Geotechnical Report

Some typical landslides which could affect residential housing are illustrated below:

 Rotational or circular slip failures (Figure 1) - can occur on moderate
 to very steep soil and weathered rock slopes (Table 1). The sliding
 surface of the moving mass tends to be deep seated. Tension cracks
 may open at the top of the slope and bulging may occur at the toe.
 The ground may move in discrete "steps" separated by long periods
 without movement. More rapid movement may occur after heavy
 rain.                                                                                                     Figure 1

 Translational slip failures (Figure 2) - tend to occur on moderate to
 very steep slopes (Table 1) where soil, or weak rock, overlies stronger
 strata. The sliding mass is often relatively shallow. It can move, or
 deform slowly (creep) over long periods of time. Extensive linear
 cracks and hummocks sometimes form along the contours. The
 sliding mass may accelerate after heavy rain.


                                                                                                           Figure 2

 Wedge failures (Figure 3) - normally only occur on extreme slopes, or
 cliffs (Table 1), where discontinuities in the rock are inclined steeply
 downwards out of the face.

 Rock falls (Figure 3) - tend to occur from cliffs and overhangs (Table
 1).

 Cliffs may remain, apparently unchanged, for hundreds of years.
 Collections of boulders at the foot of a cliff may indicate that rock falls
 are ongoing. Wedge failures and rock falls do not "creep". Familiarity
 with a particular local situation can instil a false sense of security since
 failure, when it occurs, is usually sudden and catastrophic.
                                                                                                           Figure 3



 Debris flows and mud slides (Figure 4) - may occur in the foothills of
 ranges, where erosion has formed valleys which slope down to the
 plains below. The valley bottoms are often lined with loose eroded
 material (debris) which can "flow" if it becomes saturated during and
 after heavy rain. Debris flows are likely to occur with little warning;
 they travel a long way and often involve large volumes of soil. The
 consequences can be devastating.

                                                                                                           Figure 4

More information relevant to your particular situation may be found in other Australian GeoGuides:

          GeoGuide LR1     - Introduction                                         GeoGuide LR7 - Landslide Risk
          GeoGuide LR3     - Soil Slopes                                          GeoGuide LR8 - Hillside Construction
          GeoGuide LR4     - Rock Slopes                                          GeoGuide LR9 - Effluent & Surface Water Disposal
          GeoGuide LR5     - Water & Drainage                                     GeoGuide LR10 - Coastal Landslides
          GeoGuide LR6     - Retaining Walls                                      GeoGuide LR11 - Record Keeping


The Australian GeoGuides (LR series) are a set of publications intended for property owners; local councils; planning authorities; developers;
insurers; lawyers and, in fact, anyone who lives with, or has an interest in, a natural or engineered slope, a cutting, or an excavation. They
are intended to help you understand why slopes and retaining structures can be a hazard and what can be done with appropriate professional
advice and local council approval (if required) to remove, reduce, or minimise the risk they represent. The GeoGuides have been prepared
by the Australian Geomechanics Society, a specialist technical society within Engineers Australia, the national peak body for all engineering
disciplines in Australia, whose members are professional geotechnical engineers and engineering geologists with a particular interest in
ground engineering. The GeoGuides have been funded under the Australian governments’ National Disaster Mitigation Program.


February 2019