Reference Documents

                                             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

Document Set ID: 22826802
Version: 1, Version Date: 23/02/2024