slope stability · MCU22/0172

Integra Partnerships                                                                                22339-001-Rev0
                                                                                                 13 September 2022

4.5     Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.4 above. Retaining
structures greater than 1 m high, or those required to support surcharge loads or sloping backfill, will require
engineer design and certification of construction. Retaining wall design should be carried out in accordance
with AS4678-2002 Earth retaining structures.
The parameters outlined in Table 4 may be adopted for retaining wall design. The values in Table 4 assume
that the walls are fully drained and no allowance is included for hydrostatic pressures, construction loading,
up-slope footing loadings, the effects of sloping ground and backfill compaction stresses. Any temporary
shoring required to support excavations during construction may also be designed using the parameters in
Table 4.
Table 4: Retaining Structure Design Parameters

                                                                      Lateral earth pressure coefficients
                                Unit       Friction
 Retained material            weight       angle Ø
                                                                 Ka                      Ko
                              (kN/m3)         (°)                                                           Kp
                                                          (Cantilever wall)      (Non-yielding wall)
 Very Loose/ Loose
                                17            28                 0.36                    0.53               2.77
 Sands
 Medium Dense (or
                                18            32                 0.31                    0.47               3.25
 Denser) Sands
 Future Fill                     *             *                  *                       *                  *
Note:    *       depends on fill material type and level of compaction

4.6     Drainage
Given the sandy nature of the site, rainfall is generally expected to rapidly infiltrate any unsealed surfaces. If
necessary, diversion drainage should be constructed to divert seepage and/or runoff away from the building
pad areas and batter slopes to prevent saturation and erosion.
Permanent site drainage must be installed to ensure that stormwater runoff is directed away from proposed
building walls, footings and retaining walls (e.g. positive gradients and/or channel drains). Future building
and landscaping must be carried out to maintain good drainage. Surface diversion drainage should be
constructed above the crest of any new cut or fill embankments (e.g. grassed or landscaped swales or
diversion mounds).
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.
All excess stormwater collected around the residence (including overflow from rainwater tanks, pool etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Grant Street.
Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.




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