Degree Name

Doctor of Philosophy


Department of Civil and Mining Engineering


This thesis is concerned with understanding the engineering geologic factors influencing the stability of slopes along a coastal escarpment in the northern Illawarra region. The particular area covered by this study lies between Coalcliff and Clifton where a number of known or visible areas of slope instability are present. Slope instability in the study area is a function of the stratigraphy, structural geology, petrology, geomorphology, climate and mechanical properties of the rock and soil.

In the northern Illawarra region the essentially flat-lying lower Narrabeen Group conformably overlies the upper Illawarra Coal Measures and the strata consist of repeated beds of sandstone, shale, claystone and coal seams. The lower Narrabeen Group consists of thick sequences of weak fine-grained rocks which are rather more easily eroded than the associated sandstone strata and hence relatively rapid rates of recession occur. Undermining along this the contacts between claystone and sandstone reduces the support for the overlying vertically-jointed sandstone and eventually leads to stabs falling off along the vertical joint faces. Thin marker beds (coal seams) in the Illawarra Coal Measures and sandstone beds in the Narrabeen Group commonly act as aquifers, with claystone beds acting as aquitards. Slope instability is usually related to the presence of the aquifers which are the source of high pore water pressures:- Perched water tables have been found to be quite common in the study area.

The topography along the escarpment is mainly steep and highly irregular because of past and present marine and fluvial erosion:- Large colluvial deposits have accumulated at the base of the steeper slopes. Generally, colluvial deposits are clay-rich containing abundant mixed-layer clay and smectite (montmorillonite).

Based on the petrological study, the Narrabeen group was derived from the New England Fold Belt to the north and consists predominantly of volcanic detritus. The volcanic detritus is present in both the sandstone and shale units either in form of detrital grains of volcanic rock or as fine volcanic ash- During post-depositional alteration and diagenesis the original volcanic glass in the ash and matrix of larger grains has devitrified to produce smectite clays. These clays not only cause swelling and shrinkage near the surface as a response to wetting and drying, but also reduce the permeability of the near the surface rock mass. This latter factor increases the aqueous pore pressures and hence increases the likelihood of surficial mass movement of both the rock mass and the adjacent talus deposits.

Based on X-ray diffraction, carbonates are mostly rare in the talus deposits. The natural reduction in carbonate cement due to weathering is one a cause for talus slope instability in the Illawarra area.

The high horizontal stress environment known to exist in the Illawarra area is an important factor which also influences slope failure. The resulting joint strike maxima for the lower Narrabeen Group show that the most prominent joint set exposed at the surface, with a direction between 005° and 025°, has a significant effect on slope stability in the study area.

Fracture permeability is also the most important feature of groundwater movements with it most of the fractures occurring in areas of stress relief near the face of the escarpment. It is quite obvious from studying the rainfall figures and periods of prevalence of landslides that the most unstable periods are those when the rainfall is above 400 mm per month.

A significant decrease in durability was found to accompany changes in mineralogy and an increase in weathering from fresh to weathered rocks. Moderately and highly weathered claystone and shale in the Narrabeen Group rocks have low to very low durability; it is dependent on their mineralogy, and especially on the type and quantity of clay minerals present. Claystone samples interbedded in the Bulgo Sandstone also show very low durability. In contrast, claystone interbedded in the Scarborough Sandstone shows a medium durability whereas claystone in the Coal Cliff Sandstone has a high durability. The differences in the behaviour of samples is that slake durability is sensitive to the abundance of clay minerals as opposed to carbonate in these samples.

Claystone interbeds in the Bulgo Sandstone and the highly weathered Stanwell Park Claystone both have very low durability. This has a significant effect on slope stability in the Bulgo Sandstone especially where the Stanwell Park Claystone acts as the bedrock for the talus mantle between Clifton and Stanwell Park.

The Wombarra Shale and Stanwell Park Claystone, two units of the Narrabeen Group, appear to dominate the study area as being the units most prone to instability problems. Failure surfaces of landslides are located at or near the base of highly weathered shale or claystone sequences.

A significant decrease in strength was also found to occur with an increase in weathering from fresh to weathered rocks. The geotechnical properties of the talus most related to its stability, are clay content, plasticity index and residual friction angle. These parameters and the angle of natural slopes show the talus is unstable in the longterm at slopes above 10-12°.

Man's construction activities have also caused some landslides in the northern Illawarra, especially along the excavations for the railway and road. Two main transport routes, the Illawarra Railway and Lawrence Hargrave Drive, pass through the study area. Along Lawrence Hargrave Drive major movement in gently sloping land has been triggered by high pore-water pressures in highly weathered Wombarra Shale covered by a talus mantle. Increased urban development has and will continue to complicate the issue in the future. Seven landslides have been detailed in this thesis. The majority of these have or are presently undergoing block type movements at creep rate. Detailed geotechnical investigations with survey monitoring is often necessary to identify these failures.

The area has also been extensively mined for coal, resulting in minor subsidence. This has usually caused fracturing of the rock strata and opening of the joint system which have increased water ingress, resulting in higher subsurface flows and altered groundwater regimes. Based on observations, mine subsidence has been one contributory factor to slope instability in the northern Illawarra