3. review of work on geology and land …web/@cedir/documents/doc/uow060561.pdfreview of work on...

Chapter 3: Review of work concerning the study area

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3. REVIEW OF WORK ON GEOLOGY AND LAND INSTABILITY

CONCERNING THE STUDY AREA

3.1 INTRODUCTION

Documentation and investigation of landslides in the Wollongong area has been dated

back to 7 January 1879 (Illawarra 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

Sydney. 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 Sandstone 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 Hacking River 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 Macquarie Pass 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 Lake Illawarra 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 (Young1976), (b) Annual average rainfall 1931-1960 (Young 1976), (c) Average annual rainfall Bureau ofMeteorology 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 Sydney Basin, 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 New South Wales 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 rock

descriptions of the rock formations between the Hunter River Valley in the north and the

Shoalhaven River 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 Permian

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 Mount Keira, 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 ‘WollongongSandstone 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 Carboniferous to Triassic. 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 Jurassic, 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 Australia 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 ofPermian sedimentation over eastern Australia; (b) Sydney-Gunnedah-Bowen Basin within New SouthWales; (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 Cretaceous 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 Illawarra Coal Measures (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 weathering, 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 quarry 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 sandstones comprising volcanic and lithic fragments, and thin interbeds of coal

and shale.

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 Mount Kembla 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 shales 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 Lawrence Hargrave 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 Lawrence Hargrave Drive (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 Bald Hill 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 Royal

National Park, within and east of Metropolitan Colliery 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 soil 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 Illawarra Escarpment.

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).”


3-23

• “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


3-24

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

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

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

1:50,000 * Wollongong Geological SeriesSheet 9029-11& 9028-1&1V NSW Department of Mines 1974

1:25,000 *Geology & Natural Slope Stability in the City of

Greater Wollongong, in Records of the Geological Survey of New South Wales Volume 14, Part 2

1972

1:25,000 Maps of the Coal Seam Structures in the Southern

Coalfield of NSW Australian Coal Industry Research Laboratories Ltd

1989

1:6336Geology Sheets - City of Greater Wollongong.

Geological Survey of New South Wales, Plans 5250-5286, 5545 a thesis by H. Bowman

1972

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


3-25

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


3-26

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 spatialreference is limited cadastre of main roads and the coastline.


3-27

Figure 3.6 Portion of Bowmans original scale 1:6336 Land Stability maps. Scarborough Sheet. The top ofthe escarpment crosses the upper left side of this segment, and Buttenshaw Drive diagonally crosses thepage.

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


3-28

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


3-29

Figure 3.7 Portion of Bowmans Geology maps enlarged and superimposed onto a 1:4000 scaleorthophotograph map of the Balgownie area. A clear lack of correlation between the geology and contourlines is visible.

of the area, which still remain essential reading for any geologist working in the area.


3-30

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 schoollandslide. 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 DescriptionStable land No landslip problems

Stable land with areas of minor slope instability

Normally moderately level land which is underlain by soil which is unstable in unsuitable topographic positions

Less stable Most of the land may be safely utilised although some areas are unsuitable. Generally more topographically elevated than land in the categories above

Moderately unstable Thorough investigation required before development. Generally topographically high - relief land underlain by potentially unstable material

Topographically unstable Topographically unstable for development owing to steep slope and/or topographic position and nature of soil

Essentially unstable Essentially unstable land. Best left undeveloped. Some areas may be developed 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


3-33

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


3-34

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

Zone I essentially stable (none found within study area). Detailed investigation of land within other zones may enable them to be reclassified as zone 1

Zone II most of the area has been apparently stable recently, but is potentially unstable Some areas identified on aerial photographs show some signs of recent instability

Zone III Potentially unstable bench area. Adjacent parts already affected by instability. This shows that a high potential exists for similar instability to develop

Zone IIIu Existing instability, bench area. Areas already affected by instability in historical past or as interpreted from aerial photographs

Zone IVa Potential instability - slopes mostly steeper than about 25º, forms a steeper slope area between adjacent bench area. Located on Scarborough Sandstone

Zone IVbPotential instability - slopes mostly steeper than about 25º, forms a steeper slope area

between the bench area above the flatter slopes below Located on or below the Coalcliff Sandstone

Zone V Potential instability - steep talus slopes. Located beneath the Hawkesbury sandstone cliff line on steep active slopes.

Zone VaPotential instability - bench area. Located on Stanwell Park Claystone and it has been

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 PartnersInternational (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

Bulli Pass. 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.


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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 Illawarrarailway line at chainage 33 miles (now approx. 53.8km) south of Sydney; a) Location plan showingposition 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


3-42

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, atapproximately 12.30am during heavy rainfall; (a) The rear main scarp and damage to the pavement ofMacquarie 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 debrisslump near the top of Macquarie Pass. The remedial works cost $250,000 and were completed inapproximately 3 weeks.

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


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

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


3-46

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

Plate 3.9. Slot drainage remedial works underway to repair a landslide on Lawrence Hargrave Drivebetween 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


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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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