Sur 'aces:a 0j i:y in areas unc er ain syoc coa wor

MORE THAN 70000 old mine workings have areas created as a result of the Industrial (a) Partial extraction —leaving pillars of been charted in Britain, but at least an- Revolution (Fig, 4). These areas are again coal of some form to support the mine other 30000 unrecorded workings exist as expanding rapidly, and simultaneously un- roof, and legislation making it compulsory to keep dergoing active central redevelopment, (b) Total extraction —the forerunner of plans of old workings and shafts was not which may involve the construction of modern longwall in which the introduced until 1872. These workings are buildings founded over strata partially re- coal was completely removed —with largely the product of coal mining opera- moved by mining. or without the use of art ficial roof tions in the 17th, 18th and 19th Centuries, It seems probable that as these workings supports such as pit props or packs when coal pillars were left to support the age they m'ight become more susceptible which remained after coal extraction. overlying strata. It is important to recog- to collapse, due, for example, to the slow nise that old workings due to the mining but steady erosion of material in the work- Geometry of old workings of silver, lead, shale and salt also exist, ings by ground water. Operations on the There w'as little systematic mining until and at the outset of the Industrial Revolu- ground surface such as piling can also the 15th Century, when it became clear that tion iron minerals and refractory clays were affect the stability of the workings. The coal pillars remaining would prevent sub- extensively mined. The following text, construction of new structures over mined sidence. By the end of the 17th Century however, concentrates on the major prob- areas therefore demands that caution however, regional variations in partial ex- lems related to coal mining. should be observed, and more than usual traction started to emerge depending on At shallow depths coal pillars may care in site appraisal and ground investi- such geological factors as seam thickness, continue to support the overlying strata for gation (Price et al, 1969). and nature of roof and floor strata. Since a considerable time, perhaps several hun- At the initial planning stage reference the geometry of old workings has a con- dred years, before gradual deterioration should be made to the Institute of Geo- siderable influence on surface stability, leads to pillar collapse and uneven subsid- logical Sciences for information concerning some of the more common methods are ence at the surface (Fig. 1). On the other the geology of the district within which it illustrated in Figs. 6a-e, and local variations hand roof strata between pillars may coll- is proposed to erect any structure. Refer- in terminology are shown in Table I. apse leading to the upward migration of ence should also be made to the National In Shropshire the longwall method of cavities. Where cavities reach the surface Coal Board whose Regional Offices often mining had by now also become well "plump" holes may be formed, often quite possess considerable and detailed informa- established (Fig. 7). All the coal was ex- suddenly (Fig. 2). tion about former coal workings. tracted, support being effected by timber Access and air shafts associated with props and stone packs. In Scotland the former mineral workings have often been Old coal workings "short" longwall system was used exten- partially and loosely infilled and covered Development of coal mining methods sively until early this century and typically, such that their exact location is extremely Where coal mining in its most primitive panels of 12 metre clear width between difficult to establish. In some cases the form advanced from the Simple removal of roadways were extracted, with full caving mine shafts were completely infilled and/or outcrop material with perhaps limited ex- being allowed behind the working area at sealed at the surface of the bedrock by cavation of surface soil and rock it became the advancing face. Sidegates or roadways means of a reinforced concrete slab. In dependent on four parameters: supported by waste packs and timbers many cases, however, a timber platform was (i) Access 'by shaft and/or adit, provided access for men and materials, erected at the surface of the bedrock and (ii) Roof support, Local experience suggests that the heterogeneous fill materials deposited be- (iii) Drainage, and widths of these roadways lie in the range tween the platform and ground surface. (iv) Ventilation 1-2m, the majority being of sufficient width With the passage of time the timbers de- all of which were governed to some extent for one tub. In addition, it was common cay, and sudden failure can be experienced by the dip of the seam and the topography practice to construct crossroads at 140 to in the form of a local and rapid draw-down of the surface. 200m centres. These roads, inclined at an of surface materials. In this respect it is Where the coal outcropped at surface, angle to the direction of advance, inter- noteworthy that an estimated 80000 old access would be primarily by adit. Com- sected several gate roads, and today sur- shafts remain in Britain, many of which are monly flooding, gas, or poor ventilation face collapses may occur initially at such uncharted. would prevent further working allied with intersections. The disturbance experienced at ground inexpert roof support technique. It is noteworthy that for the same min- surface by a structure due to collapse of The advent of the bell pit method of ing method the percentage of coal ex- shallow workings or old mine shafts is working (Fig. 5) occurred around the 13th tracted can vary considerably; thus for often related to the nature and condition of Century. Generally unsupported, shafts (1.0 room-and-pillar workings, 2-2jtm pillars at the superficial deposits. Loose water-bear- —1.2m dia.) were sunk to depths of up to 4.5-5m centres are observed in parts of ing essentially non-cohesive deposits such 12m as often as required to extract coal, England, whereas 1.2-2m pillars at 6-9m as sands and silts may be expected to and then widened at seam level (8 —20m centres may be found in West Scotland. accentuate the ground disturbance caused diameter) until roof collapse appeared im- Surface stability of old workings by shallow mineral workings (Fig. 3), in minent. With deeper shafts, permanent Old coal workings are prone to collapse direct contrast to the relatively better support had to be introduced but in gen- and, as the term implies, the surface ground support conditions provided by a eral the size of workings was limited. Even strains and strain rates produced are high, stiff cchesive deposit such as glacial till. in the 16th Century, 8 or 9m bell pits were creating in turn severe effects on surface Serious structural failures are possible, considered fairly deep although by this structures. involving danger to life, if buildings are time shafts of up to 30m were being suc Collapse may occur sporadically over a founded above hidden unstable shafts or cessfully constructed. period of time on a site, and at any time collapsing room-and-pillar areas. However, It was not until the invention of the after mining is abandoned. little inform'ation has been published on the steam-driven pump by Newcomen in 1712 The main types of failure concern roof, subject and yet the coal seams in question that greater depths could be drained. pillar and floor collapse. Roof collapse with are extensive, often underlying major urban Winding, haulage and ventilation methods associated void migration is illustrated in were consequently improved and at the Fig. 8. Bulking of the spalled material may close of the 18th Century shafts of 250m choke the cavity (Fig. 9), or beam action were being installed. The actual pattern of of strong overlying strata may terminate 'Technical Director, Colcrete Ltd., Strood, Roches- two forms: the upward migration (Fig. 10). No reliance ter, Kent. coal extraction took 22 Ground Engineering Fig. 4. Correlation between the development of the major conurbations and coalfield areas mined during and after the Industrial Revolution (after Willis, 1976)

Fig. 1. Council house, Tranent, East Lothian, constructed in 1938. A sudden collapse of workings in 1960 caused severe structural damage (courtesy, J. W. H. Ross ff> Co)

',tJ SWA

Fig. 5. The mining of a bell pit (after Stewart, 1973)

TABLE I. LOCAL TERMINOLOGY FOR —9 I'(IF'0'f P,. PARTIAL EXTRACTION I'»'ETHODS —,jg >,' I 1 i il ~ Term applied Area

Fig. 2. "Plump" hole produced by void migration from a 3m seam at 30m depth. The Stoop-and-room Scotland surface hole is 4m x 8m x 5m deep Bord-and-pillar Newcastle area —this method shows nar- I II t L~s»t— ~~ I III rowing of the head- ings at functions to 4Ct Y»s Aorta~ ta> prevent pillars crushing Post-and-stall Wales ~ I

>try> Pillar-and-stall Room-and-pillar Used generally Post-and-bank

Square work Used in the Staffordshire Ten Yard Seam where spontaneous combus- 't ~ I 1 tion of the coal was a tf, hazard Fig. 3. Crater caused by collapse in 1956 of two adjacent shafts, which had been previously backfilled, at Northwood, Stoke-on-Trent (after Dean, 1967) (a fte r Stewe r t, 1973) March, 1979 23 should be placed upon either action, If quent performance of a surface structure. Bord Headway important surface structures are envisaged. Rules of thumb used by some engineers Where void migration cannot be terminated to assess the likelihood of surface subsi- a surface breach or "plump" hole dence require close examination. The "safe >/lP//Ã//gg///// naturally, is the end result (Fig. 2). depth" approach where it is assumed that Where pillars collapse or punch into the no damage will result if old workings exist underlying seatearths the effects are less some 15m or so beneath rock head is dramatic, but the resulting irregular subsid- erroneous. Without considering the geo- ence at the surface can still cause severe metry of the workings, this rule logically tstsFltsrt tstsv/p//~ 8 damage (Fig. 1), implies that cavities of almost any ex- The collapse of old workings is not nec- tent can exist beneath the "safe" depth essarily dependent upon surface load being without giving rise to ground movements applied by foundation construction, al- at the surface. though such engineering activity may have Another dubious approach relates to the a marked detrimental effect on the condi- "100 year old" rule where it is assumed p,vf(q p v ppvzy~~~ tions leading to instability, Fig. 11 shows that if no subsidence has occurred within a coal pillar which has suffered gradual 100 years, then no subsidence will occur spalling due possibly to eccentric loading thereafter. These rules may be in error, and / or chemical activity. the consequence could be very serious. Uncollapsed or partially collapsed work- ings, if dated before 1872, are not likely to Shafts be recorded. Even where records exist Geometry of mine shafts there is no guarantee of accuracy in the Knowledge of the shape, plan dimen- ~~

ll M~ I r y Fig. 6b. Post-and-stall workings —South Wales, 17th Century (after Stewart, 1973)

I

Fig, 6c. Stoop-and-room workings— 1 Scotland, 17th Century (after Stewart, 1973) The photograph seen on the right shows Ih I an exposure of an 800mm thick seam at OCEAN Cow glen, Glasgow (courtesy J W. H. Ross & Co) Fig. 6d (left j. Staffordshire square work, developed for condi- Pillars Fire rib tions involving spontaneous combustion. Air-tight stoppings could l placed in the narrow to prevent of fire '.„.',di>!'4XXXX be access ways spread ~'g (after Stewart, 1973) Fig. 6e (below j. Room-and-pillar workingin seams folded into a vertical plane —Scotland, Lancashire, Staffordshire and m Somerset (after Stewart, 1973)

QQC7/='",=-i 'nJOC3CI~ Qatar="~:-" >DGOOI: )OO~Z~ )~OCll: ir head IOOMI:I JQO~( 0 ~OR~ I I WMMQQ~) ii i OMH~~[ Horse Gait Way

March, 1979 25 knowing the function of the shaft, it may be possible to infer its proximity to other shafts or the workings. With the development of the pillar-and- stall method in the 15th and 16th Centur- ies the small collieries were worked by one shaft, with either no ventilation shaft or with a connection to another colliery to allow a flow of air. During the 17th and 18th Centuries the use of two or three shafts became common practice, the spac- ing varying from less than 3m apart to being on opposite sides of the working. As the depth of coal workings increased there was a reversion to the use of a single shaft in the 19th Century. This ten- dency combined with frequent explosions led to an Act in 1852 enforcing two shafts per mine. In 1863 an Act stipulated a mini- Fig. 7. The longwall method introduced in Shropshire in the late 17th Century mum separation distance of 3m of natural (after Stewart, 1973) material between shafts and this distance was increased to 13.6m by an Act in 1887. In practice, the separation of shafts con- structed in the 19th Century, before and :Massive Sandstone . after the legislation, varied considerably '.' (Silkstone rock) (Nelson-Boyd, 1879) . Mine shaft linings I Since the 17th Century, mine shafts have l Void 1 been lined, the type of lining depending Mudstone the nature of the ground and the upon Shale= locally available materials. Wooden linings consisted of a frame of wooden planks Void arranged like a barrel. The timber, usually oak, was expected to be sound for about Seam Seam 15 years. An expensive alternative for cir- cular shafts was a wooden block lining. 0 10 20ft. For the rectangular shafts vertical plank- lal (b) ing formed the main lining held in place Fig. 8. The effect of discontinuities in the overlying strata on void migration: (a) Joint-determined breakdown arrested by change in lithology above workings in Silkstone seam, Sheffield; (b j beam-determined failure (after Pnce er ah 1969)

action from (I jigI Fig. 10, Example of beam strong overlying strata

Fig. 9. Example of a choked cavity due to bulking of spalled material (after Wardell & Wood 1965)

't,i )lg

~ ~ dL ~ Fig. 11. Example of irregular geometry of coal pillars, dating from 1715 (courtesy. J. W H. Ross & Co) 26 Ground Engineering 1 100

1 000

900

800

700

0 600 E K 500 ct 400

300 0 1600 1700 1800 1900 Year

Fig, 12. Diameters of coal mine shaft —generalised (after DoE, 1976)

1600 1700 1800 1900 2000 Year Fig. 13. Maximum depths of coal mine shafts —generalised (after DoE, 1976)

Fig. 14. View of potential shaft locations 14a 8 b, 16, 16a & 18 (after DoE, 1976)

by regularly spaced horizontal wooden was restricted to near the surface, but wood replaced by an open grid of iron frames. shortly afterwards this method was re- joists. In spite of these recommendations, In the 19th Century it was normal for placed by segments, which bolted together experience indicates that in many shafts shafts of about 3.5m diameter or less to within the shaft and formed the lining larger objects such as mine cars or trees have bricks moulded to the curvature of (Taylor, 1858). Metal linings have been in were thrown in to form an obstruction the shaft (Boulton, 1907), The normal con- common use through the 19th and 20th upon which to start filling, and sometimes struction method was to excavate for the Centuries. large objects have accidently bridged the shaft sections and to build up the brick- The introduction of concrete with its shaft creating an obstruction for the fill work from a timber or iron kerb. In aban- non-corroding qualities was an important (Dean, 1967). doned shafts, timber kerbs would certain- innovation, although it was not until the It is noteworthy that until recently (All- ly rot leaving the brickwork unsupported. 1890's that concrete linings were used in sop, 1970), there has been no standard Various construction types have been mines. practice of closing off galleries and drifts. used for brick linings: Mine shaft fill Thus fill could migrate from the shaft bot- (i) In strong rocks a skin of single bricks Old shafts will be encountered in three tom. Also no evidence exists to show that laid as stretchers. conditions of abandonment i.e. completely fill was deliberately compacted when be- (ii) In loose ground, stronger linings were filled, partially filled or open. ing placed. Any tendency to consolidate required, typically 350 —500mm thick, Many shafts have only been partly filled would be due to self weight, leading to in one bond of, say, three rows of in, the fill having been placed on a staging surface depressions (Fig. 14). stretchers to one row of headers. (usually wood) within the shaft. Although Fill materials can be described only in (iii) Where the brickwork was being used the level varies in practice, the staging is general terms but commonly took the form to exclude water, linings of up to often found just below ground level or of colliery and building waste, the former 0.9m thick were used, laid completely in the vicinity of a rockhead. On occasions sometimes containing up to 25% unburnt in stretcher courses. Each ring of the the upper shaft lining was removed so coal. This fill would also include debris lining was separated by a layer of that the staging could rest upon a secure such as blocks of sandstone, iron, pit mortar or clay, or, from the middle rim. wood and ropes. Following the introduc- of the 19th Century, concrete. In 1871 the Mines Inspectorate recom- tion of the Newcomen engine, large quan- Simple metal linings were first used in mended that substantial wooden logs of tities of ash would also be available. In the early 18th Century for tubbing, cast sufficient length to extend for a consider- some cases, the shaft lining above a plat- iron tubbing being employed in the latter able distance on each side be fixed toge- form within the shaft would be with- part of that century (Galloway, 1969). The ther and laid across the shaft. Altemn- drawn, and the superficial deposits allow- use of tubbing in the form of cylinders tively, later 19th Century shafts had the ed to cave in. Where the shaft structure March, 1979 29 has collapsed the fill may consist of the collapse may occur in the supporting shaft (b) Instrumentation capable of locating shaft lining (brick, stone, wood or iron) lining or rock (Fig. 17), because of grad- and defining underground cavities e.g. followed by blocks of the various strata ual deterioration combined with no lateral borehole radar, should be developed, through which the shaft was originally restraint within the shaft, and/or an im- because present techniques which sunk. posed loading from a new surface may involve vertical drilling over the Capping structure. suspected workings can be dangerous. Shafts that were not filled have tradition- Clearly, the possibility exists that dis- (c) Rational procedures should be univer- ally been secured by fencing or capping, In used shafts were not effectively filled upon sally agreed for the planning and ana- practice many methods of capping have being abandoned. Even if their position and lysis of site investigations and grout been noted including the use of abandoned construction technique were faithfully re- consolidation schemes in order to im- haystack boilers from Newcomen engines, corded, the process of deterioration of prove current practice and raise safety and brick cupolas, the shaft fill, lining and staging can sel- standards. Until the nationalisation of the coal in- dom be quantified, At the present time (d) Case histories concerning collapses or dustry there were few legal requirements no predictive capacity exists in relation localised subsidence are required from concerning the protection of shafts (DoE to the time, magnitude or extent of surface engineers and local authorities, in or- 1976). A form of capping recommended by subsidence due to shaft co l la pse. der to provide basic data for the as- the NCB (Dean, 1967) is shown in Fig. 15, sessment of the risk and degree of Surface stability of old shafts Future damage to surface property, given key Experience indicates that loss of support With the exception of a few experienced ground and mining conditions. It from a large area of the surface may oc- mining work in- consultants who tend to should be mandatory that such rec- cur suddenly when abandoned shafts col- dependently, little care and attention has ords be kept in a central register. lapse. Where rockhead is near the surface been directed towards the problems of old It is unlikely that anyone will ever suc- and the collapse arises from disintegration workings to date by the engineering pro- ceed in of the staging supporting the fill, then the fession at large, perhaps for the simple bringing the problem of subsi- dence into a of ex- subsidence will be severe but localised reason that the problem is complex in that system capable being (Fig. 16), In the case of a breakdown of a thorough understanding of the mechanics pressed by exact formulae. Nevertheless the shaft lining, or where a thick deposit of subsidence demands some knowledge guide recommendations based on actual of unconsolidated material overlies the of mining methods, rock mechanics, soil case records combined with closer liaison shaft, a large crater may be formed as the mechanics and foundation engineering. between mining, geotechnical and struc- deposits flow into the shaft (Fig. 3), To advance our knowledge and practice tural engineers could improve current prac- Where a shaft has been considered too in the field four outstanding problems tice considerably. deep for economic filling, a capping of the should be tackled immediately: type shown in Fig. 15 may be the only se- (a) The validity of rules of thumb, cur- References cure measure. In these circumstances if rently used some by engineers to Allsop, P. l. (1970): "Shaft incident at Ackton the superficial deposits are insufficiently assess surface stability, must be Hall Colliery''. The Mining Engineer 119 Pt. 10, 569, investigated, there is still a possibility that verified. Boulton, W. S. (1907): "Practical Coal Mining" Gresham Publishing Co. Dean, J. N, (1967): "Old mine shafts and their hazards" The Mining Engineer 124 (March), 368- 12in R.C. slab 376. 6in Finish 36in Mass concrete 1-2-4 mix 1-1'/o-3 mix Department of the Environment (1976): ''Reclama- tion of Derelict Land —Procedure for locating Ground level abandoned mine shafts" Report by Ove Arup Partners (Contract DGR482/13) Planning, Regional and 1 o o Mineials Directorate, DoE., London. P o "A oo * Galloway, R. L. (1969): I 12in Sand cushion over in Great Britain". (Reprint) David & Charles. top of piles Nelson-Boyd, R. (1879): "Coal mine inspection: its history and results". W. H. Allen & Co., London. o—Selected fill material. stone 5in to 0 well compacted in Price, D. G., Malkin, A. B, & Knill, J L, (1969): "Foundations of multi-storey blocks on the cioal 12in layers with sand in 4in layers measures with special reference to old mine workings'' Quarterly '= II Journal of Engineering Geo- I= —.-=—.———— logy 1, 271 o - slab 1-1'/o-3 mix with o 48in R.C. Stewart, K. (1973): "Stability of old coal work- o 1'/oin i bars at 6in crs. laid in top ings''. B.Sc(Honours) thesis, Department of En- I o,~ U and bottom gineering. University of Aberdeen. Rockhead 'o ~(f<:0O Rockhead < g Taylor, T. J., (1858): "The Archaeology of the Excavated down to rockhead and Coal Trade". Republlshed 1971 by Frank Graham, 0 t7 0= Newcastle-upon-Tyne. Sheet piling forming 28ft-6in sealed immediately with 6in layer square round shaft. 'i of concrete Wardell, K. & Wood, J C. (1965): ''Ground in- stability problems Down to rockhead — arising from the presence of old, 10in 6in dia. shaft 4in Shuttering timbers over top shallow mine workings". Proc of Midland S.M. & previously filled of shaft walling F E. Soc 7. Willis, A. J. (1976): "Surface stability in areas Fig. 15. Details of rafting underlain by old coal workings". M.Sc Thesis, used to cover an old shaft at St. Helens in 1965 (after Dean, 1967 Department of Engineering, University of Aberdeen.

Capping capable of spanning shaft Superficial deposits

A A o..o ~Y

Rock mass e~

Shaft

Fig. 17. The potential mechanisms of failure of the supporting ground beneath a shaft capping. Wedge failure of A-B produced by either (ij loss Fig. 16. Kirkless No. 2 Shaft, Wigan. The collapse of support to weak rock mass due to collapse of infilling, (ii j failure of occurred during site investigation work The top of the rock mass by load from capping. Such collapses are even more likely rig Can be Seen abOVe the COllaPSe (after Dean, 1967) where the capping is founded on superficial deposits (after willis, 1976) 30 Ground Engineering