Slope Stability Assessment

Jane & Cameron Blake                                                                                       22178-001-Rev0
                                                                                                              26 May 2022

4.4       Retaining Walls
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.
Preliminary design for retaining structures could be carried out using a pressure distribution applicable to the
wall type and based on the parameters listed in Table 3. Please note these values must be confirmed
following review of a suitable soil test. These values do not include allowance for hydrostatic pressures nor
transient (e.g. construction) or adjacent footing loadings.

Table 3: Retaining Structure Design Parameters

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


 Stiff (or stiffer) Clays         1.8          25                0.41                      0.58                2.46


 Weathered Rock                   2.0          35                0.27                      0.43                3.69


 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
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction areas.

Permanent site drainage must be installed to ensure that stormwater runoff and pool overflow is directed
away from structures and footings (e.g. positive gradients and/or channel drains), and also not allowed to
flow over adjacent fill areas. Uncontrolled flow of stormwater over fill batters (e.g. from pool overflow during
high intensity storms) has the potential to saturate and weaken the fill, potentially leading to erosion and
slumping failure. Specific drainage recommendations are as follows:

          To prevent the migration of stormwater into the proposed effluent disposal area, installation of
           diversion drainage (e.g. landscaped swale) must be completed along the upslope side of the effluent
           proposed disposal area(s).
          Diversion drainage (e.g. landscaped swale) should be constructed upslope of the central
           development areas.
          Formalised drainage systems (e.g. concrete culverts etc.) will be required across the driveway
           alignments if natural drainage lines are crossed. Concrete or rock lined channel drains should be
           constructed along the cut side of the proposed driveways.
          If gravel driveways are proposed, installation of ‘water bars’ across the driveway at intervals of not
           more than 10 m is recommended, with water directed into the channels drains.
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 include a
hydrostatic pressure relief valve, which must daylight at the ground surface.




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