210533 - 108 Black Mountain Range Road, BLACK MOUNTAIN - Slope Stability Report

Total Concept Homes                                                                              20332-003-Rev0
                                                                                                    16 June 2022

Table 3: Retaining Wall Design Parameters

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

Fill Embankment
                            *           *                 *                     *                     *
Materials

Stiff (or stiffer)
                           18          26               0.39                  0.56                   2.56
Natural Clays

Very Low Strength
(or stronger)              21          35               0.27                  0.43                   3.69
Weathered Phyllite
*depends on type of material and compaction achieved

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
Adequate site drainage should be installed to ensure that stormwater runoff is directed away from the
building walls and footings (e.g. swales in landscaped areas or grated drains across pavements up slope of
structures including retaining walls for the driveway). Diversion drainage should be installed above the crest
of all cut/fill batters, and toe of all cut batters. Uncontrolled flow of stormwater over embankment batters (e.g.
from gutter overflow during high intensity storms) has the potential to saturate and weaken the embankment
materials, potentially leading to slumping failure. Future building and landscaping must be carried out to
maintain good drainage.

Specific drainage recommendations are as follows:

          Install diversion drainage (e.g. turfed, rock lined or concrete spoon drain) along the upslope side of
           the proposed granny flat.
          Install diversion drainage along the upslope side of any effluent disposal areas to prevent stormwater
           migrating into shallow subsurface trenches.

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.
All excess stormwater collected on site must be directed by pipes to the legal point of discharge, at least 5 m
down slope from any proposed/existing structures, embankments or effluent disposal areas. Surface
dissipation, such as a rock weir or similar, must be installed at the stormwater outlet(s) to minimse the risk of
soil erosion at the discharge point.

Subsurface disposal of stormwater must not be undertaken.



5.0        SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
impacting the proposed granny flat. This advice is subject to implementation of the recommendations given
in this report.




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