220239 - 44 Williams Road, KIN KIN - Site Classification Report

                      AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE)
HILLSIDE CONSTRUCTION PRACTICE

Sensible development practices are required when building on hillsides, particularly if the hillside has more than a low risk of instability
(GeoGuide LR7). Only building techniques intended to maintain, or reduce, the overall level of landslide risk should be considered.
Examples of good hillside construction practice are illustrated below.




WHY ARE THESE PRACTICES GOOD?
Roadways and parking areas - are paved and incorporate kerbs which prevent water discharging straight into the hillside (GeoGuide
LR5).
Cuttings - are supported by retaining walls (GeoGuide LR6).
Retaining walls - are engineer designed to withstand the lateral earth pressures and surcharges expected, and include drains to prevent
water pressures developing in the backfill. Where the ground slopes steeply down towards the high side of a retaining wall, the disturbing
force (see GeoGuide LR6) can be two or more times that in level ground. Retaining walls must be designed taking these forces into
account.
Sewage - whether treated or not is either taken away in pipes or contained in properly founded tanks so it cannot soak into the ground.
Surface water - from roofs and other hard surfaces is piped away to a suitable discharge point rather than being allowed to infiltrate into
the ground. Preferably, the discharge point will be in a natural creek where ground water exits, rather than enters, the ground. Shallow,
lined, drains on the surface can fulfil the same purpose (GeoGuide LR5).
Surface loads - are minimised. No fill embankments have been built. The house is a lightweight structure. Foundation loads have been
taken down below the level at which a landslide is likely to occur and, preferably, to rock. This sort of construction is probably not
applicable to soil slopes (GeoGuide LR3). If you are uncertain whether your site has rock near the surface, or is essentially a soil slope,
you should engage a geotechnical practitioner to find out.
Flexible structures - have been used because they can tolerate a certain amount of movement with minimal signs of distress and
maintain their functionality.
Vegetation clearance - on soil slopes has been kept to a reasonable minimum. Trees, and to a lesser extent smaller vegetation, take
large quantities of water out of the ground every day. This lowers the ground water table, which in turn helps to maintain the stability of
the slope. Large scale clearing can result in a rise in water table with a consequent increase in the likelihood of a landslide (GeoGuide
LR5). An exception may have to be made to this rule on steep rock slopes where trees have little effect on the water table, but their
roots pose a landslide hazard by dislodging boulders.
Possible effects of ignoring good construction practices are illustrated on page 2. Unfortunately, these poor construction practices are
not as unusual as you might think and are often chosen because, on the face of it, they will save the developer, or owner, money. You
should not lose sight of the fact that the cost and anguish associated with any one of the disasters illustrated, is likely to more than wipe
out any apparent savings at the outset.

                                       ADOPT GOOD PRACTICE ON HILLSIDE SITES

Australian Geomechanics Vol 42 No 1 March 2007