Slope Stability Assessment

Paul & Janelle DeSouza                                                                                   22198-001-Rev1
C/- Sarah Waller Architecture                                                                               25 July 2022

Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
                                                              Assessed
 Hazard                          Likelihood   Consequence
                                                                Risk
                                                                         Comments

                                                                         The likelihood of a relatively shallow failure through
                                                                         the fill embankment or otherwise in stiff natural
                                                                         residual soils is assessed as Unlikely due to the
                                                                         site preparation having removed all loose/softened
                                                                         upper soils; the limitations on fill height and
 1: Shallow landslide proposed
                                                                         recommended quality, consistency of the residual
fill or through stiff natural     Unlikely      Medium             Low
                                                                         clay soils, lack of permanent groundwater, and
residual soils
                                                                         anticipated minimal surface water ingress to the site.
                                                                         Given footings are to be founded in sandstone rock,
                                                                         a potential Medium impact from such instability is
                                                                         assessed, with the resultant risk to be Low as per
                                                                         AGS 2007.


                                                                         The likelihood of a deep failure through the more
                                                                         competent weathered sandstone (below bucket refusal
                                                                         depths, based on Ground Drill excavation logs) is
                                                                         assessed as Barely Credible because of the strength
 2: Deep failure below
                                  Barely                                 of these materials and no evidence of such instability
proposed foundation depth in                     Major        Very Low
                                 Credible                                within this geology in similar conditions nearby.
weathered sandstone
                                                                         Although the consequence of such a failure could be
                                                                         Major, given that the entire residence could be
                                                                         affected, the resultant risk is Very Low as per AGS
                                                                         2007.




The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts.



4.0     RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.

It is noted that potentially significant fill earthworks would be proposed as per the current drawings (Ref.
Section 1.1), with fill up to 3 m possibly indicated to the north-western, northern and north-eastern fringes of
the footprint. The ground slope on the flanks of the knob range from about 10° to 20° in areas affecting the
building envelope.
From a stability perspective, no fill greater than 1 m height on this ground would be permitted without full
face support with an engineer designed retaining wall. It is not anticipated that the extensive retaining works
required would be acceptable from a cost or aesthetic perspective, and thus optimisation of the earthworks
should be undertaken as part of the detailed design as the project progresses. It is envisaged that
undertaking a greater depth of cut earthworks may provide some gains (including cost savings on footings),
possibly in conjunction with tiered retaining walls in area of slightly deeper fill as part of landscaping in
deeper fill areas.

Finalised plans including proposed earthworks should be reviewed by Tectonic prior to construction.

4.1   Earthworks
All fill greater than 1 m height should be fully supported by an engineer designed and certified retaining wall.

Any loose or soft soils, organic rich topsoil and severely root affected soils, must be removed from the
proposed construction area. Any fill materials utilised should be free of organic or otherwise unsuitable
materials and compacted at moisture contents within the range of -2 % to +2 % of optimum moisture content
for Standard Compaction.




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