Appendix F - Geo-Environmental Investigation Report · RAL24/0003

                                PROPOSED SUBDIVISION
                                41 TALLOWWOOD PLACE, BLACK MOUNTAIN

Table 3: Retaining wall design parameters
                                                                           Lateral earth pressure coefficients
                                Unit         Friction
 Retained material            weight          angle                                       Ko
                                                                  Ka
                              (kN/m3)       (Degrees)                                (Non-yielding               Kp
                                                           (Cantilever wall)
                                                                                         wall)

 Very Stiff (or Stiffer)
                                 19             27                  0.38                  0.55                   2.66
 Residual Clays


 Hard XW Materials               20             32                  0.31                  0.47                   3.25


 Very Low Strength (or
                                 21             35                  0.27                  0.43                   3.69
 Stronger) Phyllite


 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 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
We anticipate that construction activities should be minimally affected by groundwater, however some
seepage may occur. Control of seepage during construction activities may be achieved by grading the base
of the excavations towards a suitable discharge point.

During construction temporary open swales and diversion drainage should be installed to divert seepage
and/or runoff away from the building areas, to prevent saturation of the subgrade.

Specific drainage recommendations are as follows:
        Construct diversion drainage along the upslope edge of the building area (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.
        If the proposed driveway runs sidelong across the natural slope, diversion drainage be constructed
         above the proposed driveway cut(s).
        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.

All excess stormwater collected around the proposed development area, including the effluent disposal area,
must be directed by pipes or surface channels (e.g. rock lined ‘dry creek’ beds) to discharge at least 10 m
away from and downslope of any structure or embankment, and not towards the effluent disposal area. A
surface dissipation device such as rock weir or rock lined contour drain must be installed at the stormwater
outlet(s).
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).




22 May 2023
Report No. 23099-001-Rev0                                   18