Slope Stability - lindsays

AUSTRALIAN GEOGUIDE LR3 (LANDSLIDES IN SOIL)

an,

eel ~~
Landslide due to poorly supported excavation,
excessive fill depth, steep excavated
cut face or inadequately designed
shallow foundations

   

Landslide due
to erosion by
sea or river

Ancient landslide reactivated by ~~
extended rainfallor major reshaping

of the landscape
Figure 3
Some of the more common causes of landslides in soil are:
1) Falls of the parent material or residual soil from above, due to natural weathering processes (Figure 2).
2) Increased moisture content and consequent softening of the soil, or a rise in the water table. These can be due

to excessive tree clearance, ill-considered soak-away drainage or septic systems, or heavy rainfall (Figure 2).
3) Excavation without adequate support, increased surface load from fill placement, or inadequately designed
shallow foundations (Figure 3).
4) Natural erosion at the toe of the slope due to scour by a river or the sea (Figure 3).
5) Re-activation of an ancient landslide (Figure 3).

Most soil slopes appear stable, but they all achieved their present shape through a process of weathering and erosion
and are often sensitive to minor changes in the factors that affect their stability. As a general rule, human activities only
improve the situation if they have been designed to do so. Once this idea is understood, it is probably easy to see why
the following basic rules are so important and should not be ignored without seeking site specific advice from a
geotechnical practitioner:

° Do not clear trees unnecessarily.
e Do not cut into a slope without supporting the excavated face with an engineer designed structure,
° Do not add weight to a slope by placing earth fill or constructing buildings with inadequately designed shallow

foundations (Note: in certain circumstances weight is added to the toe of a slope to inhibit landslide movement,
but this must be carried out in accordance with a proper engineering design).

. Do not allow water from storm water drains, or from septic waste or effluent disposal systems to soak into the
ground where it could trigger a landslide.

More information in relation to good and poor hillside construction practice is given in GeoGuide LR8. With appropriate
engineering input it is often possible to reduce the likelihood, or consequences, of a landslide and so reduce the risk to
property and to life. Such measures can include the construction of properly designed storm water and sub-soil drains,
surface protection (GeoGuide LR5) and retaining walls (GeoGuide LR6). Design should be undertaken by a
geotechnical practitioner and will normally require local council approval.

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

GeoGuide LR1 - Introduction
GeoGuide LR2 - Landslides
GeoGuide LR4 - Landslides in Rock
GeoGuide LR5 - Water & Drainage
GeoGuide LR6 - Retaining Walls

GeoGuide LR7 - Landslide Risk

GeoGuide LR8 - Hillside Construction

GeoGuide LR9 - Effluent & Surface Water Disposal
GeoGuide LR10 - Coastal Landslides

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 whal 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 Sociely, 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.

Australian Geomechanics Vol 42 No 1 March 2007 165