smiths lake planning study volume 1: text - Great Lakes Council

SOILS AND GEOTECHNICAL 22
2.6 Urban Capability And Development Guidelines - SLVA
2.6.1 Foundation Conditions
Subsurface conditions in the SLVA generally comprise clay soils overlying rock at a relatively
shallow depth, typically 0.5m to 1m with some areas of surface rock outcrop. The clay soils
and weathered rock materials noted in the SLVA are generally considered to be of adequate
bearing capacity for residential development.
Specific geotechnical assessment of soil bearing capacity should be undertaken in sand soil
areas of the SLVA (terrain units S1, S2, S3 and S4).
Excavation problems are likely to be encountered where rock is encountered in earthworks.
Testing of rock core recovered by the Department of Public Works for the sewer reticulation
indicates that the rock is of high to very high strength with tested unconfined compressive
strengths in the range of 50 to 150 MPa. The orientation and spacing of rock joints and
fractures will have a major influence on rock excavatability.
Based on the results of linear shrinkage testing and the highly plastic nature as presented in
Table 2-4, the clay soils present in the SLVA are likely to be reactive (expansive). Reactive
clays are soils that swell on wetting and shrink on drying, resulting in ground movements that
can damage lightly loaded structures founded on inappropriately designed footing systems. The
amount of ground movement is mainly related to the physical properties and depth of the clay
and environmental factors such as climate, vegetation and watering. A higher probability of
damage can occur on reactive sites where abnormal moisture conditions occur, due to factors
such as:
• Growth of trees too close to a footing or removal of large trees prior to construction.
• Lack of maintenance of site drainage, failure to repair plumbing leaks and excessive or
irregular watering of gardens adjacent to the house.
• Unusual moisture conditions caused by removal of structures, ground covers (pavements),
drains and dams etc.
Foundation design should be undertaken in accordance with AS2870 – 1996 Residential Slabs
and Footings. AS2870 – 1996 establishes performance requirements and specific designs for
common foundation conditions as well as providing guidance on the design of footing systems
using engineering principles.
2.6.2 Slope Stability
Slope stability zonation for the SLVA is shown on Figure 2-2 with zones defined on the basis
of risk of instability as defined in Table 2-14. Four zones have been defined:
• High Risk - steep (>25-40%) sand covered slopes (ie. in the area of Amaroo Drive and
along the foreshore below Ski Cove Street. Terrain units S3 and S4.
• High Risk - steep (>40%) soil and rock slopes located along some foreshore areas of Symes
Bay. Terrain unit R1.
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