Chapter 3: Review of work concerning the study area

3. REVIEW OF WORK ON GEOLOGY AND LAND INSTABILITY CONCERNING THE STUDY AREA

3.1 INTRODUCTION Documentation and investigation of landslides in the area has been dated back to 7 January 1879 ( Mercury, see Appendix 2), and 27 May 1889 (Shellshear 1890). The area was, no doubt, experiencing natural land instability for many thousands of years prior to 1879. However, it is only since the late 1800’s that geological and other investigation has been undertaken consequent to colonial settlement of the area and the development of roads and rail communication with . The aim of this chapter is to present a background of the study area including its geomorphology, climate, geology and recent work on land instability.

3.2 GEOMORPHOLOGY AND CLIMATE The district comprises four Terrain Patterns following the Pattern-Unit-Component- Evaluation (PUCE) terrain analysis system (Finlayson, 1984). These four terrain patterns are; the highland Hawkesbury plateau areas to the west of the escarpment, the near vertical sandstone cliff lines and their lower slopes which together comprise the escarpment, the coastal plain, and the generally north flowing drainage channels of the in the northern section of the study area. The highland plateau to the west of the escarpment is the eastern margin of the Woronora Plateau (Herbert and Helby, 1980). The plateau area typically comprises gently undulating slopes, locally underlain primarily by the Hawkesbury Sandstone, variably incised (sometimes deeply) by drainage channels. The elevation of the plateau areas and hence the top of the escarpment ranges from approximately 50m near Garie Beach in the north to around 450m near the top of in the south. The escarpment generally comprises an upper vertical cliff line up to 50m in height, below which very steep slopes develop into terraced and faceted slopes which end on the coastal plain which is bounded by the ocean. The lower slopes of the escarpment have been variably incised by drainage lines such that they now comprise a series of spurs and valleys trending near perpendicular to the escarpment. The slopes of the escarpment below the cliff lines are almost completely covered by temperate climate slope debris deposits of colluvium of variable depth often reaching 10m deep.

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Exposures of bedrock are very limited due to the colluvium and forest cover. The geological processes which characterise the downslope movement of these slope debris deposits, or colluvium, are many and may be broadly referred to as land instability. These processes associated with erosion and transportation of debris which in turn lead to the development of the colluvial slopes probably date back to the early Quaternary period (1.65 million years ago), and possibly even to the Tertiary period (starting 66.5 million years ago). The coastal plain extends south from Coledale and Austinmer including Wollongong and to Kiama. The coastal plain is widest at Macquarie Pass, where it is 16km across. Here, the flow of the Macquarie River has exploited relative weaknesses within the geological sequence and accelerated scarp retreat has occurred. Wollongong experiences a cool temperate climate with an annual average rainfall of approximately 1200mm at the level of the coastal plain. The orographic effect of the escarpment on rainfall is quite pronounced, as shown in Figure 3.1. The annual rainfall is closer to 1600mm on the higher ground immediately to the west of the escarpment south of Bulli, and approximately 1500mm on the intermediate to upper escarpment slopes (Young 1976 - rainfall 1890-1974, Ghobadi 1994). Rainfall represents an important natural hazard affecting the Wollongong City Council (WCC) area. Whilst the average annual rainfall is approximately 1200mm, there are many intense rainstorms. In one instance, at Wongawilli, a rainfall of 803mm was recorded over 48 hours to 9.00am on 18th February 1984 (Shepherd and Colquhoun, 1985) These extreme rainfall events often trigger landslides and other types of slope instability particularly if antecedent rainfall is significant (see section 3.11). Mount Kiera Scout Camp in 1950 recorded an annual rainfall of approximately 3000mm. Young’s 1976 map of maximum annual rainfall, Figure 3.1 (a) shows a peak of 3500mm around Helensburgh. With such high levels of rainfall the flooding and associated scouring of local watercourses is another hazard affecting the WCC area. Scouring and slumping of adjacent land along the creek banks is a common problem. In most cases the instability of banks is only localised. However, long-term effects can be significant. Instances of instability of this type have been included in the land instability mapping phase of this

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Figure 3.1. Rainfall contours for the Illawarra; (a) Maximum annual rainfall, period unknown (Young 1976), (b) Annual average rainfall 1931-1960 (Young 1976), (c) Average annual rainfall Bureau of Meteorology records (Ghobadi 1994)

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project. The natural vegetation on the escarpment comprises Eucalypt forest and rainforest.

3.3 A BRIEF HISTORY OF GEOLOGICAL KNOWLEDGE IN THE ILLAWARRA The geology of the , and in particular the northern Illawarra, has been researched and documented in considerable detail. Volumes have been written regarding the tectonic setting, structural geology and stratigraphy of the area. Coal was first reported in the Illawarra by survivors of the grounding of the boat named the Sydney Cove, on the south coast of in 1797. The survivors made a fire with loose coal found on the beach at Clifton. James Dwight Dana, of the U.S. Exploring Expedition from 1838-1842, and the Reverend W. B. Clarke conducted geological mapping and compiled field descriptions of the rock formations between the Hunter River Valley in the north and the in the south (Viola and Margolis 1985). In the first recorded geological map of the Illawarra (Figure 3.2), Dana distinguishes three major sedimentary units which he and Clarke called the Sydney Sandstone Formation, the Coal Formation, and the Wollongong Sandstone Formation. The rock formations were correlated from the Illawarra, through Sydney and north to the Hunter River Valley district. From careful examination of the fossils in the lower sandstone and coal formations, Dana was able to show that these two lower formations were of age, with the upper Sydney Sandstone also of Permian age, or slightly younger. Whilst he noted the area was not a region of active volcanoes, he did record the presence of basalt layers between sandstone beds south of Wollongong, and noted the heat alteration effects on the underlying sandstone and its irregular surface. From this he deduced the basalts were flows, not intrusions. This work was a remarkable pioneering achievement, given that the expedition was in New South Wales for only three months during the period late November 1839 to February 1840, and Clarke stayed only eight months longer. The first coal mine in the Illawarra, at , opened in 1849. Thus began a long association between geologists and the Illawarra area. Following several progress reports, Harper (1915) wrote the report “Geology and Mineral Resources of the

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Figure 3.2. Dana’s Geological Map of the District of the Illawarra dated 1848 (Viola and Margolis, 1985) Yellow represents the ‘Sydney Sandstone Formation’, Purple the ‘Coal Formation’, Red the ‘Wollongong Sandstone Formation’ and brown is ‘Basalt’.

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Southern Coalfield” in which he described the structure and stratigraphy of the area, including details of some of the coal seams. Hanlon (1938, 1952 and 1956) discussed the geology of the southern coalfield and reviewed the stratigraphic nomenclature. Hanlon identified and described many of the individual geological units and his nomenclature is still used today. Numerous other workers have contributed since the comprehensive works of Hanlon. Bowman (1972 a and b, and 1974), presented several detailed discussions on stratigraphy, stratigraphic nomenclature, structural geology, petrology and geological maps at various scales, 1:6336, 1:25000 and 1:50000. Bowmans work remains the definitive Wollongong geology. Some problems of scale and accuracy associated with his mapping work are discussed in section 3.9. After Bowman, more workers have added to the geological knowledge of the area (Adamson 1974, Herbert and Helby 1980, Chestnut 1981, Sherwin and Holmes 1986, to name only a few).

3.4 REGIONAL AND STRUCTURAL SETTING The study area is situated within the geological feature known as the Sydney Basin. The Sydney Basin is the southern part of the larger Sydney-Gunnedah-Bowen Basin, a major geologically defined, structurally controlled continental margin sedimentary basin, as shown in Figure 3.3. The larger basin extends north from Bateman’s Bay, to central coastal Queensland. The sedimentary sequences included within the Sydney Basin range in age from to . In the southern areas of the Sydney Basin, including the study area, Carboniferous age sediments are absent. Here the Permian to Triassic age sedimentary sequences lie directly over the subsided Palaeozoic basement which is comprised of Lachlan Fold Belt sequences. Cessation of deposition within the basin probably occurred in the Late , as remnants of Early Jurassic deposits have been recorded where they collapsed into Jurassic volcanic breccia pipes, known as diatremes (Herbert and Helby 1980). In New South Wales, the Sydney-Bowen Basin is bounded to the south and west by the Lachlan Fold Belt, to the northeast by the New England Fold Belt (Herbert and Helby 1980). The east and southeastern extent was terminated at the outer edge of the then Gondwanaland continental shelf (after Veevers et al., 1991). The structural definition of the Sydney Basin evolved as deposition proceeded, with the final definitive movements taking place in the Late Triassic. Veevers et al (1991) have shown that the

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existing east coast of developed approximately 84 million years ago with the onset of sea floor spreading and the consequent opening of the Tasman Sea in the Late

Figure 3.3. Regional Structural Geology of the study area (Herbert and Helby, 1980). (a) Extent of Permian sedimentation over eastern Australia; (b) Sydney-Gunnedah-Bowen Basin within New South Wales; (c) Structural subdivisions within the Sydney Basin.

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Cretaceous. The uplift and erosion of the Sydney Basin sequence is thought to have commenced in relation to this Late event (Ghobadi 1994). Extensive warping occurred in Tertiary time (Herbert and Helby 1980). The sedimentary sequence was deposited in a relatively stable tectonic basinal environment, hence the sequence is largely conformable and remains essentially horizontal to the present day. However, clear field and regional evidence exists of some synsedimentary structuring. In the field, locally, such evidence includes micro faulting with variable sequence thickness on either side of the fault. Mapping at various scales, including work undertaken by coal mining companies shows some large scale faulting, with occasional throws up to 60m or so, which are laterally continuous up to several kilometres or more, with diminishing throws indicated vertically by field mapping. In addition regional mapping (ACIRL 1989) clearly demonstrates the presence of minor folding. This structuring is the result of tectonic movement both during and after deposition (Herbert and Helby 1980, Branagan et al 1988).

3.5 LOCAL GEOLOGY The study area lies on the south eastern margin of the Sydney Basin, as shown in Figure 3.3. The geological bedrock sequence of the Illawarra district is essentially flat lying with a low angle dip, generally less than five degrees, towards the northwest. This gentle northwesterly dip, whilst superimposed by relatively minor syn-depositional and post-depositional structuring (folding and faulting) is a result of the relative position of the district on the southeastern flanks of the Sydney Basin. Normal faulting within the Illawarra area is common, although the fault throws infrequently exceed 10 metres. Only seven laterally extensive faults, mapped during coal extraction from the Bulli and Wongawilli coal seams, have throws in excess of 20m, namely the Metropolitan Fault (65m), Clifton fault (66m), Scarborough Fault (60m), North Bulli Fault (60m), Bulli Fault (to 90m), Corrimal Fault (28.5m max) and the Wongawilli Fault (to 50m), (ACIRL 1989). The geological units encountered within the district, in ascending order, include the Shoalhaven Group, the (both of which include the intrusive/extrusive bodies collectively known as the Gerringong Volcanic facies), the Narrabeen Group and the Hawkesbury Sandstone. Brief stratigraphic descriptions of each of the formations within these groups, encountered within the study area, extending

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from the Budgong Sandstone up to Hawkesbury Sandstone, is presented in section 3.6. The Illawarra Coal Measures contain numerous economically significant coal seams. Of these, the most notable are, in descending stratigraphic order, the Bulli Seam, the Balgownie Seam, the Wongawilli Seam and the Tongarra Seam. These coal seams have an important influence on the local groundwater pressures and groundwater flows, and often include thin very weak tuffaceous claystone bands. The presence and location of these coal seams which may act as aquifers has been considered significant in several cases of land instability. In most parts of the district, extending from the base of the upper cliff line to either the waters edge or the coastal plain, the ground surface is covered by alluvial and colluvial slope debris deposits (see section 3.6.6 and 3.7). Bedrock exposures are limited by this cover and the thick vegetation. Therefore, geological mapping in the area represents a significant challenge. Furthermore, as the bedrock does not usually outcrop, all of the geological maps of this area are, technically subcrop geology maps. That is, they are drawn as if the geological sequence extends to the surface. In reality, the mapped geology will be found a certain distance ‘down dip’ of the mapped location. This distance ‘down dip’ is dependant on the thickness of the colluvial sequence at each location.

3.6 STRATIGRAPHY OF THE ILLAWARRA REGION In this section brief stratigraphic descriptions of the geological groups and, in particular, the specific formations which underlie the subject area are presented. For detailed geological discussions, interested readers should consult Bowmen (1972 and 1974) and Herbert and Helby (1980). The stratigraphic nomenclature employed herein follows that described by Bowman (1970, 1972, and 1974) and synthesised (for the Narrabeen Group) by Herbert (1970), and presented by the Standing Committee on Coalfield Geology of New South Wales (1971) in a report of combined subcommittees for Southern and Southwestern Coalfields (for the Illawarra Coal Measures). Each geological unit has a ‘type area’ or even a ‘type location’. These are areas or locations (which may be borehole intercepts, but are typically outcrop locations) where the geological unit or formation is ‘typically’ or ‘well’ developed. Geological units are, of course, spatially quite variable in terms of composition, form, rock type and thickness. However, the type areas or locations are where the geological units descriptions are

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determined and their locations are very useful for field reference. Hence, where possible, the type location for each unit described below has been included, and referenced as appropriate. The descriptions presented here are, unless stated otherwise, as presented by Bowman (1972, after numerous workers), although abbreviated considerably. They are presented here for completeness and to provide a framework for some later sections of this thesis which discuss the geological field mapping work undertaken as part of this research project. They also provide a framework for discussing correlations between the presence of landslides and particular geological formations. A generalised stratigraphic column of the Illawarra Region (after Bowman 1974), including the study area is shown in Figure 3.4. The abbreviation that appears in brackets after the first italicised appearance of each formation name, is the tag (label) applied to the mapped area of each formation on the accompanying Geotechnical Landscape Map series developed as part of this research project, discussed in Chapter 6.

3.6.1 The Shoalhaven Group

3.6.1.1 The Budgong Sandstone (BS) The top of the Budgong Sandstone marks the top of the Shoalhaven Group. It is lithic to felspathic lithic in composition, and is mostly plane bedded in laterally discontinuous units varying from several centimetres up to two metres in thickness, and contains abundant marine fossils. Some cross bedding does exist. The upper Budgong Sandstone comprises massive bedding, making it clearly distinguishable from the overlying Pheasants Nest Formation. In the Wollongong area, the thickness of the sandstone is approximately 180m. The Budgong Sandstone was encountered in Roads and Traffic Authority cuttings adjacent to the F6 freeway, immediately south of where the Princess Highway intersects the F6 and along the coastal cliffs above Wollongong’s North Beach. The Budgong Sandstone is not encountered in the subject area. It contains the lower five tabular basic igneous flows and or sills of the Gerringong Volcanic facies.

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Figure 3.4. A generalised stratigraphic column of the Illawarra Region (after Bowman 1974).

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3.6.1.2 The Gerringong Volcanic Facies Five tabular, laterally extensive basic igneous rocks in the Shoalhaven Group and two in the Cumberland Sub-Group of the Illawarra Coal Measures have been described by Bowman as comprising the Gerringong Volcanic Facies. Only the uppermost of these, the Berkeley Latite member has been possibly identified along the southern boundary of the subject area. According to Bowman, it varies in composition from aphanitic to porphyritic in plagioclase laths to 10mm, pyroxene phenocrysts to 5mm across, and some spherical white phenocrysts possibly are possibly zeolites. It possesses weak columnar jointing and is up to 30m in thickness. The intrusive and/or extrusive characteristics of Gerringong Volcanic Facies remain conjectural. According to Bowman, the Berkeley Latite Member (Pib) has a poor outcrop since, upon , it breaks down into small prisms with weathered surfaces. Despite limited attention to this interval by the author, its presence was inferred at two locations. The first of the two locations is an abandoned small excavation, surrounded by residential development, 200m east of the south end of Mountain View Crescent, and the second is on the south side of Cordeaux Road, approximately 450m east of its intersection with Stones Road.

3.6.2 The Illawarra Coal Measures

3.6.2.1 The Cumberland Sub-Group The Pheasants Nest Formation (PNF) overlies the Budgong Sandstone, and lies at the base of the Illawarra Coal Measures. The Pheasants Nest Formation is lithologically similar to the underlying Budgong Sandstone, except for the absence of marine fossils. It consists of coarse grained, poorly sorted, thinly bedded light yellow-grey to mid grey- green comprising volcanic and lithic fragments, and thin interbeds of coal and . Two coal members, two contemporaneous igneous bodies and a tuff member have been defined within this formation. The Unanderra Coal Member (US) and the Figtree Coal Member are only developed in the area, near the top of the formation along with thick carbonaceous claystones. The Unanderra Coal seam (7m thick maximum) has been mapped during this research project, while the Figtree Coal seam (2m maximum thickness) has not. The type section for the Unanderra and Figtree

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Coal Members lies in a creek east of the Nebo Colliery haulage portal (Wollongong 1:63360 sheet, grid reference 796457, Standing Committee on Coalfield Geology of NSW, 1970) The two igneous bodies included within this formation are the Berkeley Latite Member and the Minnamurra Latite Member. The Berkeley Latite Member has been discussed above, while the Minnamurra Latite Member does not occur within the subject area. The Erins Vale Formation (EVF) is distinguished from both the underlying Pheasants Nest Formation and the Budgong Sandstone by the absence of carbonaceous material, the flat bedding, burrowing and bioturbation. The formation comprises a coarse to medium grained light yellow brown to mid grey volcanic sandstone with some finer grained phases. Bowman indicates that the unit is up to 37m thick. There is no outcrop type section for this interval. The type section is defined from a Department of Mines borehole, Wollongong 35 (Standing Committee on Coalfield Geology of NSW, hereafter referred to as the SCCG, 1970).

3.6.2.2 The Sydney Sub-Group The basal formation of the Sydney Sub-Group is the Wilton Formation (WF) which disconformably overlies the Erins Vale Formation. The Wilton Formation varies widely in thickness, although its usual outcrop thickness varies from only 15m to 30m. The formation comprises laminites composed of mid to dark grey siltstone to fine sandstone and light to mid- grey fine sandstone. Claystones, sandstones, and minor coals are interbedded within the unit. Their is no outcrop type section for this unit, due to lateral facies changes. The type section is defined from a Department of Mines borehole, Wollongong 35 (SCCG, op cit). The Woonona Coal Member (Won), occurring within and near the base of the Wilton Formation comprises up to 3m of interbedded coal, carbonaceous mudrock, and mudrock. Below this coal member, the rocks of the Wilton Formation are coarse grained to conglomeratic cross bedded sublithic sandstone. Above the coal member, the formation consists of laminites with some fine cross-bedding. The outcrop type section for the Woonona Coal Member is midway along the cliffs at the south end of Thirroul Beach, near the old rock pool (SCCG, op cit). The Tongarra Coal (Tong) overlies the Wilton Formation. It is of relatively uniform thickness, in the order of 2m to 3m in most outcrops. The Tongarra Seam has a

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distinctive section, being subdivided into approximately four equal carbonaceous sections by thin off-white, buff to light grey, very persistent claystone bands. The upper section of the seam normally is of better quality, and provides the working section for extraction. The upper section of the underlying Wilton Formation does contain some large roots. The outcrop type section for the Tongarra Coal is the southern side of the headland at the northern end of Austinmer Beach (SCCG, op cit). The Tongarra Coal is overlain by the Austinmer Sandstone Member of the Bargo Claystone north of Wollongong. The Austinmer Sandstone Member comprises interbedded light yellow-grey lithic sandstone and claystone, which all weather rapidly on exposure, hence outcrop is poor. The outcrop type section for the Austinmer Sandstone Member is on the coastal cliff section near Coledale Hospital (Bowman 1974, p 49). The Bargo Claystone is quite variable in thickness, from several metres up to near 40m. The sandstone fines upward from a medium grained sandstone at the base, to a very fine grained sandstone at the top, with claystone interbeds increasing towards the top. Mid-grey claystone and siltstone-claystone laminite overlie the Austinmer Sandstone Member and comprise the remainder of the Bargo Claystone. The outcrop type section for the Bargo Claystone Member lies in a creek east of the Nebo Colliery haulage portal, Mount Kembla (Bowman 1974, p 49). The Darkes Forest Sandstone is approximately 10m thick in the study area, increasing to 24m in a borehole near Camden. It weathers upon exposure such that sedimentary structures are difficult to observe. The outcrop type section for the Darkes Forest Sandstone is situated east of the Nebo Colliery haulage portal at Nebo Colliery, Mount Kembla (SCCG, op cit). This sandstone sequence is overlain by the Allans Creek Formation which comprises of shale, carbonaceous shale, minor coal, and lithic sandstone in horizontally interbedded units to 0.3m in thickness. The interval characteristically contains coaly intervals at the top and bottom, the upper one being the American Creek Coal Member. As with the two units above, the Allans Creek Formation is of variable thickness, averaging about 7m to 15m in outcrop. The outcrop type section for the Allans Creek Formation is situated east of the Nebo Colliery haulage portal at Nebo Colliery, Mount Kembla (SCCG, op cit). The Kembla Sandstone, which overlies the Allans Creek Formation consists of

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very fine to medium grained, cross-bedded quartz lithic sandstone. It becomes very fine grained near the top, just below the Wongawilli Coal, where it is often ripple marked. In outcrop thickness, the Kembla Sandstone ranges from 10 to 15m. The outcrop type section for the Kembla Sandstone is situated on the escarpment at west Dapto, Water Board pipeline from Avon Dam, Wollongong 1:63360 sheet, grid reference 747437 (SCCG, op cit). The four units described above, the Bargo Claystone (including the Austinmer Sandstone Member), Darkes Forest Sandstone, the Allans Creek Formation, and the Kembla Sandstone have not been individually distinguished, due to their variable thickness and the almost complete absence of outcrop of this interval, during the mapping work carried out by the author. This interval between the top of the Tongarra Seam and the base of the Wongawilli Coal has been mapped by the author as one unit, and assigned the label (KADB) in the geological maps prepared during this research project. The Wongawilli Coal (Wong) generally consists of 3m to 9m of coal, carbonaceous shale and interbedded thin tuffs, with some sandstone and shale interbeds. The Wongawilli Coal has, as does the Tongarra Coal, a distinctive cross section being subdivided into two thick coal/carbonaceous sequences separated by one major and several smaller intermediate off-white, buff to light grey, very persistent tuffs, and claystones of tuffaceous origin. The major central tuffaceous band is known as the three foot band. Upon weathering and exposure to water these tuffaceous bands become soft, and appear to be practically impermeable. Within the Illawarra area, it is usually the lower coal section, below the three foot band, that is economically worked. The outcrop type section for the Wongawilli Coal is situated on the escarpment at west Dapto, Water Board pipeline from Avon Dam, Wollongong 1:63360 sheet, grid reference 747437 (SCCG, op cit). The Eckersley Formation, a unit of variable lithology, overlies the Wongawilli Coal. Whilst the thickness of this unit reaches approximately 122m near Camden (Department of Mines borehole, Camden 75), in outcrop along the coast it varies in thickness from 20m to 40m. Their is no outcrop type section for this unit. The type section is defined from a Department of Mines borehole, Camden 78 (SCCG, op cit). Bowman (op cit.) has subdivided the formation into several upwards fining cyclothems

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(a recurring sedimentary cycle culminating, ideally, in coal development). The most significant of these cycles, within the study area, culminates in the development of the Balgownie Coal Member. This coal member comprises a variable thickness up to 2m of coal and carbonaceous shale. In some of the smaller, and older mines, the Balgownie seam has been worked. The outcrop type section for the Balgownie Coal Member is situated at South Bulli Colliery (SCCG, op cit). The Balgownie Seam has been encountered several times during field mapping, so it has been possible to subdivide the Eckersley Formation into an Upper (UEF) and Lower (LEF) Eckersley Formation separated by the Balgownie Coal Member. The Balgownie Coal Member is separated from the Bulli Coal Seam (the top most formation of the Illawarra Coal Measures) by 5 to 15m of Eckersley Formation strata. The Bulli Coal Seam averages 2 to 3m thickness, and is underlain by carbonaceous claystones. The outcrop type section of the Bulli Coal Seam is situated at sea level, adjacent to the Coalcliff Colliery Tunnel (SCCG, op cit) between Coalcliff and Clifton. The roof of the Bulli Coal Seam comprises carbonaceous and interbedded thin sandstones which are not always present. It is assumed that the erosive environment at the onset of deposition of the overlying Coalcliff Sandstone (base of the overlying Narrabeen Group) explains the variable roof conditions of the Bulli Seam. In the accompanying Geotechnical Landscape maps, the Bulli Seam is represented by the boundary between the UEF and the Coalcliff Sandstone.

3.6.3 The Narrabeen Group

3.6.3.1 The Clifton Sub-Group Forming the basal unit of the Narrabeen Group, is the light grey, fine to medium grained, quartz-lithic, massive Coalcliff Sandstone (Rnc). The Coalcliff Sandstone disconformably (parallel bedding above and below the contact, but an irregular erosive contact is indicated) overlies the shale facies at the top of the Illawarra Coal Measures. In outcrop, this unit varies from 6 to 20m throughout the Illawarra, but at its type location at Coalcliff, it is 10m thick. Angular siderite fragments 10cm in size, are common near the base of the formation. The type section of the Coalcliff Sandstone was measured near the old adit of the Coalcliff Colliery (Hanlon 1956, p 30). The Wombarra Claystone (Rnw) overlies the Coalcliff Sandstone. The

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Wombarra Claystone varies in thickness from 36m at the type location above the Coal Cliff mine adit between Clifton and Coalcliff (Hanlon, op cit), to 17.4m in the southwest of the study area in the borehole AIS Wongawilli DDH 27. The unit comprises mid-grey to green-grey to chocolate claystone with sandstone interbeds. The colour of the claystone varies from green grey to grey with sporadic chocolate at the top to grey at the base. The sandstone interbeds are generally quite thin, lenticular, fine grained, carbonate cemented, lithic sandstones with lateral facies changes into claystone. The prominent Otford Sandstone Member lies near the top of the claystone comprising tabular cross-sets to 0.6m thick, with planar tops and bases, totalling 6.9m in thickness at the type locality for the claystone. Another, less persistent sandstone band is situated near the base of the claystone. The Scarborough Sandstone (Rns) overlies the Wombarra Claystone, and at the type location is 25.5m thick. In the borehole AIS Wongawilli DDH 27, in the southwest of the study area, the sandstone is 10m thick. In outcrop, however, Bowman suggests, and recent field mapping by the writer supports that the unit is about 24m thick. The sandstone is conglomeratic in a distinctively colourful collection of cherts, commonly up to 5mm in diameter. It consists of cross bedded planar cosets several metres in thickness, each of which are graded, fining upwards. This is typical towards the base of the unit. Indurated, ellipsoidal mid to dark grey claystone fragments are also common, as are thin carbonaceous partings. The measured type section for the Scarborough Sandstone is located on the cliffs overhanging Drive above the Coal Cliff mine adit (Hanlon, op cit). The Stanwell Park Claystone (Rnsp) consists of three main claystone intervals and two sandstone intervals. The colour of the claystone grades from chocolate or mottled chocolate with some areas of purple and olive green at the top, to olive green at the base. The sandstones are composed of weathered lithic fragments and are generally light to mid greenish-grey in colour. The type section, where it is 36.6m thick, is located in the gully adjacent to the Harbour Fault, above (Portion 18, Parish of Southend, county of Cumberland about 12 chains north of the southern boundary, Hanlon op cit). At Bulgo Headland, on the coast just to the north of Stanwell Park, the unit is 53m thick, while it lenses out completely south of the southern boundary of the study area.

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The Bulgo Sandstone (Rnb) overlies the Stanwell Park Claystone and is the thickest by far of the seven Narrabeen Group formations. At Bulgo Headland, the type location for this unit, the Bulgo Sandstone is approximately 119m thick. In the borehole AIS Wongawilli DDH 27, in the southwest of the study area, the Bulgo Sandstone is approximately 114m thick. Thus it would seem that the unit does not change much in thickness across the study area. In fact it makes up more than half of the thickness of the Narrabeen succession. The Bulgo Sandstone can be (but has not been during this study) subdivided into three distinct facies in the coastal district of the northern Illawarra (Ward, 1980). Each of these facies occupies approximately one third of the section in the type area. The three facies are the basal pebbly facies, the middle volcanic facies and the upper shaly facies. Due to the thickness of this sandstone, each of these three facies is discussed briefly in the following paragraph. The basal pebbly facies, resting on the underlying Stanwell Park Claystone with a slight disconformity, comprises a sequence of pebbly sandstone and lithic conglomerate with green, red, black and grey rounded pebbles which are loosely described within the Sydney Basin as chert. This lower facies is similar to parts of the Scarborough Sandstone, and south of the study area, where the Stanwell Park Claystone has lensed out, it is difficult to distinguish between the two. It is exposed in cliffs along the coast between Werrong and Era Beaches. Overlying this pebbly facies is a succession of sandstone, shale, and conglomerate, all of which have a characteristic green colour in the field. As this green colouration is due to the presence and weathering of volcanic sediments, the interval is referred to as the volcanic facies. It crops out along the coastal cliffs, headlands and walking tracks between South Era Beach and Little Garie Point. The sequence between the top of the volcanic facies and the base of the overlying Claystone is known as the shaly facies. This interval has a considerably higher proportion of shale than the lower two facies of the Bulgo Sandstone. In contrast to the underlying volcanic facies, the sandstones of the shaly facies are more grey-brown. This facies is exposed in cliffs along the north side of Garie Beach. The Bald Hill Claystone (Rnbh) overlies the Bulgo Sandstone and the top of this unit marks the top of the Clifton Sub-Group of the Narrabeen Group. It comprises distinctive chocolate, red and purple-brown siltstone and claystone, with some

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discontinuous sandstone beds. It consists almost entirely of haematite and kaolinite, with minor amounts of quartz, anatase, and siderite (Ward, op. cit). While massive siltstone and claystone are the most common rock types, pelletal, oolitic and brecciated textures are also found. The Bald Hill Claystone is 15m thick in its type locality, a section above Bulgo Headland, in the vicinity of Bald Hill. This interval is exposed in numerous outcrops and roadside cuttings in this area. Other significant exposures exist at the intersection of the south end of Lady Carrington Drive and Sir Bertram Stevens Drive in the , within and east of along the western side of the South Coast Railway Line in the vicinity of Helensburgh, at approximate railway chainage 49.300km, and above Balgownie along Clive Bissell Drive, within two kilometres south of its intersection with Mount Ousley Road. In addition, it has been mapped in numerous creek lines within the Royal National Park, as it is a clear marker horizon due to its thickness and characteristic chocolate brown colour in outcrop.

3.6.3.2 The Gosford Sub-Group The Gosford Sub-Group includes all the strata from the top of Bald Hill Claystone to the base of the Hawkesbury Sandstone. The Garie Formation is a thin 0 to 3m transitional zone between the Bald Hill Claystone and the overlying Newport Formation. A horizon has been identified at the top of the Bald Hill Claystone, representing a hiatus in the sedimentary sequence. Subsequent transgression eroded and resorted the soil horizon forming the clay pellet sandstone which grades up into the Newport Formation (Bunny and Herbert, 1971). The Newport Formation is defined (Herbert 1970) as the unit occurring below the Hawkesbury Sandstone and above the Bald Hill Claystone and, where present, above the Garie Formation. It consists of interbedded quartzose to quartz-lithic sandstones and siltstone/sandstone laminite sequences. A shallow estuarine and salt marsh environment into which fluvio-deltaic sands periodically encroached is indicated in a regional analysis within the southern coal field (Bunny and Herbert, op cit). Most of the upper Newport Formation has been shown to be the lateral basinward equivalent of the fluvio- deltaic Hawkesbury Sandstone. The former Undola Sandstone (of Hanlon, 1956) is incorporated into the upper Newport Formation (Bunny and Herbert, op cit). The type section for this formation is 3km north of Garie Beach, on the coast near Eagle Rock,

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where it is 18.4m thick. These two formations and the Gosford Sub-Group have not been differentiated in the mapping project undertaken during this research project and the Geotechnical Landscape maps included herein. Instead, they have been incorporated into the area mapped as the Hawkesbury Sandstone. Whilst not geologically correct, differentiating these units would have proved time consuming, due to the lack of outcrop and extremely difficult access to much of this area of the sequence. In addition, this differentiation would have been of little ultimate benefit in the context of this research project.

3.6.4 The Hawkesbury Sandstone (Rh) The Hawkesbury Sandstone overlies the Narrabeen Group within the study area, and at its base, interfingers with the underlying Newport Formation. Where the Newport Formation does not exist, it disconformably overlies the Garie Formation and the Bald Hill Claystone. The Hawkesbury Sandstone is a flat lying Middle Triassic mature quartz sandstone with an aerial extent of about 20000 km² (Conaghan, 1980). While it has a maximum thickness of about 250m, it is approximately 180m thick near Stanwell Park, thinning to the south, to about 120m at Macquarie Pass, south of the study area. It does include some thin siltstone and claystone interbeds, but sandstone exceeds mudstone by about 20:1. It underlies the entire western margin of the study area and the plateau to the west, and forms the upper cliff line along most of the . The Hawkesbury Sandstone has been subdivided into two contrasting intervals, the sheet sandstone facies and the massive sandstone facies (Conaghan and Jones 1975) with a minor mudstone facies. It is suggested that these lithosomes repeatedly recurred during the deposition of the Hawkesbury in a fluvial type environment. While this has been the subject of much debate in the literature, it is of no relevance to this project.

3.6.5 Intrusive Dykes and Sills Various intrusive rocks were encountered in the field during the mapping work, and whilst some were recorded on the field maps, those other than the Berkeley Latite Member have not been reproduced on the final Geotechnical Landscape Maps. Bowmans (1974) Mount Nebo Monchiquite and Rixons Pass Teschenite were observed. In addition, a basalt of unknown composition and approximately 3m thick, was

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encountered in an east flowing creek to the south of Joanne Street, at an elevation approximately equal to the south western end of Joanne Street. Numerous near-vertical dykes have been encountered in the field. These have not been included on the final Geotechnical Landscape Maps. However, as is discussed in a later chapter, dykes mapped during mining activities and recorded by ACIRL (1980) have been included on the final Geotechnical Landscape Maps.

3.6.6 Slope Debris deposits The slopes of the Illawarra Escarpment are almost completely mantled with a cover of slope debris, either of an alluvial or colluvial origin (see section 3.7 for technical descriptions of these and other related terms). This material restricts outcrops of the underlying bedrock to cliff lines (along the top of the escarpment, coastal and localised intermediate cliffs), incised water courses, and the occasional spur lines which have either not been inundated, or alternatively those that have been denuded of cover by erosion. These slope debris deposits make geological mapping of the underlying bedrock sequence very difficult. All geological maps of this area involve considerable interpolation of geological boundaries between known outcrops and borehole locations. Locally, the colluvium comprises a variable mixture of sandstone, siltstone, claystone and coal bedrock debris (grading from a slightly to completely weathered state) in a matrix weathered, again variably, to sand, silt and clay. The rock component is variable depending on the bedrock sequence contributing to the colluvium, and the distance from the source which any given deposit of colluvium has moved. Of course, in any natural colluvium deposit, bedrock incorporated within the colluvium can only come from an elevation higher than that of the colluvium deposit. The bedrock sequence is dipping at shallow angles commonly below 5°, usually into the slope. This, combined with the interbedded character of the sequence gives rise to a blocky and fragmented character to the typically moderately weathered bedrock fragments. Sandstone fragments or boulders range in size from less than 1m3 up to 8m3. Occasional much larger sandstone boulders, usually Hawkesbury Sandstone, do exist. Siltstone and claystone material usually enters the colluvial cycle in a residual or completely weathered state. In cliff situations where rockfalls and toppling failures occur, and in some alluvial situations, siltstone and claystone fragments may enter the slope debris cycle in a fresh to slightly weathered state.

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During the writers engineering geological experience within the study area and the Sydney basin in general, the depth of weathering below the colluvium/bedrock interface in Narrabeen Group siltstone and claystone rocks, will often extend several metres or more to slightly - moderately weathered rock. In jointed, blocky sandstone, this depth is variable and may reach up to more than 10m along joints. This thesis is predominantly concerned with colluvial materials and the upper completely to moderately weathered zones of bedrock and their association with land instability.

3.7 REGARDING COLLUVIUM, TALUS, TALLUVIUM OR SLOPE WASH? Available geotechnical literature regarding the Wollongong area, includes research theses, books, journals, and consultants geotechnical reports. The writer believes that often there has been mis-use of terminology concerning the slope debris deposits (material deposited above the insitu bedrock) on the Illawarra Escarpment. Therefore, various terms are defined below.

3.7.1 Definitions of terms used in the literature regarding gravity driven slope debris deposits Upon reference to the Collins Dictionary of Geology (1990), the following definitions are found; • “alluvial, adj. 1. Composed of or pertaining to ALLUVIUM, or deposited by running water.”

• “alluvium, n. The general term for detrital made by rivers or streams or found on ALLUVIAL FANS, flood plains, etc. Alluvium consists of gravel, sand, silt, and clay and often contains organic matter that makes it a fertile soil. It does not include the subaqueous sediments of lakes and seas.”

• “colluvium, n. unconsolidated material at the bottom of a cliff or slope, generally moved by gravity alone. It lacks stratification and is usually unsorted: its composition depends upon its rock source, and its fragments range greatly in size. Such deposits include debris and talus. Compare SLOPE WASH (see below).”

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• “debris, n. 1. also called rock waste, any surface accumulation of material (rock fragments and soil) detached from rock masses by disintegration.”

• “scree, n. a heap of rock debris produced by weathering at the base of a cliff, or a sheet of coarse waste covering a mountain slope. Scree is frequently considered to be a synonym of TALUS, but is a more inclusive term. Whereas talus is an accumulation of debris at a cliff base, scree also includes loose debris lying on slopes without cliffs. The term scree is more commonly used in Great Britain, whereas talus is more commonly, but often incorrectly, used in the United States.”

• “slope wash, n. 1. earth material moved down a slope principally by the action of gravity, aided by non-channelled running water. Compare COLLUVIUM. 2. the process itself by which such material is moved.”

• “talus, n. a heap of coarse debris, a result of weathering (frost action), at the foot of a cliff. Compare SCREE (see above). The slow downslope movement of talus or scree produces talus-creep.”

• “talus cone, n. a steep-sided pile of rock fragments lying at the base of a cliff from which they have been derived. Talus cones are formed primarily by the movement of materials aided by gravity. See COLLUVIUM (see above).”

Whilst not appearing in the Collins Dictionary of Geology (1990), the following two terms are defined in the American Geological Institute Glossary of Geology (1980); • “colluvial Pertaining to colluvium; e.g. ‘colluvial deposits’.”

• “talluvium, A term introduced by Wentworth (1943) for a detrital cover consisting of talus and colluvium; the fragments vary from large blocks to silt (US Geo book) Obsolete.”

From the above definitions, combined with some experience by the writer with exposures and the composition of gravity driven slope debris deposits within the subject area of the escarpment, it is clear that the most appropriate term for these deposits, locally, is colluvium. Talus is a term that is widely used to describe the colluvium

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deposits within the subject area, while talluvium is less commonly used (Young, 1976). Talus is incorrect, by definition, as little or no frost or ice action is involved locally, and the deposits more often than not include a high percentage of clay. Their is no doubt that scree deposits do exist on the escarpment, however, the more general term colluvium is preferred. This latter term is used throughout the rest of this thesis. Note that the definition of colluvium states that the material is moved generally, by gravity alone, and hence does not include slope wash and alluvium, which are transported by water, under the influence of gravity. Hence the terms, colluvial and alluvial.

3.8 COMMERCIALLY AVAILABLE GEOLOGY MAPS OF WOLLONGONG Geological mapping of the Wollongong area is currently available at several map scales as tabulated below in Table 3.1. In addition to these maps, some larger scale, smaller area maps are available. Of note are the works of F.N. Hanlon in the 1950's, C.L. Adamson (1974), Coffey and Partners Pty Ltd (1985) and S. Pitsis (1992). These maps are not extensive in their coverage, some lack cadastral base information, and all suffer from lack of availability and copy quality.

SCALE TITLE DATE

Wollongong Geological SeriesSheet 51 56-9 NSW 1:250,000 1966 Dept Mines

Wollongong - Port Hacking Geological SeriesSheet 1:100,000 1985 9029-9129 NSW Department Of Mineral Resources

Wollongong Geological SeriesSheet 9029-11& 9028- 1:50,000 * 1974 1&1V NSW Department of Mines Geology & Natural Slope Stability in the City of 1:25,000 * Greater Wollongong, in Records of the Geological 1972 Survey of New South Wales Volume 14, Part 2 Maps of the Coal Seam Structures in the Southern 1:25,000 Coalfield of NSW Australian Coal Industry Research 1989 Laboratories Ltd Geology Sheets - City of Greater Wollongong. 1:6336 Geological Survey of New South Wales, Plans 5250- 1972 5286, 5545 a thesis by H. Bowman

Table 3.1. Commercially available Geology Maps showing the Wollongong Area. * Maps based on Bowmans 1:6336 Mapping

With the exception of some of these latter larger scale maps, the previously available geological mapping has been carried out and presented at such small scales as

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to make it of limited use for specific, individual, geotechnical site investigations. In such investigations it is not only essential to know the regional geological context, but it is also of paramount importance to know which geological formations underlie the land in question. Such knowledge prior to the commencement of site investigations allows ready familiarisation with other local sites in similar geological settings, and a more focused field investigation at the outset of site works than would otherwise be possible. Therefore, accurate maps covering the whole study area are vitally important.

3.9 BOWMANS MAPPING Bowmans 1:6336 mapping Geology and Land Instability of the City of Greater Wollongong (1972a) has been the basis for a lot of subsequent geological mapping in the Illawarra area. Together with his stratigraphic and structural descriptions (Bowman, 1972 and 1974), Bowmans work still remains the definitive Wollongong Geology. While Bowmans mapping work was outstanding for the time, the use of poor quality topographic and cadastral base maps (the best and most detailed available at the time), have limited its life and practical application. His mapping work was shown on 1:6336 Illawarra Planning Authority (IPA) sheets whereby it was found that the contour information was inaccurate (Bowman 1972a, p. 161). At that time, the work proceeded on the basis of these maps since these map sheets were the only ones available showing the required detail. The project was intended as a regional survey such that the results of detailed site investigations may be placed in their regional context. Bowmans 1:6336 set of maps comprised one index sheet and two map series, one geology and one of land instability zoning, each set comprising 17 maps. Each map was approximately 1.2m by 0.9m in size. The quality of base detail available on the maps was variable, ranging from some which had no contours and no cadastre, to others with detailed cadastre and some contour information, as shown in Figures 3.5 and 3.6. Few geology maps contained any cadastral detail, and only some of the land stability zoning maps contained cadastral detail. In addition, the scale, at 1:6336 is difficult to convert, via photocopying etc, to the current metric scales. As noted above, the IPA sheets contained some inaccurate contour information. This included imprecisely located contour lines, and topographic and road mismatches of adjacent map sheets along boundaries of up to several centimetres. These problems

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were understandable, and insurmountable at the time. However, such inaccuracies are

Figure 3.5 Portion of Bowmans original scale 1:6336 Geology maps. Scarborough Sheet. Only spatial reference is limited cadastre of main roads and the coastline.

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Figure 3.6 Portion of Bowmans original scale 1:6336 Land Stability maps. Scarborough Sheet. The top of the escarpment crosses the upper left side of this segment, and Buttenshaw Drive diagonally crosses the page. unacceptable and unjustified today. Transferring Bowmans geology onto the more recent Central Mapping Authority 1:4000 scale maps, as is shown on Figure 3.7, clearly highlights these problems. While this technique is possible, and is an often repeated

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process in many organisations, there is no guarantee of achieving sufficient precision. The resulting ‘maps’ require considerable interpretation and often the conclusions drawn can be misleading. For example, in this relatively simple sedimentary basin sequence where the bedrock is essentially flat lying, it is expected that bedrock outcrops and/or subcrops approximately follow the contour form. However, the maps shown in Figure 3.7 suggest that the bedrock dips steeply across some valleys and spurs. The basic problem is that ground contours have been incorrectly positioned on the original IPA sheets. The result shows the maps produced by Bowman are, in places, quite inaccurate. Bowman (1972a) recognised this during his own field work, and even stated so in his text. In 1972, Bowman published his mapping (geology and land instability) at a scale of 1:25000, and then in 1974, the New South Wales Geological Survey (for whom Bowman worked), published three 1:50000 sheets including the Wollongong sheet.

3.10 PREVIOUS LAND INSTABILITY WORK OF NOTE IN THE WCC AREA

3.10.1 New South Wales Government Department of Mines

3.10.1.1 A report by Harper (1935) Mr. L. F. Harper was one of the NSW government geologists over the period including 1910 to 1935. He was involved in the geological survey of the southern coal field, and during that time published several papers regarding geology in the annual reports of the department. In 1935 he reported briefly to the department on slope instability and land movements towards the coast at Stanwell Park and Thirroul. This report was of a general nature, was not published and did not contain any maps.

3.10.1.2 Some reports by Hanlon (1942 and 1958) Hanlon followed in Harpers footsteps becoming one of the government geologists, at intervals, over the period 1938 to 1958, and was similarly involved in the geological survey of the southern coal field. Hanlon also published numerous geological accounts

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Figure 3.7 Portion of Bowmans Geology maps enlarged and superimposed onto a 1:4000 scale orthophotograph map of the Balgownie area. A clear lack of correlation between the geology and contour lines is visible. of the area, which still remain essential reading for any geologist working in the area.

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Maps showing areas of slope instability in the Wollongong district appeared in 1942 when Hanlon first prepared plans of the area from Stanwell Park to Coledale. These maps indicated zones of instability affecting the railway and the main road. In 1958, Hanlon gave a presidential address on Geology and Transport with special reference to Landslides on the Near South Coast of New South Wales. This report set a new high standard locally for specifically documenting land instability. The report identified specific sites, discussed some in detail, and included photographs of several problem areas, including Lawrence Hargrave Drive just north of the Clifton Hotel (an area known as Clifton Hill).

3.10.1.3 Some reports by Adamson (1960 and 1962) At the request of the Town Clerk of the City of Greater Wollongong, the Geological Survey of New South Wales carried out an investigation of a landslide located at the southern intersection of Seafoam Avenue and Phillip Street, Thirroul. Following these investigations, Adamson prepared two reports summarising the findings of the Geological Survey (Adamson, 1960 and 1962). The landslide was active in 1950 and 1951, during the years 1959 to 1964, and again during the wet years between 1988 and 1990. The landslide has destroyed 5 houses and damaged a further 6 houses, including the grounds and some buildings within Thirroul Public School. Adamson (1962) prepared a detailed site plan of the landslide, clearly identifying areas of ground disturbance and damage caused by the landslide. The locality diagram from his site plan is shown as Figure 3.8. Bowman (1972), discussed below, carried out some investigations into this landslide. In 1997, the site remains dormant and sterilised for development, and no remedial works have been designed or installed. Plate 3.1 is an oblique aerial view of the site.

3.10.1.4 A report by Chesnut and Crawford (1971) Concerns associated with a proposed 2000 acre development of the Camp Creek - Lilyvale area east of Helensburgh initiated this report on the slope stability within the Otford Valley. This report was of a general nature and although several geological hazards were noted, “...no real problems of land stability...” were identified or anticipated.

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Figure 3.8. Locality diagram of the Seafoam Avenue/Thirroul Public School Landslide, Adamson 1962.

Plate 3.1. December 1997 vertical aerial view of the Seafoam Avenue and Thirroul Public school landslide. The vacant lots which used to be occupied by houses that were destroyed by the landslide are clearly visible.

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3.10.1.5 Report by Bowman (1972) and his maps of geology and land instability As noted previously, Bowman (1972), prepared 16 land instability sheets at a scale of 1:6336 to accompany his geology sheets. Bowmans land instability maps distinguished six stability zones, summarised in Table 3.2. The tabulated description of each zone presented by Bowman, and shown here as Table 3.2, does differ slightly from the zones description within his papers (Bowman, March 1972 and December 1972). Bowmans zoning considered the following factors and a little associated data; • the natural strength of rocks, • geological subcrop location, • air photograph interpretation, • slopes that are likely to fail because of topographic location, and • ground slope Bowmans work was commissioned by the Wollongong City Council to assist with town planning. The council adopted Bowmans work and started using his land stability maps as a guide for assessing slope stability aspects of development applications.

Zone Description Stable land No landslip problems Stable land with areas of Normally moderately level land which is underlain by soil which is unstable in minor slope instability unsuitable topographic positions Most of the land may be safely utilised although some areas are unsuitable. Less stable Generally more topographically elevated than land in the categories above Thorough investigation required before development. Generally Moderately unstable topographically high - relief land underlain by potentially unstable material Topographically unstable for development owing to steep slope and/or Topographically unstable topographic position and nature of soil Essentially unstable land. Best left undeveloped. Some areas may be developed Essentially unstable after detailed site evaluation (includes known slip areas)

Table 3.2. Bowmans Stability of Natural Slopes in the Wollongong area.

The zone described as “essentially unstable” included known ‘slump’ areas. No other guidelines or details were provided regarding the source of information which contributed to the classification of an area as “essentially unstable”. Several case study investigations were reported, notably the Thirroul Public school, Cope Place, Bulli and a site on the slopes of Mount Nebo. The landslide which affected the Thirroul Public has been discussed above (Adamson 1960 and 1962). Bowman examined the relationship between rainfall and landslide movement at

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Thirroul Public School during the 20 year period from 1950 to 1970. Bowman concluded that rainfalls exceeding 430mm in 1 month invariably caused landslide movement. This interesting area of research is discussed in more detail in section 3.11, and Chapter 8. Pitsis (1992) noted the presence of numerous large scale areas of instability within the zone identified by Bowman as “Less stable” in the Stanwell Park to Clifton areas (sheets E12 and F12 herein). Bowman describes this zone “less stable, most of the land may be safely utilised, although some areas are unsuitable”. Moreover, Bowmans zone “Less stable land” and even his zone “Stable land” overlap large areas of known instability on the geologically controlled terraces between Austinmer and Clifton. These observations during recent studies highlight some significant errors in Bowmans work. The technique of zoning he used is not clear and may have contributed to the errors as much as limitations to careful, detailed observation and invalid interpretation of field evidence. Contour maps in use at that time were inadequate for the task. In addition, with the benefit of 25 years of hindsight, one can comment that the significance of the geotechnical setting of Wombarra and Stanwell Park Claystones terraces was not appreciated by Bowman. Certainly, he did not have the experience of the wet years of 1974, 1975, 1981 and of the 1988-1990 wet period and therefore, the experience of observing the landslides which occurred during these wet years.

3.10.2 Research work of Young (1976) Young (1976) differentiated two groups of slope debris deposits which she collectively called ‘taluvium’, noting their composition is transitional between coarse rocky talus and fine colluvium. The two groups distinguished were defined as; • Type M, bouldery, strongly mottled relict deposits mantling the lower spurs of the escarpment, and • Type U, less mottled and presently forming masses on the benches higher up the escarpment. Young considered two properties of the deposits, clay content and plasticity indices and concluded these did not differ significantly between the two groups. In both groups, these parameters and the angle of natural slopes suggested that the taluvium is unstable in the long term at gradients above 10° -12°. With an increase in annual rainfall

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on the upper escarpment slopes, combined with a rise to the south of the unstable strata, Young concluded that instability is more likely on the higher steeper slopes of the escarpment. Young (1976) considered how the colluvium deposits may have been formed and suggested that these deposits were developed during much wetter climates in the geological past. With three case studies, Young demonstrated the close relationship between marginal natural stability in the mid escarpment slopes and urban development. An extensive stereographic aerial photograph interpretation was also conducted whereby 154 sites were identified on photographs taken in 1951, 1963, 1966 and 1974. Some sites were identified repeatedly on consecutive photographs. Young initially marked the sites (general location only) on 1957 (variously amended during the 1960’s) Illawarra Planning Authority cadastral-topographic base maps (pers com, Young 1996), reproduced at a scale of approximately 12 chains to the inch. In Young’s thesis, the sites were marked by numbers only, according to a tabulated site list on a small scale sketch map. The tabulated site list included the air photograph reference and brief description. The sites were not all field checked, and hence it is not possible to conclude that all the areas of ground disturbance identified by Young, were the result of land instability.

3.10.3 Golder Associates 1983 unpublished report for the Wollongong City Council Golder Associates (1983), prepared a report for the WCC entitled “Guide to suspect Landslip Areas, Stanwell Park to Dapto”. In this report, different areas were marked with the following descriptions; • recorded past or recent landslip areas, • suspect past or potential future landslip, • no apparent past movement and no likelihood of future movement. Twenty three 1:8000 WCC building allotment plans were marked up accordingly. The maps were prepared from Golder’s in-house records, a list of councils problem areas, and a street by street inspection by Golder Associates geotechnical engineer, Mr. R. Amaral. The copy of the maps reviewed by the writer were reduced to a scale of approximately 1:11000, a scale which makes the transfer of information from one map

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to another quite difficult. The quality of the cadastral base maps was excellent. However, the work was conducted in early 1983, so the cadastre is now 14 years old, and in parts has changed considerably. In addition, there are no contours and no geology is marked. None the less, the slips are clearly marked, and the maps are very useful. These maps proved to be a significant improvement over previous land instability mapping in the area. The marked land slip areas were incorporated into the WCC’s internal hazard maps. Many of the slips identified in Golders work have been included in the Geotechnical Landscape maps and land instability database produced during this research project. A consistent problem with all the land instability mapping work that has been completed so far, is the lack of detail provided about each recorded case of land instability. As Golder’s provided perhaps the most extensive coverage of land instability up to that time, this lack of reference detail regarding each slip site became most apparent. The first attempt to rectify this in specific localities came with Pitsis’ 1992 work (see section 3.10.5).

3.10.4 Coffey Partners International 1985 report for the WCC Coffey Partners International (CPI) in 1985, completed a geotechnical report concerning the Coledale area on behalf of the WCC. The main results of this study were presented on two plans at a scale of 1:4000. Sheet 1 shows areas of known or inferred instability, major topographic features and underlying geology while Sheet 2 shows a land instability zoning of the area. The geological mapping undertaken by CPI was based on Bowmans work, although it was found that unacceptable inaccuracies existed (CPI 1985, page 5) when enlarging and overlaying Bowmans maps onto the 1:4000 base maps. A revised geology was shown, incorporating the use of sharp slope breaks as indicated by contours that were assumed to mark the top of the Scarborough and Coalcliff sandstones, mine adits assumed to lie on the outcrop of the Bulli seam, and assuming uniform horizontal bedding and interval thicknesses. The land instability mapping undertaken by CPI was based on; • aerial photograph interpretation (1966, 1977 and 1985 photography), • areas of known or inferred instability, and • a limited walk-over survey in April 1985.

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Most of the areas of instability identified during the CPI study have been included in the Geotechnical Landscape maps and land instability database prepared as part of this research project. This CPI report distinguished five principal zones with several subgroups, as summarised in Table 3.3. The zoning scheme also outlined the likely forms of instability expected within each zone, the suitability of the zone for residential development and recommendations for development constraints. The zones were delineated on the basis of ground slope, underlying geology, slope form and potential for instability. Whilst this work was adopted by council in their town planning department, the zoning as presented has not been directly applied by the WCC. However, some of the slip areas identified have been added to the WCC’s internal landslip hazards maps.

Zone Stability Conditions essentially stable (none found within study area). Detailed investigation of land within Zone I other zones may enable them to be reclassified as zone 1 most of the area has been apparently stable recently, but is potentially unstable Some areas Zone II identified on aerial photographs show some signs of recent instability Potentially unstable bench area. Adjacent parts already affected by instability. This shows Zone III that a high potential exists for similar instability to develop Existing instability, bench area. Areas already affected by instability in historical past or as Zone IIIu interpreted from aerial photographs Potential instability - slopes mostly steeper than about 25º, forms a steeper slope area Zone IVa between adjacent bench area. Located on Scarborough Sandstone Potential instability - slopes mostly steeper than about 25º, forms a steeper slope area Zone IVb between the bench area above the flatter slopes below Located on or below the Coalcliff Sandstone Potential instability - steep talus slopes. Located beneath the Hawkesbury sandstone cliff Zone V line on steep active slopes. Potential instability - bench area. Located on Stanwell Park Claystone and it has been Zone Va apparently stable recently, but may be affected by large scale instability from above and possible instability associated with the Scarborough Sandstone below

Table 3.3. Description of five principal zones of land instability in the Coledale study, Coffey Partners International (1985).

3.10.5 Paper by Hutton, Ferguson and Jones, (1990) This paper briefly describes several types of landslip that have been prominent during the few years prior to 1990 and especially movement associated with the heavy rains experienced by the Illawarra in late April, 1988. The paper reports on the cliff areas to the north of Clifton that are traversed by Lawrence Hargrave Drive, and several areas on . The paper also provides a cursory discussion as to the causes of each of the different types of landslip. Three types of mass movement are reported to be common;

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a) rock falls, b) mud flows and, c) earth flows (slumping). A rock fall in 1987 is discussed where an estimated 400 to 500 tonnes of rock broke away from the cliff face after heavy rains. The rock fall blocked the road for several days. The rock fall developed in the Scarborough Sandstone, immediately above the Wombarra Claystone, which is weathering more rapidly. The locations and salient features of numerous mud flows and slumps near Bulli Pass and Lawrence Hargrave Drive, Clifton, are discussed. However, no individual site plans were included in the report and the information is mostly of a general nature.

3.10.6 Research work of Pitsis, (1992) In 1992, whilst employed by the State Railway Authority (SRA) Geotechnical Services as their Senior Geotechnical Engineer, Pitsis submitted his Master of Engineering Science thesis to the University of New South Wales. His work included; • detailed 1:4000 scale field-based mapping of both geology and land instability within the Stanwell Park to Clifton area (two 1:4000 scale CMA map sheets, E12 and F12 of the WCC Index), • summary 1:25000 scale mapping of geology, with known and possible areas of land instability in the Stanwell Park to Wollongong area, • a cross-referenced tabulated summary of the 53 landslides mapped on the 1:4000 scale sheets, and a brief summary of the other 111 known sites included on the 1:25000 map sheet. In total, Pitsis identified 164 sites, • limited discussion of case studies of seven prominent railway landslides with some engineering details. Pitsis presented some detailed information gathered during extensive RSA geotechnical investigations, and, in some cases, during the installation of extensive subsurface remedial works. Pitsis’s work set a new high standard in Australia for documentation of land instability. His mapping, and cross referencing of each mapped site of land instability with a tabulated text-based summary is an important step toward gaining a wider and better understanding of land instability within the region. Hence, Pitsis’s work was adopted as part of the basis for the mapping component of this research project. In particular, his site numbering was adopted, and although extended as discussed in chapter 5, this present research project accepts Pitsis’s work without reservation.

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3.10.7 Research work of Ghobadi (1996) Ghobadi worked on the geological engineering factors influencing the stability of slopes and cliff lines in the northern Illawarra region, and included a literature review of general problems of, and strategies for assessing slope stability. His field study area encompassed the area between Coledale and Stanwell Park, although his work was concentrated within the rugged coastal cliffs traversed by Lawrence Hargrave Drive between Clifton in the south and the Coalcliff terrace in the north. Ghobadi mapped several landslide sites and relied on some existing borehole data and existing geology maps. The sites mapped include; the Clifton Earth Slump, the Moronga Park Earth Slump, the Southern Amphitheatre Complex Landslide, the Northern Amphitheatre Complex Landslide, the Jetty Rock Slump, the Harbour Slump and the Coalcliff Slump. He carried out numerous index property tests on rocks and seventy five direct shear tests on colluvium samples obtained from fifteen locations within five landslide sites between Clifton and Stanwell Park.

3.10.8 Other local geotechnical investigations There are nine geotechnical engineering firms (most of them are branch offices of Sydney based firms) advertising in the 1996 Wollongong Yellow Pages telephone directory and a lot of geotechnical engineering work is being done locally. A significant proportion of this work does include the assessment and treatment of land instability. A limited amount of this information has been made available to the author during the course of this project by Longmac Associates, Coffey Partners International, Golder Associates and individuals who commissioned various investigations. However, many of the reports remain beyond the reach of the public domain for reasons which include client confidentiality and the commercial interests of the private companies and individuals involved.

3.10.9 Rail Services Authority geotechnical investigations The Rail Services Authority (RSA) has recently been established from part of a previous organisation, known at the State Rail Authority (SRA). A business unit of this authority, known as the Railway Geotechnical Services, formerly known as simply Geotechnical Services has undertaken several noteworthy geotechnical land instability investigations within the subject area over the years. These investigations specifically

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concern land instability problems associated with the passage of the dual electric South Coast Railway Line (SCR) easement through the Hacking River Valley and escarpment slopes. The stability problems they have encountered include natural instability, instability induced by placing fill, embankment construction, excavation, and some possible mine subsidence induced problems.

Plate 3.2. April 1997 view to the southeast over the reconstructed Coledale railway embankment.

Two lives were lost in 1988 during an active advancing composite extremely rapid very wet debris flow, along Rawson Street, which originated in the railway embankment adjacent to Coledale Railway Station. In addition to the two fatalities, one house was destroyed and one track of the dual line was closed for an extended period, restricting traffic flow on the line. The debris flow occurred during an intense rainfall period, during which flooding occurred on the upslope side of the line due to a blocked culvert. Following this disaster, tens of millions of dollars have been spent by the Rail Access Corporation on geotechnical investigations, upgrading of the track and remedial works along the railway easement. Whilst such works were proceeding on a lower scale before the disaster, additional funds become available following the event for an increased and accelerated effort to reduce the hazard and risk associated with slope instability. An April 1997 aerial view of the Coledale Station and reconstructed embankment with the adjoining residential area is shown in Plate 3.2. It is interesting to compare this plate with Plate 2.2. 3-39 Chapter 3: Review of work concerning the study area

The Railway Geotechnical Services has a database of past and present (active) geotechnical problem sites relating to NSW railways, totalling, as of mid 1996, in excess of 1000 sites (pers com Christie, 1997). Of these, 133 sites are situated within the subject area. Of these 133 sites, 85 have been stabilised following geotechnical investigations and installation of a range of remedial works. Whilst the Railway Geotechnical Services has and is currently conducting many geotechnical investigations, they have previously employed consultants to investigate and report on some specific problem sites and areas within the Illawarra. One of these specific investigations is discussed as a case study site in chapter 9. Reference is limited here to several significant regional studies. Smith (1964), an SRA drainage engineer, compiled a set of drainage works plans (sketches) for the south coast line between Helensburgh and Thirroul, over the period 1950 to 1964. These plans provide an excellent historical record of land instability which affected the line over this period. Smith documented some remedial works with sketches in these plans. Longmac Associates Ltd Pty (1989) prepared a report for the then State Rail Authority titled Engineering Study - Stage 1, for the South Coast Railway, Helensburgh to Thirroul stations. This study, presented in two volumes (Volume 1 - text, volume 2 - plans) detailed 50 ‘problem’ sites. This information was assessed using a risk category approach. This approach considered the site features, known history and referred to the risk of an event affecting the track and/or public safety. It did not refer to the probability of a certain landslip event actually occurring. It is appropriate here to draw attention to an important historical note. Shellshear (1890) discussed land instability which affected the original alignment of the SCR. The location which Shellshear described at chainage 33 miles on the south side of Stanwell Park, as shown on Figure 3.9, is now occupied by Lawrence Hargrave Drive. He also described the remedial works that were carried out which comprised of manually excavated trenches and drives backfilled with earthen ware pipes and hand packed stone. This was an elaborate system for underground drainage and an inspired approach at a time when the discipline of soil mechanics was unknown and the principle of effective stress had not even been discovered. This site has not experienced significant instability since that time.

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Figure 3.9. Shellshear (1890) discussed the treatment of slip land near the cliff edge on the Illawarra railway line at chainage 33 miles (now approx. 53.8km) south of Sydney; a) Location plan showing position of railway, road and subsurface drainage lines. Position of railway and road is now reversed, b) Cross sections of drainage trenches/drives 3 and 4.

3.10.10 RTA geotechnical investigations The Roads and Traffic Authority (RTA) is the State Government body which administers the major arterial roads and highways within New South Wales. Within the study area, the RTA is in charge of the F6 Freeway (which includes Mount Ousley Road), the Princess Highway (which includes Bulli Pass), Lawrence Hargrave Drive, and the Northern Distributor. Until approximately 1995, they also looked after Mount 3-41 Chapter 3: Review of work concerning the study area

Kiera Road and Clive Bissell Drive. These two roads are now under the jurisdiction of the WCC. Of these roads, the F6 Freeway and in particular the Mount Ousley Road area, and the Princess Highway and in particular the Bulli Pass area, and Lawrence Hargrave Drive from Stanwell Park to Coledale have all experienced destructive land instability. Over the last one hundred years, each road has been closed on numerous occasions for periods of one to six months, to allow for reconstruction works after landslides destroyed sections of the roads. As recently as Thursday 13th February 1997 at approximately 12.30am, during heavy and prolonged rainfall, the Macquarie Pass road (south of and outside the subject area), was affected by a debris slump as shown in Plate 3.3. This landslide affected the Wollongong lane in two places (the slide is located adjacent to a hairpin bend), and the road was closed for several weeks for repair. The remedial works cost $250,000 and included the construction of a 14m high retaining wall using gabion baskets backfilled with coarse basalt gravel, as shown in Plate 3.4. As with the RSA, the RTA (formerly the Department of Main Roads - DMR), has a Geotechnical Services Group which amongst many other areas of work, conducts geotechnical investigations on sites of land instability within the study area. These works are conducted in liaison with the District Office situated in Bellambi. In addition to their own investigations, the RTA also employs consultants to conduct investigations of some sites. The Illawarra District Office has compiled a database of 52 sites within their area of jurisdiction. Of these 52 sites, 40 are within the subject area considered during this research project. During 1988, a period of prolonged heavy rainfall, Lawrence Hargrave Drive between Clifton and Coalcliff was closed due to damage from rock falls, debris flows and other landslides. Aerial photographs of this section of Lawrence Hargrave Drive are shown in Plates 3.5, 3.6 and 3.7. The rockfall hazard is clearly evident in Plate

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(a)

(b)

Plate 3.3. A debris slump near the top of Macquarie Pass occurred on Friday 14th February 1997, at approximately 12.30am during heavy rainfall; (a) The rear main scarp and damage to the pavement of Macquarie Pass, (b) The debris slump viewed from below.

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Plate 3.4. Remedial works comprised of a retaining wall constructed with gabion baskets to repair a debris slump near the top of Macquarie Pass. The remedial works cost $250,000 and were completed in approximately 3 weeks.

Plate 3.5. Vertical aerial view of Lawrence Hargrave Drive between Clifton and Coalcliff, the northern amphitheatre.

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Plate 3.6. Vertical aerial view of Lawrence Hargrave Drive between Clifton and Coalcliff, the central amphitheatre. Note rock fall and debris flow paths.

Plate 3.7. Oblique aerial view to the northwest over Lawrence Hargrave Drive between Clifton and Coalcliff, the southern amphitheatre. Hanlon referred to this area as ‘Clifton Hill’.

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Plate 3.8. One of several dramatic landslides, a debris slump, which closed Lawrence Hargrave Drive between Clifton and Coalcliff in 1988 (Construction Australia 1988).

Plate 3.9. Slot drainage remedial works underway to repair a landslide on Lawrence Hargrave Drive between Clifton and Coalcliff, 1988 (Construction Australia 1988).

3.6. The road was closed at the end of April, and reopened in November. During this time, the RTA spent five million dollars on stabilisation and road reconstruction, on

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approximately two kilometres of roadway. One of the quite dramatic debris slump landslides is shown in Plate 3.8. A stage in the installation of one longitudinal trench or slot drain is shown in Plates 3.9.

3.10.11 Wollongong City Council internal landslide hazard maps The WCC has its own hazard maps as a series of map layers within their Geographic Information System (GIS) computer package. These maps are confidential and strictly for internal use within the WCC offices. Hence, the writer has little information regarding these maps. These plans include and distinguish between numerous local hazards such as known past land instability, potential land instability, landfill and areas subject to flooding (pers. com. Peter Tobin, 1995). Areas of recorded ‘landslip’ and ‘potential landslip’ are identified with shading of different colours. However, the writer understands that not very much information has been recorded by the WCC regarding the source of the identification of ‘landslip’ or ‘potential landslip’ areas, let alone specific technical and other information regarding each ‘landslip’ site.

3.11 RAINFALL VERSUS OCCURRENCE OF LAND INSTABILITY It is now well known that rainfall leads not only to saturation of soil but also to elevation of pore water pressures which, in turn, decreases the effective strength at different locations within a slope. It is also well known that prolonged periods of rainfall, often in combination with short duration high intensity rainfall events are common triggering events for land instability. Young (1976) summarised literature which supports this contention across a wide variety of climates-humid tropics, temperate regions, arid areas and high mountains. Locally, several authors have estimated a relationship between rainfall and the onset of land instability, whilst some have attempted to establish the relationship between rainfall events, and specific magnitudes of antecedent rainfall and the onset of land instability. Some of the conclusions are presented in the following section. This aspect of landslide research is taken up in more detail, in Chapters 8 and 9 as part of this research project.

3.11.1 Bowmans work (1972) In his documentation of ground movements at Thirroul Public School, Bowman (1972a), plotted monthly rainfall totals with reported ground movements and damage 3-47 Chapter 3: Review of work concerning the study area

over the period 1950 to 1970. In addition, he plotted rainfall against 24 reported landslips within the City of Greater Wollongong, on an annual basis over the period 1948 to 1968. On the basis of these 24 reported landslips Bowman concluded that catastrophic slides invariably occur after a rainfall of over 430mm in 1 month, and slides often occur after monthly rainfall totals of 350mm. He added that slow slides and movement of existing slides may occur with lesser amounts of cumulative rainfall. Of the few slides that did not occur after periods of heavy rain, all could be related to external disturbance of the site by engineering earthworks. Bowman attributed the lack of landslides in some years of heavy falls, such as 1952, to incomplete records concerning landslide occurrence.

3.11.2 Young’s work (1976) Young noted the difficulty in compiling a complete register of Landslip in the Wollongong area, due to the inaccessible or uninhabited condition of some sites, and the reluctance of some residents to report damage as no compensation was available and simply making the report would be expected to cause property devaluation. Hence Young supplemented her records by examining all Illawarra Daily Mercury newspapers for all the months during which rainfall exceeded 250mm. Young then used these reports, in addition to WCC records and personal observations (including air photo interpretation) to compile landslip numbers to compare with rainfall records. This work of Young’s is included as Appendix 2, and has been extended by the writer to include the period up to 1991. Whilst noting the variation in rainfall station records, and the period of record, Young selected two stations, Mount Kiera Scout Camp and Albion Park for her analysis over the period 1890 to 1974. Young concluded that Bowmans 350mm critical magnitude of monthly rainfall was probably too high and hence not conservative. She estimated a critical value of 250mm rainfall per month as being likely to initiate landslip. In estimating this value, Young noted that it would be valid for a range of socially acceptable levels of risk. Young showed that this 250mm rainfall per month critical value had a 10% chance of occurring in 4 months of any year on the coastal plain and in 8 months of any year on the escarpment. To present this information in context, Young determined the maximum probable 24 hour rainfall in any year to be about 170

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mm on the escarpment. Furthermore, falls of 380mm per day and 550mm per day occur, on average once every 50 years. Young also noted the temporal and spatial variation in the balance between precipitation and evaporation within the Illawarra. She demonstrated that heavy daily and monthly falls are most common in summer, autumn and early Winter, with February and June often being particularly wet. As evaporation is lower at higher elevations, there is more water available for runoff and to enter into the groundwater on the upper slopes, thus accentuating the instability of these upper slopes.

3.11.3 Longmac Associates Pty Ltd In studying the relationship of rainfall with respect to landslide activity over the period 1988 to 1992, Longmac (1991, as reported by Pitsis, 1992) found a poor correlation with one month antecedent totals. They concluded that a three monthly period correlated better to current large scale landslides in their study area (Stanwell Park to Coledale). In the Longmac study, the maximum monthly and three monthly rainfall totals of the Coledale and Woonona Stations (combined records cover the period 1930 - 1990) were ranked on a year by year basis. In addition, this data was plotted against yearly recurrence interval. The ranking placed years of major instability 1950, 1956, 1961 (Bowman 1972), 1974 (Young 1976) and 1988, 1989 and 1990 (Pitsis 1992) in the top 10 for the 3 monthly totals. This was not the case for the one monthly totals, although some of these years were in the top 10. The magnitude of the top ten ranked 3 month totals varied from 1171mm to 865mm, whereas the top ten ranked 1 month totals varied from 708mm to 436mm. In a geotechnical report for the WCC (Longmac, June 1991), regarding a landslide at Morrison Avenue, Coledale, a threshold 3 monthly rainfall to trigger instability between about 550mm and 650mm is quoted. This corresponds to a relatively low return period of two to three years. In a geotechnical report for the State Rail Authority of New South Wales regarding a landslide in Scarborough, Longmac Associates (April 1991) reported that the landslide was triggered by a 1 month antecedent rainfall total exceeding 350mm, and that movement of the landslide was maintained by a 1 month antecedent rainfall total 230mm.

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3.11.4 Pitsis Pitsis (1992) summarised the major periods of land instability, and noted the concurrence of these periods of land instability and periods of extreme rainfall. Pitsis noted that most documented mass slope failures appear to have occurred during high intensity events of 400 - 500 millimetres over 24 hours, which have occurred within a long duration rainfall period. Pitsis suggested that erosional scouring and flooding rather than reactivation of ‘major land slip’ result from high intensity short duration rainfall events, such as the June 1991 and February 1992 events. Based on observations over the period 1988 to 1992, he concludes that prolonged rainfall acts to ‘top up’ the phreatic water surface until a critical threshold is reached and landsliding occurs. This contrasts with a high intensity rainfall event during low rainfall periods where the water mostly runs off and tends to cause flooding, and failure by scour rather than landsliding.

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