20220109 - Geotech · OPW23/0182

                             PROPOSED RESIDENCE
                             21 MAHER TERRACE, SUNSHINE BEACH

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.

5.2.1       Quantitative Assessment
In order to estimate the factor of safety (FOS) against slope instability for the proposed development,
numerical analysis has been conducted using the software Slope/W. The ground surface profile has been
modelled using a combination of survey information, preliminary design drawings and slope angles
measured in the field by Tectonic. The analysed subsurface profile was compiled based on the results of our
subsurface investigation. Some inferences regarding subsurface conditions were made between test
locations. A minimum FOS value of 1.3 and 1.5 is the commonly adopted industry standard for short- and
long-term conditions, respectively.
Although no groundwater was encountered during the investigation, a piezometric line was modelled at
about 1 m below the future ground surface to replicate possible groundwater development for short-term
conditions. Should groundwater be encountered during the additional investigation, subsurface drainage has
been recommended in Section 6.6.

To replicate future landscaping, a surcharge load of 5 kPa has been applied upslope of the proposed
building. Although building loads were not available at the time of reporting; a gross building load of 20 kPa
has been applied to areas of the building likely founded using high-level footings. Other areas will be
supported by piled foundations in the underlying competent materials.

Soil shear strength was based upon effective parameters which were estimated based on our interpretation
of the subsurface investigation results, while also considering the values recommended for similar materials
in Appendix D of Australian Standard AS4678-2002 ‘Earth Retaining Structures’. Adopted soil parameters
are shown on the Slope/W outputs presented in Text Figures 10 and 11 on the following page. Analysis was
undertaken for a specified range of entry and exit points that could impact the proposed development.

The analysed profile represents the critical section (i.e. ‘worst case’ scenario) for the proposed development,
which covers the deepest cut. Our analysis indicates that the proposed development will achieve at least the
required minimum FOS for short- and long-term conditions, that being, 1.3 and 1.5, respectively.

It should be noted that this modelling addresses the global stability of the proposed works; however, the
stability of the retaining walls must be addressed by others as part of detailed design.




16 July 2021
Report No. 21236-001-Rev0                              9