Avocet Pde OPW - App E Slope · OPW20/0092

AUSTRALIAN GEOGUIDE LR2 (LANDSLIDES)

What is a Landslide?

Any movement of a mass of rock, debris, or earth, down a slope, constitutes a “landslide”. Landslides take many forms.
some of which are @ustrated. More information can be obtained from Geoscience Australia, or by visiting its Australian
Landslide Database at www 93 gov au/urban/factsheets/landslide jsp Aspects of the impact of landslides on buildings
are dealt with in the book “Guideline Document Landslide Hazards” published by the Australian Building Codes Board
and referenced in the Building Code of Australia. This document can be purchased over the internet at the Australian
Building Codes Board's website www_abcb.gov.au -

Landslides vary in size. They can be small and localised or very large. sometimes extending for kilometres and involving
millions of tonnes of soil or rock. It is important to realise that even a 1 cubic metre boulder of soll, or rock, weighs at
least 2 tonnes. If it falls, or slides, it is large enough to kill a person, crush a car, or cause serious structural damage to a
house. The maternal in a landslide may travel downhill well beyond the point where the failure first occurred, leaving
destruction in its wake. It may also leave an unstable slope in the ground behind @. which has the potential to fall again,
causing the landslide to extend (regress) uphill, or expand sideways. For all these reasons, both “potential” and “actual”
landslides must be taken very seriously. They present a real threat to life and property and require proper management.

identification of landslide risk is a complex task and must be undertaken by a geotechnical practitioner (GeoGuide LR 1)
with specialist experience in slope stability assessment and slope stabilisation.

What Causes a Landslide?

Landslides occur as a result of local geological and groundwater conditions, but can be exacerbated by inappropriate
development (GeoGuide LR8), exceptional weather, earthquakes and other factors. Some slopes and cliffs never seem
to change, but are actually on the verge of failing. Others, often moderate slopes (Table 1), move continuously, but so
slowly that it is not apparent to a casual observer. In both cases, small changes in conditions can trigger a landslide with
serious consequences. Wetting up of the ground (which may involve a rise in ground water table) is the single most
important cause of landslides (GeoGuide LR5). This is why they often occur during. or soon after, heavy rain.
inappropriate development often results in small scale landslides which are very expensive in human terms because of
the proximity of housing and people.

Does a Landslide Affect You?

Any slope, ciiff, cutting, or fill embankment may be a hazard which has the potential to impact on people, property. roads
and services. Some tell-tale signs that might indicate that a landslide is occurring are listed below:

trees leaning down slope, or with exposed roots
debris/fallen rocks at the foot of a cliff

open cracks, or steps, along contours
ground water seepage, or springs
bulging in the lower part of the slope tilted power poles, or fences
hummocky ground cracked or distorted structures

These indications of instability may be seen on almost any slope and are not necessarily confined to the steeper ones
(Table 1). Advice should be sought from a geotechnical practitioner if any of them are observed. Landslides do not
respect property boundaries. As mentioned above they can “run-out” from above, “regress” from below, or expand
sideways, so 3 landslide hazard affecting your property may actually exist on someone else's land.

Local councils are usually aware of slope instability problems within their jurisdiction and often have specific development

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and maintenance requirements. Your local council is the first place to make enquiries if you are responsible for
any sort of it or own or oc: on or near sloping land or a cliff.

TABLE 1 - Slope Descriptions

 

 

 

 

 

 

 

 

Appearance a ‘enum Slope Characteristics
Gentle O° - 10° 1on6 Easy walking.
Moderate 10°- 18° 1on3 Walkable. Can drive and manoeuvre a car on driveway
Walkable with effort. Possible to dnve straight up or down
Steep 18°- 27° 1on2 roughened concrete driveway, but cannot practically manoeuvre a
car.
Very Steep 27°- 45° toni Can only climb slope by clutching at vegetation, rocks etc.
Extreme 45°- 64° 10n 0.5 Need rope access to climb slope
Cliff 64°- 84° 10n 0.1 Appears vertical. Can absed down.
Vertical or Overhang | 84° - 902° infinite Appears to overhang. Abseller likely to lose contact with the face.

 

 

 

 

 

 

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

162 Australian Geomechanics Vol 42 No 1 March 2007