14169-001-Rev0 17 Webb Rd -Revised Draft · 132002.220527.1

                                     17 WEBB ROAD, SUNSHINE BEACH
                                     PROPOSED RESIDENCE

Considering the existing site conditions, and subject to the implementation of recommendations given in
Section 7, it is our opinion that there is a Low (L) Risk of global slope instability affecting the proposed
residence. 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 in Appendix C (taken from AGS 2007).


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

                                                                                   The likelihood of a failure through the medium
                                                                                   dense sands is assessed as Unlikely due to the low
1: Localised landslide through                                                     to moderate gradients, lack of groundwater and
medium dense to dense sands                                                        effective friction angle of the sand. Numerical
on 15° to 20° slope near house,       Unlikely        Medium             Low       modelling (Ref. Section 6.2.2) also indicates that an
with slide extending below the                                                     acceptable factor of safety against instability for this
foundation depth                                                                   scenario will be attained. Although the
                                                                                   consequence of instability could be Medium, the
                                                                                   resultant risk is Low as per AGS 2007.

                                                                                   Based upon current climatic conditions, it would be
                                                                                   Barely Credible that the 20 m to 30 m distant
                                                                                   foredune slope could be eroded to such an extent
2: Significant retrogressive
failure of foredune due to erosion
of toe during a high sea event(s)
leading to undermining of house
                                       Rare
                                                     T Major             Low
                                                                                   that the house could be undermined. However,
                                                                                   depending on the extent of sea level rise and
                                                                                   possible climate change over the design life of the
                                                  AF
                                                                                   building (at least 50 years), the likelihood of such a
foundations
                                                                                   failure could increase to Rare. Combined with a
                                                                                   Major consequence of such instability, the resultant
                                                                                   risk is Low as per AGS 2007.
Note:     Barely Credible = the event is inconceivable or fanciful over the design life
          Rare = the event is conceivable but only under exceptional circumstances over the design life
                                      R


6.2.2         Quantitative Assessment
                            D


In order to calculate the factor of safety (FOS) against slope instability for the proposed residence, numerical
analysis has been conducted using the Slope/W computer program for the assumed ‘worst case’ conditions.
We have assumed a ground surface profile of 10° through the building area, increasing to 20° for a 2 m high
slope, then flattening to 10°. The analysed subsurface profile comprises medium dense sand to 1 m depth,
then becoming dense sand. Note that we have not analysed the stability of the foredune slope as this is
located well east of the proposed house on public land.

As recommended in Section 7, footings and retaining walls will be founded in medium dense (or denser)
sands. A surcharge load of 20 kPa has been applied to replicate the proposed construction, which is
equivalent to a two storey house or 1m depth of fill.

No groundwater was encountered at the time of investigation. However, as a conservative measure to
account for possible ‘worst case’ transient (short-term) conditions, a piezometric groundwater level has been
included in the model at a depth of 1 m BGL.

Results of the numerical slope stability analysis are illustrated in Text Figure 10. The key points from our
analysis are summarised as follows:

         Shear strength and unit weight parameters adopted were based upon drained (or effective) soil
          shear strength parameters taken from the values recommended for similar materials in Appendix D
          of Australian Standard AS4678-2002 ‘Earth Retaining Structures’.
         Analysis was undertaken for a specified range of slip circle centroids and radii. Text Figure 10
          shows the slip circle with the lowest FOS value. Coloured zones indicate FOS contours.
         For long-term (drained) conditions, the minimum calculated FOS is 2.1.


26 February 2015
Report No. 14169-001-Rev0                                          9