Geotechnical Report

AUSTRALIAN GEOGUIDE LR8 (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.

EXAMPLES OF GOOD HILLSIDE CONSTRUCTION PRACTICE

  
  
  
  
  
 
 
 
 
  
  
  

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Vegetation retained
‘Surface water interception drainage \ of
Walertight, adequately siled and founded roof water storage \
tanks (with due regard for impact of potential leakage) —____—— \g
Flexible structure
Roof water piped off site or stored ———__——

On-site detention tanks, walertight and adequately
founded. Potential leakage managed by sub-soil
drains

"Pier footings into rock
~~ Subsoil drainage may be
fequired in slope
\ ~ Cutting and filling minimised in development
\ Sewage effiuent pumped out or connected to sewer.
Tanks adequately founded and watertight. Potential
leakage managed by sub-soll drains
~\___ Engineered retaining walls wth both surface and fs
subsurface drainage (constructed before dwelling)
©) AGS (?007)
Ss ‘Son also AGS (2000) Appanaix J

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 LR8) 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 LRS).

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

174 Australian Geomechanics Vol 42 No 1 March 2007