RFI - slope stability reporting

RECOR Pty Ltd                                                                                         22393-001-Rev0
                                                                                                      26 October 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 3 may be adopted for retaining wall design. The values in Table 3 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 3.

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)


 Loose Sand                      17           30                 0.33                  0.50                   3.00


 Medium Dense (or
                                 18           34                 0.28                  0.44                   3.54
 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 4.4 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.

4.6     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. It is assumed that the pool floors will include
hydrostatic pressure relief valves, which must daylight at the ground surface.

All excess stormwater collected around the residence (including overflow from rainwater tanks, pools etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Southern Cross Parade. Subsurface disposal of stormwater, such as by an infiltration pit, must not be
undertaken.




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