Reference Document - OPW25 0085 (AS Published)

Table 5: Retaining Wall Design Parameters

                                                               Lateral earth pressure coefficients
                               Unit            Friction
    Retained material          weight          angle           Ka                  Ko                Kp
                               (kN/m3)         (Degrees)       (Cantilever wall)   (Non-yielding
                                                                                   wall)
    Loose/Firm Residual
                                    17              23                0.44               0.61             2.28
    Soils
    Medium Dense/Stiff
                                    18              25                0.41               0.58             2.46
    Residual Soils
    Very Low Strength
    (or Stronger)                   21              32                0.31               0.47             3.25
    Sedimentary Rock
    Future Fill                      *               *                 *                  *                *

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

At a minimum, retaining structures must be founded as per the recommendations in Section 6.3.2 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.


6.5               Drainage
Construction activities may be affected by groundwater, as seepage may occur particularly through the
upper sand which would worsen if works are undertaken during or following a prolonged/heavy rainfall
event. Control of seepage during construction activities may be achieved by grading the base of the
excavations towards a suitable discharge point. Where groundwater seepage is encountered in subgrade
areas it will be necessary to install subsurface trench underdrains to promote drainage across road and
building areas.
During construction temporary open swales and diversion drainage should be installed to divert seepage
and/or runoff away from pavement and building areas, to prevent saturation of the subgrade.
Specific drainage recommendations are as follows:
      Construct diversion drainage along the upslope edge of pavement and building areas (e.g. turf,
       concrete or rock lined drain or similar) to discharge any captured stormwater from the development
       area downslope of any structure, embankment or effluent disposal area.
      Grade any building platforms (minimum 1V:100H) to achieve positive drainage into edge drains.
      To prevent the migration of stormwater into the proposed effluent disposal area, install diversion
       drainage (e.g. landscaped swale) along the upslope side of the proposed disposal area.
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. Any pools must include a hydrostatic pressure
relief valve (or similar), which must daylight at the ground surface.


Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                                 21