Avocet Pde OPW - App E Slope · OPW20/0092

AUSTRALIAN GEOGUIDE LR4 (LANDSLIDES IN ROCK)

Rocks have been formed by many different geological processes and may have been subjected to intense pressure,
large scale distortion, extreme temperature and chemical change. As a result there are many oifferent rock types and
their condition vanes enormously. Rock strength varies and is often significantly reduced by the presence of
discontinuities (GeoGuide LR1). You may think that rock lasts forever, but in reality it weathers under the combined
effects of water, wind, chemical change, temperature variation, plant growth and animal activity and erodes with time.
Rock is often the parent maternal that ends up forming soil slopes (GeoGuide LR3). Inevitably different rocks have
different physical and chemical characteristics and they weather and erode to form different types of soil.

Weathering can lead to landslides (GeoGuide LR2) on rock slopes. The type of landslide depends on the nature of rock,
the way @ has weathered and the presence or absence of discontinuities. It is hard to generalise. though normally a
specific combination of discontinuities and material types will be the determining factor and these are often underground
and out of sight. Typical examples are provided in the figures 1 to 4. A geotechnical practitioner can assess the
landslide nsk and propose appropriate maintenance measures. This often entails making geological observations over
an area significantly larger than the site and a review of available background information, including records of known
landslides and aerial photographs. Depending on the amount of information available, geotechnical investigation may or
may not be needed. Every site is different and every site has to be assessed individually.

It is impossible to icte when a landslide will occur on a rock s but failure is normally sudden and
the coi wences can be catastrophic.

 

 

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Figure 3 - Block slide on wok layer SS Wolte failure along discontinuities

if the landslide risk is assessed as being anything other that Low, or Very Low, (GeoGuide LR7) @ may be possible to
carry out work aimed at reducing the level of risk.

The most common options are:

1) Trimming the slope to remove hazardous blocks of rock.

2) Bolting. or anchoring. to fix hazardous blocks in position and prevent movement.
3) Installation of catch fences and other rockfall protection measures to limit the impact of rockfalis.
4) Deep drainage designed to limit changes in the ground water table (GeoGuide LR5).

Although such measures can be effective, they need inspection and on-going maintenance (GeoGuide LR11) if they are
to be fective for periods equivalent to the life of a house. Design should be undertaken by a geotechnical
ctitioner and will normal! wire local council approval. !t should be appreciated that it may not be viable to
Carry out remedial works in all circumstances: for example where the landslide is on someone else's property. where the
cost is out of proportion to the value of the property, or where the risk inherent in carying out the work is actually greater
than the risk of leaving things as they are. In situations such as these, development may be considered inappropmate.

166 Australian Geomechanics Vol 42 No 1 March 2007