTl o //)/ o W > O 2 « D O EH O u H U W Q H CO ra 0D0 inCO * H fe • £ STATION I *OO BAY LfWCTH o 0 UJ lOO U 1 oo 200 20O z 300 UJ ^ 300 400 - oE 400 DD <^ SOO - SOO D 600 - 600 700 - 700 3.0 3.0 2.5 UJ 2.5 0 2.0 2.0 z 1 .5 1 .5 < 1.0 I 1.0 0 ^ 0.5 0.5 K O.O O **- 0.0 id - 0.5 - 0.5 1 0 l.O - 1 .0 J w 10 10 8 8 u 6 6 Q: D 4 4 h 2 2 ? 0 -*_«____ -6—f—., 0 _ I I 2 2 u o 4 4 u 6 6 CO UJ 8 - 8 > 10 z 10 - 12 1 2 "1—r 1—i—r -i—i—i—r T—i—i—i—i—i—I—i—r i—r -i—r 1—r —I— -i 1 r 1 r —i— 1 —i— 1400 1 200 10O0 800 6O0 400 FIG. 5.9 PROFILES ALONG CHURCH STREET 150 ZOO m Plan showing lot numbers +X/ m Homes where repairs carried out FIGURE 5.10 LOCATIONS OF DAMAGED HOMES Repairs to No. 55 (Lot 28) Nos. 36, 38 and 40 (Lots 23, 22 and 21) FIG. 5.11 HOMES IN CHURCH STREET General view of house Damage to patio FIG. 5.12 40 CHURCH STREET (LOT 21) __K? " ^ .^sl HP *^ ^^ Lr ' H1 Tr w E^_^_^__> "™" IT " _^H r J 1 General view of house Be Damage to path, piers out of alignment FIG. 5.13 47 GOUNDRY STREET (LOT 5) Change of Class of Description of Typical Damage Length of Damage Structure Up to 0.03 m 1. Very slight Hair cracks in plaster Perhaps isolated or slight fracture in the building, not visible negligible on outside. 0.03 m - 0.06 m 2. Slight Several slight fractures showing inside the building. Doors and windows may stick slightly. Repairs to decoration probably necessary. 0.06 m - 0.12 m 3. Appreciable Slight fracture showing on outside of building (or one main fracture). Doors and windows sticking; service pipes may fracture. 0.12 m - 0.18 nm 4. Severe Service pipes disrupted. Open fractures requiring rebonding and allowing weather into the structure. Window and door frames distorted; floors sloping notice ably; walls leaning or bulging noticeably. Some loss of bearing in beams. If com pressive damage, overlapping of roof joints and lifting of brickwork with open horizontal fractures. More than 0.18 m 5. Very severe As above, but worse, and requiring partial or complete rebuilding. Roof and floor beams lose bearing and need shoring up. Windows broken with distortion. Severe slopes on floors. If compressive damage, severe buckling and bulging of the roof and walls. o-oos 1 • \ 0-004 \\ \\ 0003 \ \ Strain - > 0 002 v K- » V• \p \3A C FIG. 5.14 NCB DAMAGE CLASSIFICATION Page B-5 9 STUDY 6 SUBSIDENCE OVER VARIOUS MINE LAYOUTS IN THE VT SEAM 6.1 Introduction The extraction of Shortwalls 1 to 4, described in Study 4, was successful in that it demonstrated the high production of the shortwall method, in association with its adaptability to the panel and pillar layout in order to control subsidence in residential areas. The high productivity was due in part to the fact that the pillars required to remain between the shortwalls for subsidence control allowed caving to occur behind the face supports, with arching between the pillars so that excessive loading did not inhibit chock performance (Martin and Hargraves, 1972) . It was therefore decided to continue the shortwall layout into the northern part of the lease beneath mainly undeveloped bushland but with some residential areas. the following shortwalls, 5 to 8, are shown in Fig. B.10 with the corresponding surface details in Fig. B.8. Extraction of the shortwalls took place from west to east between the dates shown in Table 6.1. Pillars immediately to the north of Shortwall 8, and also to the immediate south of L Panel were extracted as Shortwall 8 was being mined. After the finish of these shortwalls, the pillars at the ends of the shortwall blocks were extracted, leaving two rows of pillars on the west of the proposed Shortwall 9, which was mined from February to July, 1977.TABL E 6.1 V.T. Seam Extraction Study Panel Dates of Mining Start Finish L Panel 7.3.73 14.12.73 Shortwall 5 19.11.74 22. 1.75 Shortwall 6 17. 4.75 5. 6.75 Shortwall 7 1.10.75 19.11.75 Shortwall 8 22. 1.76 3. 5.76 Shortwall 9 10. 2.77 12. 7.77 Macquarie Panel 10. 9.73 10. 5.74 6D F Panel 12. 3.76 11. 3.77 Page B-60 The layouts of the shortwalls were similar to the earlier Shortwalls 1 to 4, Shortwalls 5 to 8 having a rib to rib extracted width of 65 m, and separated by two rows of pillars at 18 m centres. Subsidence was monitored only along streets which were affected by these extractions, as required by the Mine Subsidence Board. The survey lines were only marginally affected by the mining, apart from the monitoring over L Panel which is discussed in Study 6A. Because of the increase in the depth of cover towards the east, Shortwall 9 was made wider than the earlier panels. A study of the earlier subsidence results resulted in the recommendation that the proposed rib to rib extracted width of 7 8 m would not cause excessive subsidence in the overlying residential area west of the Pacific Highway, Charlestown. Shortwall 9 is discussed in Study 6B. In addition to the shortwall panels and associated extractions, there were two larger pillar extraction areas mined in the northern and western parts of the V.T. Seam lease. Macquarie Panel is an the area of pillar extraction adjoining a residential area, discussed in Study 6C. EK Avenue, Charlestown passes over F Panel. Pillar extraction resulted in damage to several homes along EK Avenue as discussed in Study 6D. The limits of mining of F Panel were defined by the faulting shown in Fig. B.9. No other seams have been mined in the areas covered by the four investigations discussed in this Study. 6.2 Study 6A - L Panel The most northerly part of the V.T. Seam workings had been developed for selective pillar extraction, known as L Panel. A subsidence grid was set out along the streets in accordance with the requirements of the Mine Subsidence Board. The subsidence grid is shown on the surface map in Fig. 6.1 and the mine layout is shown in Fig. 6.2. The extracted area of each sub-panel varied from 51 to 54 m wide and 146 m long. The seam height mined was 2.4m and the depth of cover over L Panel rose from 13 0 m at station 70 to 150 m at station 90 and remained at that value to stations 109 and then 120. A single row of pillars was left unmined between each sub panel. Each row was 15 m wide (solid coal). The relevant mine geometries are therefore: extraction w/h = 0.4 to 0.35 W pillar _ = 6.25 m W pillar _ = 0.12 to 0.10 h Page B-61 Even with small panel w/h ratios, the pillars would be of marginal stability. Subsidence profiles in Fig. 6.3 show the subsidence at various stages in the extraction of L Panel. The subsidence due only to the first major sub panel was 15 mm on 4th May, 1973 and the subsidence due to the first two major sub panels was 30 mm on 27th June, 1973. Time- subsidence curves for Stations 85, 98, 108 in Fig. 6.3 show that the maximum subsidence stabilised at 46 mm but gradually increased due to the residual subsidence effects. Extraction of pillars in L Panel took place from 7th March to 14th December 1973. The extraction plan and interpolated subsidence contours at March 1975 are shown in Fig. 6.4. Concurrent with the mining of Shortwall 8, two rows of pillars to the north of the shortwall were extracted (Fig. 6.2). This pillar extraction was separated from the shortwall by two rows of pillars which remained unmined to provide support for the superincumbent strata and for the surface. From earlier experience these pillars were known to be stable. One row of pillars separated the pillar extraction area with L Panel to the north. Some of these pillars were split. The rows of pillars south of L Panel were extracted from December, 1975 to August 1976. In anticipation of this extraction a line of survey stations was established along residential streets as far south as station 262 (Figs 6.1 and 6.2). Levelling first took place on 21st August, 1975. Subsidence profiles around this loop are shown in Fig. 6.5. Subsidence of 54 mm had already taken place at Stations 99 and 109 although this lost subsidence value would have decreased significantly south to the stations between 257-267 over the solid coal (at that stage) . The pillar extraction up to August 1976 resulted in a maximm subsidence of 60 mm due only to the combined effects of the pillar extraction and Shortwall 8 to the south. The extended time-subsidence curve in Fig. 6.5 shows a gradual increase in subsidence till June 1979 most of which can be regarded as residual subsidence, but then a relatively rapid increase in subsidence to 200mm in May 1981. This is likely to be due to a gradual failure of the pillars in L Panel due to the increased loading following the removal of the end support with the extraction and splitting of the pillars to the south. There are some homes which were constructed immediately to the south of Station 262 during the period of residual subsidence. There have not been any reports of damage made to the Mine Subsidence Board. 6.3 Study 6B - Shortwall 9 Following the mining of Shortwall 8 and the associated extraction of pillars to the north, the last area to be mined in this part of the VT Seam lease lay beneath homes in Charlestown Page B-62 shown in Fig. 6.6. Pillars between the finish of the Shortwalls 5 to 8 and the proposed Shortwall 9 block were extracted from August to December 1976, starting with those to the east of L Panel and finishing with those east of Shortwall 5 (Fig. 6.7). Pillars to the north of Shortwall 9 were extracted in early February 1977. Shortwall 9 was mined from 10th February to 12th July 1977. The full width, w, was 78 m and the seam height mined, m, was 2200 mm. the depth of cover h varied from 13 5 mm to 145 m giving a w/h ratio of 0.50 to 0.54. At a localised depression in the land surface between Stations 18 and 27, the cover decreased to 130 m and the w/h ratio increased to 0.60. A grid of subsidence stations was established along streets in the area as shown in Fig. 6.6. the lines of survey stations were first levelled on 29th October and 2nd November 1976. Topographic sections and subsidence profiles across and along the shortwall are given in Figs 6.8 and 6.9. The values of subsidence from the line which runs in a general longitudinal direction above the shortwall were projected onto the straight line section above the shortwall. Some minor irregularities are due to some stations not being over the centre of the panel. In Fig. 6.8 the final subsidence profile after the finish of Shortwall 9 is at 8th June 1977, 7 weeks after the face passed beneath that cross line. The profile at 19th August 1980 shows the effects of residual subsidence, especially over the earlier mined areas to the west. Progressive longitudinal profiles are shown in Fig. 6.9, the final profile is for 20th July, 1977. Residual subsidence is shown in the profile of 5th March 1979 and in the time-subsidence curves for Stations 130 and 109 in Fig. 6.10. Subsidence was related to face advance and depth of cover for Stations 116, 109 and 102 with the results shown in Table 6.2 and Fig. 6.11. The maximum subsidence due to Shortwall 9 was 55 mm at Station 116. The slightly greater subsidence towards Station 100 was principally due to the effects of the limited area of pillar extraction towards the east. The maximum slope change was 0.08 per cent around Station 90 over the goaf edge. Strains were monitored where the upper cross line (Stations 69 to 1) in Fig. 6.6 passed through undeveloped bushland. The average maximum tensile strains were estimated to be 0.3 mm/m around Station 9, and the maximum compressive strains °.4 mm/m around Stations 50 to 58. There were no reports of damage to any of the homes in the area. Page B-63 TABLE 6 .2 Sub sidence Related to Face Advance, Shortwall I ) Face f s So,, Station Date Advance (ftffi) _S_ % f(m) h S SW 8.2.77 -495 -3.69 0 0 102 11.5.77 -112 -0.84 13 7 .3 S =178mm 27.5.77 - 45 -0.34 5 2.8 mPl44m 9.6.77 + 3 + 0.02 5 6 31.5 21.6.77 + 32 + 0.24 87 4 8.9 7 .7.77 + 98 + 0.7 3 156 87 .6 20.7.77 + 150 + 1.12 178 100.0 8.2.77 -390 -2.91 0 0 109 6.4.77 -100 -0.75 10 4.9 S_a=205mm 11 .5.77 - 8 -0.06 27 13.2 mPl44m 27.5.77 + 55 + 0.41 88 42.9 9.6.77 + 107 + 0.80 159 77.6 21 .6.77 + 137 + 1.02 173 84.4 7.7.77 + 205 + 1.53 200 97 .6 20.7.77 + 256 + 1.91 202 98.5 8.7.77 -283 -2.1 0 0 116 21.3 .77 - 80 -0.6 7 4.27 Sm =164mm 6.4.77 + 5 + 0.04 17 10.37 man=l44m 28.4.77 + 50 + 0.4 77 46.95 11.5.77 + 100 + 0.7 94 57 .3 27.5.77 + 165 + 1.2 128 78.0 9.6.77 + 215 + 1.6 148 90.2 21 .6.77 +244 + 1.8 156 95 .1 7.7.77 + 310 +2.3 161 98.2 6.4 Study 6C - Macquarie Panel The Macquarie Panel, located in the western part of the V.T. Seam lease was extracted by conventional pillar extraction methods in two periods as shown in Fig. 6.12. The n orthern and western boundaries of the extraction were defined by a line of 35 degrees to the vertical down from the surrounding residential areas. Pillar extraction was beneath undeveloped bu shland. The proposition that a panel and pillar system be us ed to mine beneath these areas was not pursued. Mining condi tions in the panel were poor and falls occurred at several inter sections in the developmenA line oft survework yprio stationr tos extractiowas establishen of pillarsd alon. g Crescent Road, Charlestown according to the requirements of the Mines Subsidence Board. This line was centred across the panel (Fig. 6.12). Subsidence and strain were both monitored. Pillars were extracted from 10th September, 1973 to 10th May, 1974. The seam height varied around 2340 mm. The surface was gently sloping and the average depth of cover was 175 m. Page B-64 Progressive subsidence profiles for the lateral line 417 to 350 are shown in Fig. 6.13. The early subsidence between Stations 414 and 403 is due to the extraction up to November 1973. The slope change and inverse curvature values were calculated along the profile for 14th January 1975. The corresponding strain profile is shown. the maximum slope change and maximum strains can be related to the maximum subsidence by the relationships G = k_. S G max +E = k, . S 1 max , and -E = k,. s 2 max With h = 175 m and S = 200 mm, and using (from Fig. 6.13) G = 0.18%, +E= 0.3 mm/m, and -E = 0.7 mm/m, the k values were calculated to be kQ = 1.58, k1 = 0.26, and k2 = 0.61 To obtain the value of maximum subsidence over the panel, a line at right angles to the main cross line was established. Subsidence profiles along this line (680-691) are given in Fig. 6.14 together with the time subsidence curves at Stations 385 and 6 85 to show the development of subsidence in relation to the time of extraction. The maximum subsidence of 250 mm occurred at Station 685 in January, 1975, 8 months after the finish of extraction (Fig. 6.14). The slow rate of increase in subsidence after the finish of extraction is due to the yielding and collapse of pillar remnants which remained after the overall poor recovery from the panel. There was some later residual subsidence. The subsidence in Crescent Road did not exceed 45 mm and the average maximum strains there did not exceed 0.4 mm/m compression. Page B-65 6.5 Study 6D - Subsidence Damages over F Panel Pillar extraction took place in F Panel, between two faulted zones, from March 1976 to March, 1977. Accompanying the subsidence which resulted there was some damage to homes, in EK Avenue and Lara Close, Charlestown. The plans locating the area in relation to the geological disturbances and the mine layout are shown as Figs B.8, B.9 and B.10. A line of survey stations for subsidence monitoring was established along EK Avenue over F Panel, shown on the surface plan in Fig. 6.15 and on the V.T. Seam plan in Fig. 6.16. The section line AB connecting Stations 218 and 231 runs longitudinally through F Panel extraction. Pillars were extracted on the western fringes of F panel intermittently from October 1974 to February 1976. However the major F panel extraction took place from 12th March, 1976 with the mining of pillars in the north, to 11th March, 1977 when the most southerly pillars were extracted. The pillars were extracted in panels, with some separating pillars but not according to a predesigned panel and pillar layout. The seam height mined was 2550 mm. The topographic and seam section along the line AB is shown in Fig. 6.17. Subsidence was first monitored on 16th December 1974. A maximum subsidence of 60 mm occurred up to 18th June, 1976 due to the adjacent extraction to the west. The next subsidence profile was obtained on 9th September 1976 and was after the wide panel below Station 216 and the later two panels had been mined. The maximum subsidence at that stage was 190 mm as shown on the profile in Fig. 6.18. At this time, cracking was first reported from two dwellings in Lara Close. Although the subsidence at Lara Close was only 3 0 mm, the surface was in the tensile strain zone and there was the additional effect of the steep natural slope up from the creek. The surface slopes up and away from the widest panel at 1 in 7, with the depth of cover varying from 110 m at Station 219 to 155 mat Station 232 (Fig. 6.17). The two sub-panels which initiated the subsidence were mined from 23rd April to 9th July 1976. They were 50 m and 80 m wide respectively and separated by a single row of pillars 12 m square solid coal giving a pillar width to height ratio of 4.7 and a pillar width to depth of cover ratio of 0.11. The work in Study 4 would indicate that these pillars would be of marginal stability, but will have failed as extraction progressed. The line of subsidence stations passes over the edge of the extraction over the two sub panels and this was not sufficient information to enable the maximum subsidence over the sub-panels to be assessed in relation to the mine geometry. The curves of increase in subsidence with time (Fig. 6.19) were used to assess the timing of repairs to homes damaged by subsidence. The curves show that even several years after Page B-66 mining, residual subsidence is taking place because of the continual deterioration of the small pillars which remained. The maximum subsidence has reached 7 90 mm. It was possible to relate the curvature of the subsidence profile along EK Avenue to the observed damages of the nearby dwellings. The homes which were damaged are shown in Fig. 6.20. The extended subsidence profiles along EK Avenue for 9th September and 14th Otober 1976, the period during which damage was first reported, are also shown in Fig. 6.21 together with the calculated slope change and inverse curvature profiles. Repairs were carried out to damaged homes as from when the primary subsidence was considered to be complete, at 3rd May, 197 8. The subsidence and related profiles are also shown in Fig. 6.21 for that date. Damages to homes and the nature and cost of repairs are listed in Table 6.3. The lot numbers are shown on the plan in Fig. 6.20. The homes were weatherboard with brick footings as high as 2.1 m, and were built in 1969 and 1970. The exceptions were two brick veneer dwellings in lots 1 and 5, EK Avenue, built in 1947 and 195 9 respectively and as such were not subjected to the approval of the Mine Subsidence Board. Damage was first reported on 14th August 1976 from homes on Lots 36 and 40 when the maximum subsidence was 200 mm, as shown on the profiles in Fig. 6.21. The homes were located generally in the area of maximum slope change and maximum curvature, and the natural ground slope being in the same direction would have exaggerated the tensile strain effects. Reports of damage continued as subsidence increased, as shown by relating the reports of damage in Table 6.3 with the locations of homes in Fig. 6.20 and with the progressive increase in subsidence of the relevant stations as shown in Fig. 6.19. Progressive extraction is shown in Fig. 6.16. Page B-67 TABLE 6.3 Damage and Repairs to Weatherboard Homes LOt Date of Cost No. Street* Claim Nature of Repairs*** 138 EK 30. 3.77 Br, Int. 1668 137 EK 31. 5.77 Br, Int., Garage Floor 2632 36 EK 14. 8.76 Br, Renew tiled floor, 730 regrout rock wall. 35 EK 19. 9.76 Adjust door 19 34 EK 19. 9.76 Br, Int., guttering, 3192 paths, patio. 40 LC 14. 8.76 Br, Int., bathroom tiles. 246 39 LC 12. 7.77 Int. 225 26 LC 1. 11.76 Int. 410 24 EK 6. 12.76 Concrete driveway 257 23 EK 28. 12.76 Int. concrete paving 1095 1** EK 18. 1.77 Int., brick veneer skin 2454 32 LC 20. 1.77 Int., concrete path, patio tiled. 33 LC 25. 1.77 Br. rebuilt, Int., patio, steps, floors, garage 5** EK 6.12.76 Footings rebuilt, brick veneer skin, patios, paths, int. * EK - EK Avenue LC - Lara Close ** Brick veneer dwellings 30 and 18 years old. *** Br - brickwork Int - interior linings repaired and repainted. Page B-6 8 Most of the damages listed in Table 6.3 were superficial. Of the 14 claims six were less than $1,000, and six cost between il.000 and $3,2 00 to repair. There were two which cost around $12,000. These were located in the vicinity of Stations 226 and 227 (Fig. 6.20) where in May 197 8 the subsidence was 500 mm, slope change 0.25% and inverse curvature was 0.4 x 10~4m-1 Although it would not be expected that the strains associated with curvatures of this magnitude would result in surface damage, the steep natural ground slope of 15 to 2 0 per cent would have contributed to any possible instability of the surface. The total cost of restoring homes damaged by subsidence movements amounted to $3 9,7 83, more than half of which ($24,012) was for the homes on lots 5 and 33. In addition to the damage to homes, the road surface of EK Avenue sustained some cracking as shown in Fig. 6.20 between stations 221 and 222 where the compressive strain was a maximum (Fig. 6.21). Road resurfacing at that locality cost $460. 6.6 Summary There were no workings in any other seams in the areas of the investigations so that the subsidence phenomena are due only to the V.T. Seam extraction. Damage to dwellings occurred along EK Avenue over F panel where the subsidence was in excess of 500 mm. This is in contrast to the contours over Shortwalls 1 to 4 which were planned on a panel and pillar basis to minimise subsidence. Subsidence is related to mine geometry in Tables 6.4 and 6.5 for the investigations in Study 6. This information is correlated with the information from the other studies in Chapter 4. Studies in 4, 5 and 6 show the importance of strategic planning in order to reduce subsidence and its effects. Without rational planning, excessive subsidence can ocur such as with Studies 5 and 6D. This can have serious effects, not only in property damage, but also in associated human effects. Page B-69 TABLE 6.. 4 Mine Geometries Extracted area Cover W 1 Panel (m) h (m) h h Width Length w 1 * LI 52 146 130 0.40 1.12 L2 54 146 140 0.39 1.04 L3 52 146 150 0.35 0.97 SW9 78 620 145 0.54 4.28 Macquarie 200 296 175 1.14 1 .69 * Ll is first sub panel-only L2 is first two sub-panels L3 represents overall panel average TABLE 6.5 Maximum Subsidence Values Seam Extraction Effective S s Panel height (%) seam height (W ____ (%) m (mm) mE (mm) mE Ll 2400 80 1920 15 0.78 L2 2400 80 1920 30 1.56 L3 2400 80 1920 46 2.40 SW9 2200 100 2200 55 2.50 Macquarie 2340 60 1400 250 17.86 o 3 o o : E o o o o-* -r-r--" 1 r- nnizinizinnii!.' uatntzii.zirzirzii: n • a • • rzi r:: r_: Dnnnnanc nnnnnnnc c 0 (LJLQJLC 0 u DD -+- o) DD c DD DD DD DD DD o o DD o DD DD 1 !' :: • - ; . | . . : . • I • • • N 1:4,000 »cc uo zoo J I I I I I > I . I • I .... I . . I ftl FIG. 6.6 RESIDENTIAL AREA, CHARLESTOWN, OVERLYING SHORTWALL 9 -•• DDDDE DDD DD DDDnDnaD nn [ DDDDDDDC ornczo _!0_B_DD maDaaanczoECE" innnDD rnnnaSrxW]DDD|][ 1:4,000 N ICO wc iOO l till I ..... t .... I .... I .... I I 1 1 1 m FIG. 6.7 SHORTWALL 9, V.T. SEAM V* O — N OOOOOOOOOOOOOOOOOOOOOOOOOOo O ——» » • 1 1 I 1 I • ...... o o o __ no - 100- 90 - 80- 7o- 60- -40r -50 60- Pillars 5V/-9 V End SW-7 «/> -70 -80L ID. 23iOl& .'. >—-I I. 50 •—-— 5«€-77 . .—• . '' 100 FIG. 6.8 TOPOGRAPHY AND SUBSIDENCE ACROSS SHORTWALL 9 i—i—I—r 1*1 0>l • 6CI Bii Lit 9£l SSI fr£l • £€t 261 \l\ 0£l-| LL-l-tZ &z\ cn SZl i Wl £71 EH 810 • O 610 oz a W 130 U uo Z no • O SZO 331 v. H 0 U 121 i 53 OZI W 7,' Q 611 H 811- CQ iy D -II • W 911 LL-+-S SiH Wl EH 211 - III o Oil EH 601 -j ll-S'U cn 801 _ £01 901 6 SOI -| H VQ\ to\ zoi • LL-9-Q ' 101 • 001 . I _ o o o o ? IA ? 2 I o £--_-2l M*'.u!df aae^Jins •sqnS (ui) SJ3A3-| pttr>pv>£ iO o o w u 53 W Q M in PQ D CO W co < H o z > M CO CO w o o fe o rH >£> M fe vO T I 1 - r -T" —i r -1 o in o i^> o o in o i^ o u N in s o in is o (uuuj)sqns (ujaj)sqns Face Advance / Depth of C over -i. 10 -- 20 -- 30 i.0 -- sw (%) SW 50-- 60-- 70 -- 80 -- 90 -- 100 1— -I 1 1 * FIG. 6.11 SUBSIDENCE RELATED TO FACE ADVANCE So.pt. +o isJov 1973 Qdflnnt • •••••[ SG CD CZI-ZiLZ3[ •zr=dr-\\^J^ m 1:4,000 ICO 2C0 IQO ' ....!«...«' ... I . . • . I . ... I . m FIG. 6.12 MACQUARIE PANEL, V.T. SEAM Bay length n r- oo in coin r> mot 01 o o oow oi OOOO n •'''''''I nn i i i • ' • ' • ' ' ' ' ' • ' ' > 1 .5 1 .5 l .0 1 .0 0.5 0.5 0 0.0 <^ft^fN>**- -»fr**S>oa 0.0 0.5 - 0.5 1 .0 . 1 .0 i .5 1 .5 2 1 1 0 y _* 0 1 1 2 2 i—i—i—r—i—i—i—i—n- i i t i i—r-r—i—i i i i i i i i i—i—i—i—i—i—i—i—i—i—i—i—i—i—r- -i—i—i—;—i—r rO -TO IT: o O MOO in O OO) Oi "i 10 FIG. 6.13 SUBSIDENCE PROFILES AND ELEMENTS OF SUBSIDENCE ! Bajy lengfK I8tn 680 681 385 682 683 684 685 686 687 688 689 690 691 Imt i a I t Oi Date: 27-6-73 O—0 15-2-74 ~ iooH _g—-a- O—B 9-5-74 E ^—&15-7-74 ~S' 20CH j O—©14-1-75 . : . ; i- . ' I . '. 30QJ O— '73 '74 *75 y .-iv 1S1O1N1DI J1F1M1A1M1J1 JIAIS 1O1N1DIJ 1F1M1A1M1 Ji J1A1S1O1N1 On E iooH E in 5 20CH 30QJ | 73 74 | 75 | 76 | 77 | 78 | Oi E 100H E 200- 3 PlUUR. 300^ H——H 1 E*r**' FIG. 6.14 PROGRESSIVE INCREASE IN SUBSIDENCE 1••»' • I • 11... t... I •,, FIG. 6.15 SURVEY GRID OVER F PANEL te / / aw^ FIG. 6.16 F PANEL, V.T. SEAM r' i • » ' CQ r '—i—i—r l£Zi CM OiZ- a 3 622 __ izz o 8 9 o LZZ o c o 0 9ZZ M 4> o L c o 0 z C SZZ o 0 H M^ EH v) U O-J W WCO c , CO *33*o Q O Z2Z Z -a '22 < u ozz 3AV HV1V3»M 2 312 fe a. < _IZ « c o E o o fe i/i o SIZ EH H • frIZ H_ fe HZ ZIZ J L I I _i i i i 8 o o 9 ? o 3 N i (_»•) "cy aoiojjng (•") T» •"oas oooooooo oooooooo • 1^92 (z.82) 992 (992) 292 (982) 192 (t.82) 092 (982) 622 (282) % fe fe 822 (182) CO ^22 (082) w H fe O fe fe 922 W u z H Q H CO EQ 022 CO rOH CO• d H fe 9 \,7, 17 fujui) 3nN3Gisans 1976 1977 ,A|M,J,J,A,StO,N,D JiF.M.A.M.J.J.A.S.O.N.D 76 77 78 79 80 81 82 ~N \ ' 6 \ • E \ s^ \ _ 0) \ 0 k - C A Q) \ 500 "- "0 feL ~~ ••H V. f) ^"G^ . -Q ^ ^. 3 ^-^t>_^ 221 - tO - —v> - 1000 L FIG. 6.19 PROGRESSIVE INCREASE IN SUBSIDENCE G3 OamaQed homes IE-1 Costs $1000- t 3000 Costs > $ 10000 •1—1———-I—1—I—I *.il-. • • I 50 »QO |5Q IQO m FIG. 6.20 PLAN SHOWING AFFECTED HOMES o •* m co r^ oooio<-n^in©Ncco)Or(Mn^invO i^ «- «- r- «- ,- ,- cv CM CMCMCMCMCNCMCNnnnnnion CN CM CM CM n '• : •••••• • CM ii i i i 11 i i i i 11 i i . i i ii111111111111iiii'i'•'''''i'1 ||11 * i 11 11111 i n ^ 1/1 io h oo 01 o <- rO"*mior-oooio*-cNn'*mioh ,_ ,. ,_ r- ,- r- CM rv cMCMCMCNfcvcvcMnfonnnronn CM CM CM CM (M CM CM CM CM CMCMCMCMCNCMCMCMCMCNCMCMCMCMCN FIG 6.21 PROFILES ALONG EK AVENUE Page B-70 STUDY 7 SUBSIDENCE IN WHITEBRIDGE AND ALONG BULLS GARDEN ROAD 7.1 Introduction By the time it came to extract coal in the Dudley Seam beneath the township of Whitebridge, the system of extracting panels of coal and leaving supporting pillars between them had been used on earlier occasions. The same general layout was continued in adjoining panels under a main road along which were located several homes. The extraction plan was varied when necessary to accommodate surface improvements. The surface and geological features, and the mine plans of the V.T., Dudley and Borehole Seams are given as Figs B.ll to B.15. Because frequent reference will be made to three of those plans, these are also included in this study as Figs 7.1 (surface topography), 7.2 (early V.T. Seam workings) and 7.3 (current Dudley Seam workings). Throughout the study area, the overlying V.T. Seam had been first worked and there were some extensive areas of pillar extraction. In part of the study area the Borehole Seam, underlying the Dudley Seam, had been first worked, leaving small pillars. Thus in the assessment of aspects of subsidence due to the Dudley Seam extraction, consideration was needed to be given to early workings in both the underlying and overlying seams. The lines of survey stations for subsidence monitoring are shown in Figs 7.1, 7.2 and 7.3. The mining and extraction were not required to be on a planned basis designed to minimise subsidence but each panel was extracted with the approval of the Mines Inspectorate and monitored for subsidence in order to correlate the movements with any possible subsidence effects. The results from each panel were used in assessing extraction in later panels. Approval for extraction by the Inspector of Collieries was based on the subsidence results from the earlier panels. 7.2 Subsidence in Whitebridge over NW and X Panels A proposal of a lift and fender pillar extraction layout was considered for mining beneath the township of Whitebridge. The final layout of the NW and X Panels is shown in Fig. 7.4. The subsidence profiles over NW Panel, along line 50 to 91 are given in Fig. 7.5. The time-subsidence curve of station 83 is also shown. The NW Panels were mined from 3rd December 1973 to 3lst March, 1974. The subsidence after the mining of the first sub-panel was 8 mm. This increased to 8 5 mm with the completion of all sub panels the NW Panel. With the mining of X Panel from 7th November 1974 to 28th April 1975, the maximum subsidence over NW Panel increased to 120 mm. The effect of mining adjacent Y Panel was to further increase the subsidence over NW Panel to 150 mm in December 197 8 as discussed later. After the July 1974 survey, many of the Page B-71 stations were severely affected by roadworks and after January 1976 all of the stations, except 80 and 83 were removed durinq road reconstruction. Distances were measured along Hudson Street from 19th November 1973 to 19th July, 1974 and the calculated strains did not exceed 0.3 mm/m tension or compression. There were no reports of damages to any homes. The maximum subsidence along line 135 to 147, along Dudley Road was 5 5 mm on 15th July 1975, the survey at which it was considered that subsidence due to NW and X Panels was complete. The subsidence contour plan over these panels is given in Fig. 7.6. 7.3 Increase in Subsidence in Whitebridge due to Y Panel Extraction The pillar extraction procedures were continued into Y Panel, adjacent to X Panel. Station Street, Whitebridge, (Line 104-109-116), and the adjacent railway line cut across the eastern corner of Y Panel (Fig. 7.4). The residential area to the east of Station Street included Barbara Street (Stations 122-129-134) and Fenwick Street (Stations 116 to 122), as shown in Figs 7.6, 7.7 and 7.8. The topographic plan over Y Panel is shown in Fig. 7.7 and, to the same scale, the extraction plan in Fig. 7.8 shows the extraction sequence from March to November 1976. The topographic section A-A in Fig. 7.9 is along the line of the greatest natural slope. Along the section line the cover varies from 135 m to 150 m. The depth of cover along Station Street is 150 m (Fig. 7.10) and the seam height mined in Y Panel was 1.7 m. Selected subsidence profiles along Station Street and the increase in subsidene with time are shown in Fig. 7.10. Extraction in Y Panel was completed on 23rd November 1976. The profiles on 12th October 1976 and 20th January 1977 show that subsidence began to increase significantly after extraction in Y Panel, and continued to increase to a maximum of 3 80 mm in January 1981. No later pillar extraction would have affected Station Street. The adjoining Z Panel extraction comes no closer than 250 m from Station Street (Fig. 7.8), and that extraction was completed in August 1978. Subsidence was monitored along each of the streets in Whitebridge. Station Street showed the greatest subsidence. The subsidence contour plan in Fig. 7.11 for 16th November 197 9 indicates a general trend of movement down the natural ground slope in the direction of the Y Panel extraction. The subsidence profile across the steepest part of the subsidence basin (Section B-B) was drawn using the results along line 134-126 as a guide. The slope change and curvature profiles are shown in Fig. 7.12. The maximum slope change was 0.21% and the maximum inverse curvature was 0.4 x 10 _«m"». Page B-72 The mine geometry of Y Panel was similar to that in X Panel with extracted panel widths of 55 to 60 m separated by rows of pillars at 22 m centres (16.5 m solid coal). The last row beneath Station Street, was at 24.5 m centres, the pillar width to height ratio was 9.7, slightly greater than for X Panel. There had been known to be a general zone of instability (creep area) in the Borehole Seam, 50 m below the Dudley Seam due to the small pillars and wide galleries which remained after the early mining in this seam (Fig. 7.3). This creep zone was located more to the north east in the area of NW and X Panels which were extracted without abnormal subsidence. It is possible that this zone has extended to beneath Y Panel. The Dudley Seam extraction will have caused some movement of the V.T. Seam goaf, 30 m above. Pillars were not extracted from the V.T. Seam above the NW and X Panels but were extracted above Y Panel in the Dudley Seam from 1964 to 1966. The Y Panel extraction caused further reconsolidation of the overlying Seam goaf giving rise to additional subsidence, the effects being further exaggerated by the natural ground slope up from Y panel and the V.T. Seam goaf to the Whitebridge residential area. No reports of damage were received by the Mines Subsidence Board, although from experience elsewhere, further subsidence could have resulted in visible surface effects. No pillars were extracted or split in the main headings to the south east of Y and Z Panels when mining was retreating from the area. This was to ensure that the stability of the area was not further jeopardised. 7.4 Subsidence along Bulls Garden Road over Y, z and 0 Panels Subsidence has occurred along Bulls Garden Road over extraction in Y, Z and O Panels, Dudley Seam. The line of subsidence stations is shown on the plan of the Dudley Seam workings in Figs 7.3 and 7.13. in the mid 1960's there was pillar extraction in the V.T. Seam, 30 m above current workings in the Dudley Seam (Fig. 7.2). Pillars were left standing to provide support beneath Bulls Garden Road in a strip 100 to 150 m wide (a minimum 5 chain barrier). The Borehole Seam has been worked below Y Panel and part of Z Panel. The dates of pillar extraction in Y, Z, 0 and P Panels and the date when extraction reached Bulls Garden Road are given in Table 7.1. Page B-7 3 TABLE 7.1 Pillar Extraction, Dudley Seam Dates of Extraction Extraction under Panel Start Finish Bulls Garden Road Y~ 1776 2lTllT76 475776~-~1276776" Z 4. 8.77 22. 9.78 20.5.78 - 21.6.78 0 2.12.77 8.12.78 8.9.78 - 15.9.78 P 7.79 1.80 10.79 The topographic section and subsidence profiles are shown in Fig. 7.14. The road dips in a southerly direction at an average slope of 1 in 20 from Station 1 to Station 30. The increase of subsidence with time in Fig. 7.14 shows that there has been a gradual and significant increase in subsidence after mining was completed in each panel. The plan in Fig. 7.13 shows that in each of Y, Z and 0 Panels there are several sub panels separated by rows of pillars left unmined as a part of the mining procedure. The mining geometries and pillar geometries are summarised in Tables 7.2 and 7.3. The purpose of leaving the pillars in Y Panel was to reduce the subsidence and the subsequent movement of the Station Street, Whitebridge area. The sub panels of w/h ratios of 0.35 should give a subsidence of 60 mm in the Newcastle District. Assuming complete collapse of the pillars separating the sub panels, a maximum subsidence of 1700 x 0.65 x 70%, or 770 mm would be obtained assuming total extraction principles. It is apparent from the subsidence results (Fig. 7.14) that there has been significant yielding of the pillars in Y Panel, possibly encouraged by the creep area in the underlying Borehole Seam. The early extraction in the overlying V.T. Seam would have also contributed to the subsidence. The profiles in Fig. 7.14 show that the subsidence of 240 mm in September 1976, three months after extraction, increased to 400 mm in December, 1978, two and a half years after extraction, a value still significantly less than the maximum possible, assuming total extraction principles, of 770 mm. Page B-74 TABLE 7.2 Panel Geometries Under Bulls Garden Road Ove rly ing Ex tr nT Extrn7 Cover w 1 Panel Stations Width Length h w 1 (m) h h (m) (m) Y 1-2 0 5 5~ 260 156 o7?5 1767' Z 20-27 55 110 152 0.36 0.72 Z 30-35 33 110 146 0.23 0.75 0 44-50 90* 300 125 0.72 2.40 * Mean width includes failed diagonally~split"pillars TABLE 7.3 Pillar Geometries Under Bulls Garden Road Overlying Pillar Mining Cover W W Panel Stations Width Height h W m (m) m h (m) (m) Y 1-20 16* 1.70 156 9.4 0.10 Z 20-27 16* 1.65 152 9.7 0.11 Z 30-35 2 x 16* 1.65 146 19.4 0.22 0-P 50-52 40 1. 80 120 22 .2 0.33 •Rows of pillars at 22 x 34 centres The progress of subsidence over Z, 0 and P Panels can be followed in relation to the extraction. Pillars from the central headings of Z Panel were not extracted, shortening the length of the extracted sub-panels below Stations 20 to 26 and significantly reducing the subsidence to a maximum value of 100 mm in Otober 1978, 4 months after extraction. This area was also away from the influence of the creep area in the Borehole Seam. The sub panel below Stations 30 to 35 was mined to a much narrower dimension with no appreciable subsidence. In 0 Panel, larger pillars beneath Stations 35 to 44 were split to form pillars at 24 m x 18 m centres with no resulting subsidence being detected. The plan in Fig. 7.15 shows the locations of the homes requiring protection. The importance of Page B-75 leaving pillars unmined to the south east of Bulls Garden road in the vicinity of station 40 is made clear in Fig. 7.16 by the sharp drop in topography away from the road. Any extraction beneath the road would have increased the tensile strain effects. Below Stations 44 to 50, away from the last house to be protected, sub panels were extracted and the intervening pillars were split so as to afford no support. The small stooks failed and the resulting subsidence of 168 mm in October 1978 increased to a maximum of 32 0 mm in September 1979. It is possible to relate the mine geometry to the maximum subsidence. The w/h ratio is 0.72 (Table 7.2). The seam height mined was 1750 mm, and with an assumed overall coal recovery of 75%, the effective seam height becomes 1750 x 0.75, or 1300 mm. The Smax/m value is therefore 320/1300, or 0.25; for a w/h value of 0.72. Changes in the slope of the surface and curvatures due to subsidence were calculated for the final profile on 16th April 1981 along Bulls Garden road. These profiles are shown in Fig. 7.17. The spacing of the stations was 20 m. The panels are related to the subsidence profile in Fig. 7.14. The maximum slope change was 0.5 per cent over the side of 0 Panel and the maximum curvatures were +1.2 and -2.0 x l0-*m-1 over 0 Panel, away from any homes. There were reports of minor damages to dwellings (Fig. 7.15) where pillar extraction in Z Panel caused an increase in the subsidence over the adjoining Y Panel. Bulls Garden Road cuts across the corner of P Panel (Fig. 7.3) and experienced a maximum subsidence of 150 mm in February 1980, increasing to 190 mm due to the effects of nearby extraction and residual subsidence. 7.5 Subsidence in Green Valley Road, Charlestown Damage occurred to two homes in Green Valley Road following pillar extraction in Z Panel, Dudley Seam. Subsidence was not measured at that location but a study of the causes of damages to these homes gives more insight into subsidence effects. Green Valley Road is located to the north west of Station 20 in Fig. 7.1. These streets which overly Z Panel are shown on the surface plan in Fig. 7.18. The area is very hilly. The mean depth of cover is 130 m and the average mining height was 1.4 m. The subsidence over Z Panel was due to the combined effect of this extraction and its effects on earlier mining in the overlying V.T. Seam. In addition, the steep surface topography has accentuated the subsidence movements and increased the effects of subsidence on the dwellings. The underlying Borehole Seam was not mined in this area. Pillar extraction commenced in Z Panel on 4th August, 1977. The V.T. Seam workings are 28 m above Z Panel, Dudley Seam and the plans of both seams in Fig. 7.18 can be co-ordinated with the Page B-76 surface plan by the section lino AD mu is shown to 9th Deceit"? 19?7 B- The extra=tion in Z Panel the resents To "thT _£*„f VS?,1."? f"" "P<«^ ^ 9 !977 while extraction was ttlTng place between* t^U" Ts ^ throughs. The Department commenced measuring the tilt Cor fc f dwellings when damage was first reported and f ™ fJ ? ;. ° Per cent^incS" coYJV* ™ ^»"" «Por& tcTe x S f 7*7 ' increasing to 1.5 per cent early in December a<- ^+-K«^ uecemDer locations over wide extract-,nn a-«r T • At other magni^de have b^ of 800 mm although the section in Fig. 7.19 shows that a lesse? value of maximum subsidence could result in significant tUtina ws seriously^ilteTwfsT' ^ £illar SUPP°rt ^e'SSuse Iticl mj n^IY tJlted w?s ^ised by wooden blocks to keep it level. Damage to services to the house were repaired. ,, . ^e SeJt!°n in Fi9' 7'19 and the plans in Fig. 7.18 show that the subsidence was caused by the following factors. (a> P™VideSt !fracti°n of the first four panels in Z Panel is 55 m. This could affect the overlying strata up to about 50 m although caving should not occur above _.i*rifra«Sei.K0f 5 timeS the mined hei9ht- 7 ni. or one quarter of the seam separation. It is likely that the old V.T. Seam workings have been affected by z Panel in the Dudley Seam, and could cause instability in the small pillars to the south west of the fault line on Section AB. (b) There is an area of pillars on either side of a fault line in the V.T. Seam which is approximately 100 m wide. Any movement which would be induced in areas in the V.T. Seam surrounding these pillars resulting in further reconsolidation of the V.T. Seam goaf would cause the overlying strata to deflect down and to hump over this area of pillar support. (c) The steep surface topography (Fig. 7.19) would accentuate subsidence movements and increase the strains further with any movement which is occurring down the slopes. The damaged homes were located at the top of 7.6 Summarthe hily l and would be subjected to tensile strains on both side slopes. Extraction took place in the Dudley Seam from NW and X Panels on a panel and pillar basis with a maximum subsidence of J20 mm occurring in the Whitebridge residential area around Hudson Street. The V.T. Seam, 30 m above, had been mined by tirst workings of a bord and pillar layout. Beneath part of the area, there was known to exist a creep area, or zone of instability due to early first workings in the Borehole Seam 50 m below. Page B-77 The layout was continued into the adjoining Y Panel but the pillar extraction sub-panels caused reconsolidation of the goaf in areas of pillar extraction in the overlying V.T. Seam and, with the effects of steep topography, resulted in new and additional subsidence in parts of Whitebridge which did not cease until 4 years later. The maximum subsidence along Station Street was 3 80 mm. There were no reports of damage to homes in the area but no pillars were extracted from later panels in the vicinity of Whitebridge to ensure that no further actively induced mine subsidence would occur. Pillar extraction took place from Y, Z, 0 and P panels from March, 1976 to January 1980. Each of these panels had some effect on Bulls Garden Road, a main trunk road. There are dwellings for a certain distance from Whitebridge along this road and care was exercised in minimising the possibility of subsidence damage by selectively extracting or splitting pillars in the vicinity of Bulls Garden road. Decisions were made in the light of previous subsidence results by Mines Inspectors. Only minor damage was reported. The maximum slope change along Bulls Garden Road was 0.4%. Relationship between subsidence and mine geometry were difficult to obtain because of (a) the irregular mine geometries below the line of subsidence stations, (b) the influence on subsidence of the old workings in the overlying V.T. Seam, (c) the steep surface topography, and (d) the failure of some of the narrow pillars which were left unmined between the sub-panels. However one sub-panel area in 0 Panel resulted in a w/h ratio of 0.72 and a corresponding subsidence of 320 mm giving a s m*„/m value of 0.25. max M i Scole I'IOOOO ©3 200 300 400 900 FIG. 7.1 SURFACE TOPOGRAPHY y^Mi^~ N 1 Scole MO 000 oo zoo 300 *oo 500 nwtrw FIG. 7.2 EARLY V.T. SEAM WORKINGS , Vaouwc TO plG. 7.3 CURRENT DUDLEY SEAM AND EARLY BOREHOLE SEAM WORKINGS ]•__ 1:4,000 ICO ICC sec 1 . > . . I ..... < .... I ... . 1.1.1 1 I . 1 ± I m FIG. 7.4 EXTRACTION IN NW AND X PANELS RIy 150- D. 16-11-73 14-1-74 3-12-73 21-1-74 15-2-74 13-3-74 to to to to 15-3-74 x- x 2-5-74 17-12-73 5-2-74 4-3-74 31-3-74 £> & 14- 7-75 1974 1975 NIDJJIFI M lAiM/JUiAiSiOiN'OljiF iM iAiM i J i J 1A1S i0 IN i D I 1501- I '74 FIG. 7.5 SUBSIDENCE OVER NW PANEL \N ^y,: T) JI irvvjn —ji iiL ^^'/•VEEV lL. J X rvjrvvK !ix]LV["jr:"E>//>T'M JLVJVV] ' ""VVLJCJLJIV id JLJLlC"2aCZlr j :: Jul.) 1975 ; A. Subsidence- Contours (mm)r .: H' 1:4/000 ' 0 ice WO loo 1 , , , , » > • ^ . I ... • » I • !•>•»• • » ' ' * 7.6 SUBSIDENCE CONTOUR PLAN, NW AND X PANELS ' / <*• % 1:4,000 ICO 100 zoo m FIG. 7.8 EXTRACTION IN Y PANEL o o o 3> 3 V) c 0 M W u o < w E § w 1 o Oi o'"fc OI « >N "" N a o a o o a CM o 00 SdJ(d|^ (uj)-ia o CM n 10 CO 0> O c\J ,vt- Id o < li. _ D 1 OO 2 90 < 80 Id 1/1 70 O CN n 10 DO 0) O rj ^~ •o (0 O O o o o o o o r- *" *- ' I ' I I I I I .D.29.8.74 o 1 oo 2O0 300 400 Ld O Z Id O [0 DQ E D V) FIG. 7.10 TOPOGRAPHY AND SUBSIDENCE ALONG STATION STREET, WHITEBRIDGE 119 1:4,000 ICC ICO 100 . 1 1 i 11 I •***•••• I .»..»•»•• 1 m FIG. 7.11 SUBSIDENCE CONTOUR PLAN, WHITEBRIDGE B B CM 0. ID 10 •* CO o N m <0 n IN N CM (N CM CM o n 0 r— o I.D 23-8-74. 50 - b0 1 OO - 1 oo 1 oo I 50 - 1 50 200 - 200 250 - 250 JQO - JOO O.J -, O. J 0.2 - 0.2 O. i - O.i O.O — O.O O. I - O. i 0.2 -jO.2 0.1 - O. I O.O - O.O 0. I - O. I 0.2 - 0.2 O.J - O.J 0.4 - 0.4 CM o 01 00 N (0 in -» FIG. 7.12 SUBSIDENCE, SLOPE AND CURVATURES IN WHITEBRIDGE / / IO CO 00 0) t— i^ r* i- £7 r^ 00 mi 0) ° f» o r- w , jf; <0 <— — •J CM in «? ^ 01 CO -- Q >- CN r- I- >- O - (oiOl) m u N < i . 1 , 1 , 1 1 LJL i i i i 33N3aisans (LU) KW3S o o o o o 0 i i i i i i t i i | i o oCN o ovf o o o o m *> o m N CN 3QN3aisans (_0 "ia 3ovj_ns < 13. (^ -«s^ ^ 60 "7 OJ /> $Z <& N 1:4,000 ICO 2CC 300 i...»i«*..i»*».'.-.-l-...'....« m FIG. 7.15 LOCATION OF HOMES ALONG BULLS GARDEN ROAD Bulls Garden Road 60T- 40-- 20-- 144m. cover to Dudley Seam Coll.datum -—0.L 500-ft.= 15Om 140 120 - First wor 80 - 35 m of i Dudley Seam 60 -- j . 40 •- 50 m 20 No extraction j | First workings Borehole Seam J Honzoa-ral Scale 1: 4OO0 FIG. 7.16 TOPOGRAPHIC SECTION OVER O PANEL oooooooo ill -tf ") 'N — O - 'N i -* ^ m o O in ^ oomomomom oooooooo o O O >-' o O ill -- - w -l -0 1 -j if o o i T -m _o oooooooooo il 1 "1 N -o--cN"i-tif)in o in o >n n momomomom o o o o ooooooo- r- o o o M r- ,- f.j g 1 10 •* ^ (,-'-U,_0 L.I fuju.i) 33N3oiS0ns f%) 30NVH0 BdOIS (c) Z Panel y Dudley Seam 4X * *\« *' 1:4,000 ICC WO JCO * ' ' ' • I » . . . t 1 ... I .... I .... I ... . I m Flfi. 7.18 EXTRACTION BENEATH GREEN VALLEY ROAD Horizontal scale:- 1:4,000 A B j Homes ^220- y J s H 210 tr s i> ./ \ U200- s Average cover to Dudley Seam 130 m 120T 110 - 100- V.T.Sea 70- Dudley Seam ' 60- FIG. 7.19 TOPOGRAPHIC SECTION OVER Z PANEL Page B-7 8 STUDY 8 USE OF THE PANEL AND PILLAR SYSTEM TO CONTROL SUBSIDENCE IN RESIDENTIAL AND LIGHT INDUSTRIAL AREAS 8.1 Introduction Mining has taken place in the Dudley Seam, Lambton Colliery, for many decades. As mining progressed in a westerly direction, the seam thinned and mining conditions deteriorated so that increasing amounts of roof support were required in first workings and development work. Also this part of the seam was located beneath the residential area of Windale. Mining was by first workings of bord and pillar, without pillar extraction. The surface plan and associated geological and mining plans are shown in Figs B.16 to B.20. The plan of surface topography and the plan showing the Dudley Seam workings are included in this study as Figs 8.1 and 8.2. Experiences at the adjoining Burwood Colliery with the success of the panel and pillar mining system were used to demonstrate to the mine management and to the Mines Inspector that similar layouts could be used under the conditions at Lambton Colliery in order to increase the recovery of coal. In addition to the then current mining under the homes in Windale, first workings of the bord and pillar layout were being mined beneath the light industrial area of Bennetts Green, on the eastern side of the pacific Highway. The panel and pillar layout was also proposed for this area. 8.2 Mining Details The panels discussed in this study are shown in Fig. 8.2. Mining according to the bord and pillar layout in 3 NW Panel was at 25 m centres according to the Coal Mines Regulation Act and resulted in an extraction of 40 per cent. It was considered that the recovery could be improved considerably by extracting pillars on a panel and pillar basis. This was achieved successfully in 3 NW Panel and the procedure was continued on into 4 NW, 5 NW, 6 NW, 6 NW Left and 4 NW Left panels in that order. The panels 4 NW, 6 NW and 6 NW Left were set out specifically for a designed panel and pillar system with the other panels having already been mined by bord and pillar first workings. The mean seam height mined varied from 1.4 m in 3 NW Panel to 2.0 m in 6 NW Left Panel. The depth of cover increased from 135 m in the northern part of 3 NW panel to 170 m in the southern part of the area of investigations. Some very early development and first workings had taken place in the V.T. Seam, located at distance which varied from 25 m to 35 m above the Dudley Seam (Fig. B.19). The amount of Page B-7 9 development work was so limited that the mining will not have been affected by the underlying Dudley Seam extraction. 8.3 Subsidence Over 3, 4 and 5 NW Panels The subsidence grid set out in the Windale - Bennetts Green area was required as a condition of mining by the Chief Inspector of Collieries in order to obtain information on the proposed panel and pillar extraction. Survey stations 110 to 187 (Fig. 8.1) were established in the grassed median strip down the centre of the four lane Pacific Highway. To the west of the Pacific Highway is a grassed reserve containing sports playing fields and a grandstand. To the north of Lake Street is Windale lawn bowling club. The subsidence grid also covered the streets around the grassed reserve and was later extended to include streets over 6 NW Panel and the light industrial area over 4 NW Left Panel to the east of the Highway. The basic grid, along the highway and around the reserve along South and Lake Streets was first levelled on 26th September 1975. No subsidence was observed due to the first workings which later passed beneath some of these lines. In addition to the BHP survey grid, the Colliery survey staff established a line of stations through the reserve across 4 NW Panel and levelled these stations as pillars were extracted. Pillars were extracted in 3 NW Panel from August to November 197 8 as shown in Fig. 8.3. The maximum subsidence along Lake Street due to 3 NW extraction was 50 mm in December 197 8 due to the 3 NW extraction (Fig. 8.4). This value increased to 60 mm in November, 1979 with the extraction in the adjoining 4 NW extraction and slowly increased to a value of 75 mm at the time of the last level run in August 1981. The subsidence around stations 348 to 350 is due to extraction in the adjoining 4 NW Panel, the maximum slope change along Lake Street was 0.05%. The relevant mining details and related subsidence values are summarised in Tables 8.1, 8.2 and 8.3. Pillars were extracted in 4 NW Panel from December 1978 to September 1979, apart from a small area in July 1978. The plan in Fig. 8.5 shows that the panels were permitted to be wider beneath the open reserve, than beneath the Highway. The topographic and seam sections along the Pacific Highway between the Lake Street intersection and the South Street intersection are shown in Fig.8.6. Page B-80 TABLE 8.1 Mine Geometry, NW Panels Dates of Extrn. Extracted area Cover w 1 Panel h Start Finish Width Length (m) h h w 1 (m) (m) 3 NW 8.78 11.78 56 180 160 0.35 1.13 4 NW 12.78 5.79 58 250 165 0.35 1 .52 6.7 9 8.79 34 250 165 0.21 1.52 5 NW 12.79 4.80 34 300 160 0.21 1.88 6 NW 6.80 11.81 82 200 170 0.48 1.18 4 NWL 12.82 8.83 60 250 150 0.40 1.67 TABLE 8.2 Maximum Subsidence, NW Panels s Seam height Extraction Effective roaX Panel m (mm) % seam height (mm7 max (%) m„ (mm) mE 3 NW 1500 70 1050 60 5.71 4 NW 1600 80 1280 35 + 2.73 + 1800 80 1440 30 2.08 5 NW 1760 70 1232 30 2.44 6 NW 1700 80 1360 85 6.25 (20+65) 6 NWL 2000 80 1600 60 3 .75 Page B-81 TABLE 8.3 Geometry of Pillars, NW Panels Depth of Rows Widths Height Width Width Panel cover of of pillars of (m) pillars (m) pillars Height Depth (m) ~3~NW 160 T~ ~Yi IT? 12 To oTII 4 NW 165 1 27 1.6 16.9 0.16 165 2 21+21 1.6 26.3 0.25 165 1 21 1.6 13.1 0.13 5 NW 160 2 21+16 1.8 20.6 0.23 6 NW 170 1 32 1 .7 18.8 0.19 4 NWL 150 2 21+21 2.0 21.0 0.28 A line of subsidence station (Station 2 to Station 7) was established across 4 NW Panel as shown in Fig. 8.7. The maximum subsidence recorded by the colliery surveyors was 35 mm between Stations W 7 and W 13 and occurred from the initial level run in 27th July 1978 to 23rd October 1979. The maximum subsidence remained at that value at the next level check on 18th April, 1980. Subsidence occurred between stations 170 and 180 along the Highway. The maximum was 3 0 mm. The original time - subsidence plots showed the subsidence occurred as a result of the mining of the narrow panels from June to September 197 9. The plan of 5 NW Panel is included as Fig. 8.8. Subsidence profiles along the Highway over 4 NW and 5 NW Panels are shown in Fig. 8.4. The maximum subsidence of 30 mm over both 4 and 5 NW Panels was associated with the maximum slope change along the Highway was 0.03 per cent. 8.4 Subsidence Over 6 NW and 4 NW Left Pillar Extraction The pillar extraction in 6 NW Panel (Fig. 8.9) lay under several streets in Windale. The basic subsidence grid was extended to cover more of the residential area. The first level run along these extensions was on 16th June, 1980. Pillars were extracted from June 1980 to November 1981. Some of the pillars which would normally have remained in a panel and pillar layout were extracted from beneath the reserve area and resulted in additional subsidence along South Street (Fig. 8.10). The maximum slope change along South Street was 0.14 per cent. Subsidence profiles are shown in Fig. 8.10 along Lachlan Street. the profile in June 1981 shows a maximum subsidence of 60 mm at Station 526 over the first complete sub panel mined, shown in Fig. 8.9. The following subsidence profile in August Page B-82 1981 shows the effects of the iaf0r 4-.,~ u particularly the next fun sufpanel over ™hi_V„™"„,bE along Lachlan Street was i « per « ! maXlmU,° Sl°pe change Selected time-subsidence curves in Pin « n OK~. ~U *. subsidence was complete in January if82. Flg-8-11 show that ,o«o ^ pillars extracted from 4 NW Left Panel starting December "/S. a ?anel and PiUar basis are shown in Fig 8.ii The layout dimensions were based on the earlier work in the NW ?an5 ^ ?illar extraction v/as wholly beneath privately owned land and thus not covered by a subsidence gridP A line of subsidence stations was set out down Statham Street in the liqht industrial area of Bennetts Green shown in Fig. 8.12. Details of mine geometries and subsidence are given in Tables 8.1, 8.2 and 8 3 The maximum subsidence of 60 mm was recorded in November 1983, as shown in the subsidence profile in Fig. 8.13. No S^iSSnSS .^SfenCe daM9G haV6 b8en CeCeiVed by the Mines 8.5 Summary The examples discussed in this study are further confirmation of the effectiveness of the panel and pillar layout in reducing subsidence to acceptable levels. The ratio of the width of extraction to the depth of cover varied from 0.21 to 0.48, resulting in maximum subsidence values from 3 0 mm to 85 mm. There was additional residual subsidence. Strains were not measured but there were no reports of damage to the Mines Subsidence Board. yy - g&j|fef FIG. 8.1 SURFACE TOPOGRAPHY, STUDY 8 yyyMmv '-mm i ^MME^ Scale I FIG. 8.2 CURRENT DUDLEY SEAM WORKINGS, STUDY 8 \~yy>- ;'../ . T N -a y**k\ \\* , -v •_» « l^v^ .^^SS^Svy #>, * xy&i\W P • VX ^^-_4^b \W 1:4,000 ICC wc 3C0 <.•.•>>•. . » > . . • i i . • , I • i t i I m FIG. 8.3 PLANS OF 3 NW PANEL, DUDLEY SEAM 5 -o cn - 0 CN 00 IT) CM o l£> — cn CN io -J o (N 0> g 9 c "a 0) Q O 6 © o 'E CO O to £ UJ in z: UJ < _i LD 2: a. UJ < h- 2 III <. m UJ U_ CE icr— =J UJ l/> z > CO UJ UJ o •x:CZ < UJ S _i > I o O I X in o X LL. o o X < o CL LL U I ' ' • ' I o ° ' UJ UJ •T I •» ] r o o o m o UUUJ r-T-r-r- o Ln -uuuu Xn,W m #*•; •*? .<••' • ->r* B^.-r. X r S_B_9W' *^3 _> **_#>.- •_E3# _t *_• »^JrV' \4!KX ** >FtfH ^v^^p^'^'JfeiV^*-1^ ^^^ff^-fl m Jr__l i_^B^" 3S_7__w^ ^^^^^K^^^fggn &/UiG99» ^JrTfrMrniiril^T£fciuV_MHi_ir^ JjfSc I rXTr/flTlTTTTTTTn I" 1:4,000 o ICC wo iCC l....l....l....l..l.l«'lllll'»l m FIG. 8.5 PLANS OF 4 NW PANEL, DUDLEY SEAM o uoipesjajui m IS a>Hn ~T o no u H PL, M C_> < PM U o O in E •J o in o M H U o W o in CN1 w o m o P o < o u o o M in re D PH U _ o in o o H oo U M o I I >- -J uoipesjejuj —I r- —I 07E 0c"£ ooe ooz 081 ozt OSl (UJ; -J-^J aoojjns (cu; -|-^ suiDas *. 1:4,000 0 ICC 100 100 i«»»»it...ii.«.i i • • * . t • » * . t m FIG. 8.7 COLLIERY SURVEY LINE ACROSS A NW PANEL XJQF •' 1 # j ... 1 / _V* •' -#CT^^AOs.X "^_P • ^b_. DQD^ *. 1:4,000 ICO wc ICC I . I • » • • ...ll.Illl.tll m FIG. 8.8 PLANS OF 5 NW PANEL, DUDLEY SEAM Sgfcr ^aom^aamanmmuwso 1:4,000 ICC 2CC ICO m FTfi. 8.9 PLANS OF 6 NW PANEL, DUDLEY SEAM o CN O CO 8-o So co E CN co oo Lf) . «— CM m 00 ao oo LO CO CM o CM n m 00 ^- n CM CN *~ l£> O CM r— r~~ cu *— 0/ -*o— "a (p c] >: c Q cn Q - ©x _j "a d) c] )< a c CD in o CM w z \ < PL) \ Pi s 6 > -J o o w CN hJ M UJ Pn LU UJ o UJ cr Pi _ H- PH r- m w m u x z x X o w I o a in < H tn PQ £3 0C/03 M Pn / 7 o IN o CM in (M — CM f" I III 1 i i |—r "T " o I o o o o o o in o o o in m CN • UiW ' UJ uu J I I L Oi 7/ZJCJCJCJCJCJCL 71 icjnjnjcjcjEJi JOTfl EjeaiDo DDn DO on Bfl _!••_! umwnn DSj&rasswraoQM n_a_iDn _2nnr _JI_]L_[_I_IE__]O_ mmau2_23Dn__3_Ss P i izza cn czi Bzza _a czi cn EHI nn_aEncs ] • m m • • m m • n m •n_^_Ov= ]n_inana_i_iaa OO^^UPUODDOQ ] DIM Ezr0001X1a n era n0 ~lt777771 02 .czroosi-zinnncnDDDtz: uy u ]CZZ)CZ OGrannera~xi mi in nmm/-__ i• -i«Mffi=i rmrrn T i i»r»i» ni r BOTC ~i rrrr-n i 1:4,000 ICC 2CC SCO l....l...fl...*t..l.l.»l«lll m FIG. 8.12 PLANS OF 4 NW LEFT PANEL o O o O 00 o CM LD EH W 3 EH W o W CM LD z EH Ld u < oo EH LT < W H Q_ on CO ,o h> z O < < x o H h- h- U < < 13 i- \- (f) W Q H W m H 00 O H fe o 4- (LUUJ) 30N3C]iScEinS Page B-83 STUDY 9 SUBSIDENCE OVER PILLAR EXTRACTION IN THE VT SEAM OVER A LONGWALL IN THE UNDERLYING BOREHOLE SEAM,' AND ITS EFFECTS ON A SEWERAGE TREATMENT WORKS 9.1 Introduction Coal has been mined by longwall methods in the Southern Coalfield since the mid 1960's. The method was considered for use in the Newcastle District to obtain a greater production of coal than conventional pillar extraction methods especially at increasing depths of cover such as at John Darling Colliery. One area suitable for mining by longwall was in the Borehole Seam located between some early pillar development work, and a faulted and a geologically disturbed area. Although the resulting face length of 67 m was rather short it presented a good opportunity to study the performance of the first longwall panel in the Borehole Seam. It was proposed to develop the longwall block out to sea and then extract the panel back towards the main headings. Extraction would first take place beneath the Pacific Ocean and proceed beneath the shoreline and the Belmont Wastewater Treatment Works recently constructed by the Hunter District Water Board (HDWB). The Victoria Tunnel Seam had already been mined above the proposed Borehole Seam longwall at the site of the Treatment Works. A study was carried out into the possible subsidence effects from the proposed longwall based on earlier subsidence work at Burwood and Lambton Collieries, and following a meeting which included the Chief Inspector of Coal Mines, application was made to that Department to mine the longwall panel. Following the Department's approval, there was correspondence between the Company, the Mine Subsidence Board and the HDWB, with the HDWB being provided with an estimate of the subsidence, associated slope changes and strains. A requirement of the Department was that subsidence surveys be carried out over the longwall. In addition the HDWB carried out surveys around the various structures making up the Treatment Works. Mining of the longwall block was completed with the shoreline and Treatment Works having been lowered with no reports of damage. Subsidence was greater than predicted. The slope changes, which are more important when considering the effects on the Treatment Works, were less than predicted. 9.2 Geology and Mining The longwall panel being considered in this report is shown in Fig. 9.1. It lies beneath the Pacific Ocean, coastal sand dunes and the HDWB Wastewater Treatment works. The longwall was mined in the Borehole Seam, 2.3 m high, at a depth of cover of 280 m. The Victoria Tunnel Seam also 2.3 m high is 70 m above Page B-84 the Borehole Seam at that location and was mined in the early 1970's by a series of shortwall extraction panels with extraction of the intervening pillars. The VT Seam workings in Fig. 9.2 can be correlated with the Borehole Seam plan in Fig. 9.3 by means of the subsidence grid shown on both plans. The nature of the strata is given for the bore N927 in Fig. 9.4. Up to the base of the Charlestown Conglomerate, 80 m above the VT Seam at bore N927, the strata consist of beds of sandstone, shale, claystone, tuff and coal. The conglomerate is favourably located in the upper portion of the strata and is 86 m thick. It is an important part of the stratigraphic sequence and includes beds of sandstone, shale and coal. The small maximum subsidence and flat subsidence troughs which have been observed over subcritical panels in areas of similar stratigraphy indicate that individual beds deflect and bridge within the strata. The plies within the Charlestown Conglomerate assist as the conglomerate deforms in specific horizons. There are 50 m of unconsolidated sandy alluvium at the surface. A significant thickness of alluvium would ensure that if any irregularities occur in the strain profile at rockhead, these would not be transmitted through to the surface. Thus a given subsidence profile would result in smooth profiles of the slopes and strains which are developed at the surface so that any structures on the surface would not be subjected to anomalous peak values of strain. The cross section through bores N827 and N927 (Fig. 9.5) show that the dip of the strata is to the west. Thus for a panel of constant extraction width, the maximum subsidence should decrease from the shoreline inland towards the main headings as the depth of cover increases. Conversely the maximum subsidence and maximum strains at rockhead would increase out to sea as the depth of cover decreases . Mechanised shortwall mining took place in the VT Seam at John Darling Colliery. Subsidence investigations were carried out over the mining of the Shortwalls 8 to 10 and associated pillar extraction. The results of these investigations were provided to the Chief Inspector of Coal Mines to assist in evaluating the application to mine the longwall in the lower Borehole Seam. The Borehole Seam longwall block (Fig. 9.3) was located between early workings to the north east and a geologically disturbed area to the south west. With due consideration for pillar sizes either side of the longwall, the maximum face length was determined to be at 72.5 m centres to give an overall width of extraction of 78 m. Production commenced from the longwall face on 18th June, 1982 and was complete on 19th November, 1982. The length of the longwall block was 1500 m and there were 366,000 tonnes mined. The average production rate was 4500 tonnes/day (4 shift production) and the maximum was 7800 tonnes/day. The record for one shift was 2800 tonnes. ) Page B-85 9.3 Subsidence Due to VT Seam Shortwalls ^.^.Te?,.Short^1^ Panels were mined in the VT Seam under difficult conditions, from March, 1969 to August 19??. Five shortwalls are to the north and five to the south of the main intake and return headings which service the VT Seam undersea workings. As shown in Fig. 9.2 a now disused privately ownld Shortwall s"6 1S l0Cated on the surface just to the south of A line of survey stations was established to monitor the subsidence resulting from the mining of the Shortwalls 8 to 10 and the pillar extraction to the south (2 North East Panel) The Borehole Seam longwall and the HDWB Treatment Works were both later located over the 2 North East extraction. The survey line extends from the railway line to the southern tip of the 2 NE extraction. The initial survey was on 28th June 1972, just after the start of Shortwall 8, and so the full effects of Shortwalls 8, 9 and 10 and the later pillar extraction will have been monitored. Sufficient information was obtained to enable an accurate assessment to be made of the subsidence effects of the shortwall and pillar extraction areas. The VT Seam workings in the vicinity of the line of subsidence stations is shown in Fig. 9.6. The depth of cover is 210 m and the seam height mined was 2300 mm. The width of the extracted area is variable and is in a direction at right angles to the line of subsidence stations and along the length of the shortwall blocks. The minimum width of 600 m results in a supercritical extraction w/h value of 2.9. The last level survey along the subsidence line was in February - March 1975. The plots of subsidence, calculated slope change values and inverse curvatures are shown in Fig. 9.7. The initial level run was on 28th June 1972, sometime after the series of longwalls commenced. Although the true maximum subsidence value was monitored, the true slopes and curvatures were not obtained along the northern part of the profile because of the subsidence which had already occurred to the north of Shortwall 8 before the initial levels were taken in June 1972. However the observed radii of curvature for both the tensile and compressive zones were 40 km. The time-subsidence relationships at Stations 109 and 145 are shown in Fig. 9.8. Most of the stations along the line were later removed (trail bikes, etc) and were not replaced. The only stations remaining in 1982 were 108 and 109, near to the site of the HDWB Treatment Works. The final subsidence at the stations due to the VT Seam workings would be 550 mm. Station 109 was one of only two remaining at the time the grid to monitor the Borehole Seam longwall was established. Levelling of this station was resumed. Fig. 9.8 shows the time-subsidence relationship extended to include the effects of the Borehole Seam longwall, discussed later. The subsidence profile in Fig. 9.7 does not include the effects of Shortwalls 6 and 7 which would extend to Station 83, bottowith msom eof tailinthe g subsidencoff to Statioe troughn 100, .ther Aet woulStatiod nb e 80a atsligh thte Page B-86 additional subsidence due to Shortwalls 6 and 7, and also due to residual subsidence effects after February 1975. The first extraction to affect Station 109 was Shortwall 9 which commenced in October 1972. Shortwall 10 was completed in August 1973. The pillar extraction commenced in September 1973, below Station 150, and progressed towards Shortwall 10. Station 109 was first influenced in mid 1974. Pillar extraction was completed with the extraction of pillars in 3 South East Crosscuts to the west of Shortwall 10, in September 1975. 9.4 Failure of Pillars and Pillar Remnants The subsidence profile is smooth, with no irregularities. This indicates that no pillar remnants have remained in place. The likelihood of failure is shown by examining the pillar geometry, and by comparing calculated subsidence, assuming pillar failure, with measured subsidence. The rows of pillars between Shortwalls 9 and 10 are each 9 m and 18 m wide and extend for the lengths of the shortwall blocks. The subsidence line crosses these pillars around Station 100. The geometric relationships for the depth of cover of 210 m and pillar height of 2300 mm, are shown in Table 9.1. TABLE 9.1 Geometric Relationships for Two Pillar Sizes Width/Cover Width/Height 9 m pillar 0.04 3.9 18 m pillar 0.08 7.8 A comparison was made with values for these geometric ratios in other areas where subsidence work has been carried out. The 9 m pillar has crushed out and the 18 m pillar will not have remained stable. If the pillar had remained stable, the slight undulation in the subsidence profile at Station 100 is more likely to have been a pronounced hump. The maximum calculated subsidence is given by S = 0.5 x 0.6 x 2300 max = 690 mm Page B-87 where (a) the recovery factor of 0.5 is considered conservative, (b) the subsidence factor is assumed to be 0.6, and (c) the seam height mined is 2300 mm The maximum observed subsidence was 600 mm in February 197 5 at Station 80. The time-subsidence curves in Fig. 9.7 show that some additional residual subsidence will have occurred, and there will have been some slight additional subsidence at that location before the initial survey in June 1972 due to Shortwalls 6 and 7. Thus the observed and calculated maximum subsidence values are in reasonable agreement, indicating that no pillar remnants remain standing. 9.5 Subsidence Over the Borehole Seam Longwall Lines of survey stations were established over the proposed longwall extraction in the Borehole Seam to enable the subsidence, slope change, and developed strains to be determined. The grid layout is shown in Fig. 9.9. Where distances were measured for strain calculation, a bay length of 15 m was adopted, based on the National Coal Board (UK) recommendation of 0.05 of the depth of cover. Where levels only were monitored along particular lines this distance was increased to 30 or 60 m. The grid was established and the initial observations were made on 13th May 1982, before the start of extraction beneath the ocean well before any effects from the longwall will have been evident on the coast. In addition to the grid of the BHP Survey Department, the Hunter District Water Board (HDWB) established stations around the various structures which make up the treatment works. These are shown in Fig. 9.10. Levels were taken on a weekly basis while subsidence was occurring. The changes in slope of the structure were calculated from the level observations. The slope changes were considered by the HDWB as being more important than the actual subsidence to the continued safe operation of the treatment works. To summarise the mine geometry of the longwall block, the overall width of extraction w was 7 8 m, seam height mined was 2.3 m, and the depth of cover h of 2 80 m near the site of the treatment works included 50 m of unconsolidated sandy alluvium. The w/h ratio was 0.28. The weekly face advance is shown in Pig. 9.9. The final subsidence profiles for 14th December 1982 over the coastal lines. Stations 20 to 48 and 52 to 69 are shown in Pig. 9.11. There will be some continuing residual subsidence. The profiles of slope change and inverse curvature were calculated from the subsidence profile. The corresponding Page B-88 profile of measured surface strain is also shown. The final profiles along the inland lines, Stations 134 to 116 and 112 to 100 are shown in Fig. 9.12. Both these sets of profiles cross the extraction area at right angles, in the direction in which the maximum slope, inverse curvature and strain values would be recorded. The maximum values are listed in Table 9.2. TABLE 9.2 Maximum Values of Subsidence Features on 14th December, 1982. Unit Coastal Line Inland Line 20-48, 52-69 134-116, 112-100 Subsidence mm 43 108 Slope Change % 0.03 0.06 Inv. Curv + lO-'m"1 0.02 0.05 Inv. Curv - lO-'m-1 0.07 0.07 Radius of curv. + km 500 200 Radius of curv. - km 143 143 Tension mm/m 0 0 Compression mm/m 0.45 0.74 The maximum subsidence around the HDWB treatment works tailed off to 104 mm on 26th January, 1983. This maximum subsidence occurred at Station 10, shown on Fig. 9.10. The maximum calculated slope change between any two stations around the treatment works was 0.03%. The HDWB has made no damage claims from the Mines Subsidence Board. The subsidence profiles can be compared with profiles from other studies by constructing non dimensional profiles. Features of the subsidence profiles are listed in Table 9.3 and shown plotted in Fig. 9.13. The profiles are discussed in relation to profiles from other studies in Chapter 4. Page B-89 TABLE 9.3 Features of Subsidence Profiles over Longwall 1 Coast Line Coast Line Inland Line Inland Line 20 - 41 52 - 69 134 - 117 112 - 100 Feature 14.12.82 14.12.82 14.12.82 14.12.82 of Profile d d d d (%of) _ (Smax> d The additional maximum subsidence is 65 mm, and although it X % f he fc tal subsid L-f- ? ,n« £ ? ence of 108 mm, it represents an additional 10% of the maximum subsidence of 600 mm due to the earlier VT Seam extraction. The subsidence contour plan in Fig. 9.14 is drawn from the results of the BHP level survey on 14th December, 1982 and the survey around the HDWB Treatment Works on 9th December, 1982. The longwall face had advanced 900 m before it reached the coastal subsidence survey line and it was assumed that the maximum subsidence of 43 mm was an indication of the maximum subsidence which would occur without the influence of the old VT Seam workings. It was also assumed that the maximum subsidence of 108 mm along the inland line gave an indication of the maximum subsidence in the area where the VT Seam had already been mined. 9.7 Rate of Subsidence Development Stations on the surface are affected by an advancing line of extraction before the face arrives beneath the station. The first stations to be affected by the advancing longwall face were those over the longwall on the coastal line (around Station 41). Stations around the Treatment Works (Fig. 9.10) which lay directly over the longwall were affected in the following order: 2, l, 9, 10, 13. Thereafter, Stations 116 and 111 on the main BHP grid were finally affected as the longwall face approached. The plan in Fig. 9.9 shows weekly face positions. The average rate of advance of the face was 100 m per week. Time-subsidence relationships for selected stations 41, 2, 9 and 116 are shown in Fig. 9.15. Subsidence observations were generally made on a monthly basis on the BHP grid and on a weekly basis by the HDWB around the structures of the treatment works. The dates at which the face passed beneath the various stations are indicated on Fig. 9.14. The maximum rate of subsidence shown for Station 10 is 6 mm per day. The subsidence of any station at a particular time can be related to the maximum subsidence and the position of the face with respect to the station. This results in a dimensionless relationship between subsidence and face advance and is used as a comparison with other locations both in the Newcastle area and elsewhere. 41,2,9,116 and the ratios station With rfeapeuc tu tue uuiiawe fun cover. These values are shown plott< Page B-91 TABLE 9.4 Subsidence Related to Face Advance. Face Face Advance srTT sTT, (mW _™ Station Date Advance Cover* STTT (m) 41 23T 7.82 -7 80 "177 9 ~3~ 0.07 SLW=43mm 17. 8.82 -540 -1.93 0 0. OK) 24. 9.82 -50 -0.18 1 0.02 20.10.82 222 0.79 34 0.79 26.11.82 640 2.29 43 1.00 2 7.10.82 -85 -0.30 1 0.01 SLW=9 3mm 14.10.82 -20 -0.07 8 0.09 22.10.82 44 -0.16 15 0.16 29.10.82 130 0.46 32 0.34 4.11.82 225 0.80 58 0.62 12.11.82 300 1 .07 74 0.80 19.11.82 370 1 .32 79 0.85 26.11.82 440 1 .57 81 0.87 26. 1.83 660 2.35 93 1 .00 9 7.10.82 -130 -0.46 1 0.01 SLW=102mm 14.10.82 -67 -0.24 4 0.04 22.10.82 -2 -0.01 14 0.14 29.10.82 87 0.31 27 0.26 4.11.82 180 0.64 58 0.57 12.11.82 257 0.92 77 0.75 19.11.82 330 1 .18 84 0.82 26.11.82 400 1.43 87 0.85 26. 1.83 630 2.30 102 1 .00 0.00 116 17. 8.82 -840 -3 .00 0 SLW=105mm 24. 9.82 -350 -1 .25 0 0.00 20.10.82 -80 -0.29 3 0 . 03 26.11.82 338 1.40 98 0 . 93 14.12.82 500 1.70 105 1 .00 * Depth of cover - 2 80 m Station 41 responded slightly sooner than Stations *•'*** 116 which are located over the fully goaved area of the VT Seam. It thus appears that although the VT Seam goaf resulted in significantly more subsidence over the Borehole Seam longwall, the presence of the VT Seam goaf slightly retarded the progress of subsidence through the strata to the surface. It can be noted from Fig. 9.16 that Page B-92 l. subsidence commenced when the face was at a distance of half the depth of cover before the surface point, 2. when the face was directly below the surface point, 15% of the maximum subsidence had occurred, 3. 50% of maximum subsidence occurred when the face had advanced to a distance equal to half the depth of cover from the surface point, and 4. subsidence was effectively complete when the face had advanced to a distance of between 1.5 and 2.5 of the depth of cover beyond the surface point. Residual subsidence is still occurring at stations around HDWB Treatment Works as shown by the time-subsidence curves in Fig. 9.15. Since the residual subsidence is considerably less at the BHP stations away from the treatment works, this could be an indication of the settlement of the structures in the unconsolidated sandy alluvium. 9.8 Summary Shortwall mining and pillar extraction took place in the VT Seam, 2.3 m high, at a depth of cover of 210 m. Extraction took place over a supercritical area with an estimated coal recovery of 50%, and resulted in a maximum observed subsidence of 600 mm. Mining was complete in September 1975. The longwall in the Borehole Seam passed beneath the old VT Seam workings and the recently constructed HDWB Wastewater Treatment Works, in October, 1982. The Borehole Seam is 70 m below the VT Seam at that location and the mining height was 2.3 m. The longwall resulted in an overall width of extracted coal of 78 m to give a width to depth of cover ratio of 0.28. The resulting maximum subsidence was 108 mm, due mainly to the reconsolidation of the old VT Seam workings. Residual subsidence is continuing. Associated with this maximum subsidence was the maximum slope change of 0.06%, zero tension, and a maximum compression of 0.74 mm/m. At the site of the treatment works, the maximum subsidence was 96 mm on 9th December, 1982, the date used for the HDWB Stations in the contour plan of Fig. 9.14. Since then residual subsidence has occurred and in March, 1984 the maximum subsidence increased to 115 mm. The maximum slope change was 0.03%. Strains were not monitored around the structures of the treatment works. The effect of the old VT Seam workings was to spread the influence of subsidence over a greater area, thus reducing the slope changes, radii of curvature, and strains. Along the coast of the Pacific Ocean, where the influence of the old VT Seam workings was minimal, the maximum subsidence was 43 mm, maximum slope change 0.03%, zero tension and a maximum Page B-93 compression of 0.4 5 mm/m. longwan L^ wL^i^rbe^ that *•» the the maximum subsidence of the station ttT °n the surface> 15% of could be considered olci°,.uOCCUrred' Subsidence a position around ?wice the depth o_ JJJ*6 fac\had advanced to which took about 6 weeks V6r past the station, r \ *^ • '•--• ~\a> 1I ••" V Maf <(M~| II 1 Hi'*!*, r./.0/^>sS? IT--2-* \ -I i -IILHOKT MM J o Belmon\ Bay ill irHWf ^t_i » JV* »•/ /y Bav 1:25,000 ICCO 2CC »III J_i i i _-i- • 1 m FIG. 9.1 PLAN SHOWING BOREHOLE SEAM LONGWALL 1:10,000 0 5oa IOCO L _i _J L. j - _J m FIG. 9.2 VICTORIA TUNNEL SEAM WORKINGS 0 1000 L _JL_ J I FIG. 9.3 BOREHOLE SEAM WORKINGS 20- Sand 40- 60- 60- 100- ^r 120- o O •> uo- Al J3 160- Q. o 180- 200- i^XX^ 220- V.T. Seam 240- fcTr-WTTm 2 60- 280-J Borehole Seam FIG. 9.4 STRATA SECTION, BORE N927 FIG. 9.6 V.T. SEAM SHORTWALLS AND SUBSIDENCE LINE If FIG. 9.7 PROFILES OVER VT SEAM SHORTWALLS o o o o o o o o o~ o o o o o CI Nr fl 1I fl"T X) o o o Oi fl -J OO o •* cn 0) cn (0 o o o o o o _ o o o o o o 01 O 1—I—i—r- CM ,*) -rf fl 0 oo cn CM > T3 (0 cn O co 01 o> oo ^ cn (0 m n cn CM CN J l_ _l_ o o o o o o o o o o o o o o o o o o o o o o o o o o ~ N fl -t in io - CM fl * fl (0 (UJUJ) (liJUi) t3u»p»«qn5 t^ 1:4,000 i . i . . i . . . . i « . i i i i.ii 1 IIII i ri_„ 9.9 LCMCWALL ADVANCE BENEATH TREATMENT WORKS 1:2000 0 so too I i i—i—• * • • * i t FIG. 9.10 WASTEWATER TREATMENT WORKS m i/3 o in c n o o o o o —< o O r-l n m o o O O o 11 i im i i i 11 oi o o I I ' I ' I I I I IJ ' I I I I I I I I , i i < i I w H < O u o o < u w u z w p 0) H so c rt u a o o CN i i i i I i • i i I ii M 11 111 11 i i i i i i i 11 i . i • i i i 11 i i i o o o mom o m o in o m o in m o -HO o —i o o o oo o o o o •dlUOQ (uiui) 01X) •su&x (%) (T_UV (U1/UIUI) aouapTsqns UTBJ^S o m m o • • • . I 11 , , 7*1 i i iV i i i rf i 11 I""j Q, o HJ o S3 H C/1 u . M CM CN H 00 O O^ wH r-l Pi cn W Pi H o CJ w m S o u Srii u Qw w 10 CN CN o CN H fc cn • ••'••''' i 11 111 i i 1111 11 1111, i i o o o -o (%) (l-^V-OTX) • SU9X " dUIOQ aouapxsqns a2uBtp adoj_g ajnrjBAjriQ 3SI9AU] (ttl/UIUl) UTIBJ^S X a E £CO o o LD O oo CM • TDl-C O II XI \ 3* r UIU I o 1 i UII U O O O O m in rH O rH rH II II II II X X X X rd rd rd rd a> 00e OJe W e 00£ SH "w V Q •» N CD QJ D cu CD G C EH C •H -H 00 •cH •H rH rH H H in T3 ra +J +J W CO rcd rcd o rd rd rH H 0 0 c c U U H H o a + o -ol-c IT) O O X ^ O a £ 0s l^ FIG. 9.14 SUBSIDENCE CONTOUI? PEAN 1982 1983 Sep. Oct. Nov. Dec. Jan. 5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30 5 OJ uc 40 OJ •a J3 60 3 Date face beneath Station 41 ioo: -i *—r- -i 1 r- -i 1—i—i—i Sep, Oct, Nov. Dec. Jan. FIG. 9.15 PROGRESSIVE INCREASE IN SUBSIDENCE OVER LONGWALL Face Advance / Depth of Cover JLW SLW 7' = 'O 100 FIG. 9.16 SUBSIDENCE RELATED TO FACE POSITION