Attachment 5 - Slope Stability Assessment · RAL20/0005

Mr Peter Loxton                                                                                            16162-001-Rev0
                                                                                                              22 July 2016

Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is our opinion that there will be a Very Low (VL) Risk of global slope
instability affecting the proposed addition. Regulators (NSC) normally require that a Very Low or Low Risk of
landslide affecting property must be demonstrated to enable development approval. Summarised in Table 2
below is our qualitative assessment of landslide risk for the site. A summary of qualitative terminology for
use in assessing risk to property is attached (taken from AGS 2007).
 Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)

                                                               Assessed
 Hazard                         Likelihood   Consequence
                                                                 Risk
                                                                          Comments

                                                                          The likelihood of shallow erosion of the loose sand
                                                                          is assessed as Possible on this site given the
 1: Shallow failure/erosion                                               geomorphological position. However, subject to
through loose sand above the    Possible     Insignificant     Very Low   implementation of the recommendations made in
foundation depth                                                          Section 4, the consequence of such shallow erosion
                                                                          would be Insignificant, with the resultant risk to the
                                                                          addition being Very Low.

                                                                          The likelihood of a deep failure through the medium
                                                                          dense (or denser) sand is assessed as Barely
 2: Deep failure through
                                 Barely                                   Credible due to the near level ground slope through
medium dense (or denser) sand                   Major          Very Low
                                Credible                                  the building area. Although the consequence of such
below the foundation depth
                                                                          a failure could be Major, 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.

4.1   Earthworks
New fill should not exceed 0.5 m depth and must be fully enclosed by an engineer designed structure (e.g.
under the carport slab as proposed). Given the position of the addition at the edge of a watercourse,
permanent cut batter slopes are not recommended.

Any organic rich topsoil and severely root affected soils must be stripped and removed from the building area.

Due to the wet and loose nature of the natural subgrade, it will likely be necessary to place a ‘rock blanket’
across any fill subgrades (as proposed in the structural design under the carport slab). This could comprise a
non-woven geosynthetic filter fabric (e.g. Bidim) on the natural soil (to prevent upward migration of ‘fines’ by
‘pumping’ during earthworks), then an imported rock fill product (e.g. 75 minus).

Fill materials should be compacted at moisture contents within the range of -2% to +2% of optimum moisture
content for Standard Compaction. Confirmatory testing must be carried out at regular intervals and further
details for control and testing of fill are given in Australian Standard AS 3798-2007 “Guidelines on Earthworks
for Commercial and Residential Developments”. Select fill should have a maximum particle size of 100mm for
an uncompacted layer thickness of 200mm and shall be compacted by repeated rolling with a small
compactor to achieve a dry density ratio of at least 95% of the Standard Maximum Dry Density for cohesive
soils, or 70% Dry Density Index for any imported cohesionless soils.

Temporary excavations in natural sands may be battered at a maximum slope of 1V:2H. Flatter batters may
be required if groundwater seepage is encountered, or if excavations are carried out during periods of wet
weather. Depending on the locations and depths of any excavations relative to existing structures, it may be
necessary to install temporary shoring (e.g. sheet piles).




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