Geotechnical Stability Report · OPW24/0017

GMW Projects Pty Ltd                                                                                  24008-001-Rev0
                                                                                                      31 January 2024

Table 3: Retaining Structure Design Parameters

                                                                        Lateral earth pressure coefficients
                                Unit       Friction
 Retained material            weight       angle Ø                                     Ko
                                                                Ka
                              (kN/m3)         (°)                                 (Non-yielding               Kp
                                                         (Cantilever wall)
                                                                                      wall)


 Very Loose/ Loose
                                 17           28                 0.36                  0.53                   2.77
 Sands


 Medium Dense (or
                                 18           33                 0.29                  0.46                   3.39
 Denser) Sands



 Future Fill                     *             *                  *                      *                     *


Note:    *       depends on fill material type and level of compaction

As a minimum, retaining structures must be founded as per the recommendations in Section 4.3 above with
the footings also designed to account for lateral loads imposed by the retaining wall. Detailed design must
include an assessment of internal and global stability, together with required bearing capacity and founding
depth, by the retaining wall design engineer.

4.5     Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment (e.g.
grassed or landscaped swales or diversion mounds). Adequate site drainage must be installed to ensure
that stormwater runoff is directed away from buildings, walls and footings. Although unlikely, if groundwater
seepage is encountered during excavations, Tectonic must be notified to assess the impact on stability.

Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure. It is assumed that the pool floor will have a hydrostatic
pressure relief valve, which must daylight at the ground surface away from the proposed and existing
footings.
All excess stormwater collected around the residence (including overflow from rainwater tanks, pool etc.)
must be directed by pipes or lined channels to discharge at least 5 m downslope of any structure or
embankment and towards the vegetated Reserve Area to the east.

Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.

5.0      CONCLUSION
It is assessed that there would be a Low Risk to the proposed extensions and carport from slope instability
subject to implementation of the recommendations given in this report.




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