Slope Stability - lindsays

AUSTRALIAN GEOGUIDE LR3 (LANDSLIDES IN SOIL)

Landslides occur on soil slopes and the consequences can include damage to property and loss of life. Soil slopes exist
in all parts of Australia and can even occur in places where rock outcrops can be seen on the surface. If you live on, or
below, a soil slope it is important to understand why a landslide might occur and what you can do to reduce the risk it
presents.

It is always worth asking the question ‘why is this slope here?", because the answer often leads to an understanding of
what might happen in the future. Slopes are usually formed by weathering (breakdown) and erosion (physical
movement) of the natural ground - the "parent material. Many factors are involved including rain, wind, chemical
change, temperature variation, plant growth, animal activity and our own human enthusiasm for development. The
general process is outlined in Figure 1.

The upper levels of the parent material progressively weather over thousands, or millions, of years, losing strength. This
can result in a surface layer which looks similar to the parent material (although its colour has probably changed) but has
the strength of a soil - this is called “residual soil". At some stage the weathered surface layer is exposed to the
elements and fragments are transported down the slope. In this context a fragment could be a single sand grain, a
boulder, or a landslide. The time scale could be anything from a few seconds to many thousands of years. The
transported fragments often collect on the lower slopes and form a new soil layer that blankets the original slope -
"colluvium". If material reaches a river or the sea it is deposited as "alluvium" or as a "marine deposit". With appropriate
changes in river and sea level this material can again find itself on the surface to commence another cycle of weathering
and erosion. In places often, but not only, near the coast, this can include sand sized fragments which form beaches and
are sometimes blown back onto the land to form dunes.

As

 
 
 
    
 

* r *s
Weathered parent material
(residual soil)

Fragments of parent material
transported down slope

Parent .
jure 2

raatarlal Remnant

ancient
landslide

  
 
 
 
 
  
  

River

Collected weathered orsea

fragments (colluviu -
Water or wind deposited soils
(alluvium, marine deposits, or dunes)

Figure 1
Landslides can occur almost anywhere on a soil slope. Slides can be rotational, translational, or debris flows (see
GeoGuide LR2) and may have a number of causes.

‘ea.
away ws
Z Me

 

Landslide from parent material
or residual soil further uphill

 
 
 
 

Landslide due to increased
water pressures, or
softened soil

  

Rise in water table, or an

increase in water content can
be due to tree clearance, installi
soakaway drains, or a period of heavy rain =

Figure 2

164 Australian Geomechanics Vol 42 No 1 March 2007