Response to Information Request - MCU12/0184

                          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.


        EXAMPl-ES OF GOOD HILLSIDE CONSTRUCTION PRA’CTICE

                                                            Surface water interception drainage

               Watertight, apequately sited and fo,unded roof water storage
               tanks (with due regard for impact of potential leakage)

                                                       Flexible structure

                     Roof water piped off site or stor d


           Do-site detention tanks, w tertight and adequately
                                                                      .




           founded. Potential leakage manageq y .stib-!oil
           drains

                                                                                                                             OF SOIL AND
                                                                                                                  ROCK FRAGMENTS
                                                                                                                  (COLLUVIUM)
                                                                                                         Pier footings into rock

                                                                                                            drainage may be
                                                                                                    required in slope
                                                                                          Cutting and filling m n mised in development
                                                                                   Sewage effluent pumped out or connected to sewer.
                                                                                   Tanks adequately founded and watertight. Potential
                                                                                                                                      t
                                                                                   leakage managed by sub-soil drains

                                                                            Engineered retaining walls with both surface and
                                                                                                                                 .




                                                                            subsurface drainage (constructed before dwelling)
                                            ,BEDROCK
                                                                                                                                @AGS(2007}
                                                                                                                 See also AGS (2000)Appendix 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 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 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 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