Changes Application - MCU15/0023.02

                    AUSTRALIAN GEOGUIDE LR8 (CONSTRUCTION PRACTICE)
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IHlllSIDE     CONSTRUCTION            PRACTICE

Sensible development practices are required when building on hillsides, particularly if the hillside has more than a low
rlsk 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
                                                                                               Vegetation retained




                                                Flexible structure ~

                 Roof water piped off site or stored ----~                        '"

                 . detentlon
            On-Slte        . tan k's, watertight and adequate Iy          \ '""        '
            founded. Potential leakage managed by sub-soil          ,.~
            drains -------

                                                                                                                     '-     MANTLE OF SOIL AND
                                                                                                                            ROCK FRAGMENTS
                                                                                                                            (COLLUVIUM)
                                                                          tI                              '-  Pier footings into rock
                                                                       .'! ~           \ I              Subsoildrainagemay be
                                                                                           l            required in slope
                                                                ,         ~                    Cutting and filling minimised in development
                                                                                  Sewage effluent pumped out or connected to sewer.
                                                                                  Tanks adequately founded and watertight. Potential
                                             :---........'---...                  leakage managed by sub-SOil drains

                                                            ~       Engineered retaining walls with both surface and
                                                                    subsurface drainage (constructed before dwelling)
                                       BEDROCK
                                                                                                                                            @AGS(20071
                                                                                                                          See llIsa AGS (2000) AlJfM'nQiJcJ


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 indude
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 likeiy 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 tum
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 iandslide 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 anyone 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