Appendix G - Geotechnical Report

Total Concept Homes                                                                             20332-001-Rev0
                                                                                              15 December 2020

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


Stiff to Very Stiff
                          18           26                0.39                  0.56                   2.56
Natural Clays


Weathered Phyllite
                          21           35                0.27                  0.43                   3.69
Rock



These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).

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, or if formal arrangements are not in place for the retaining
wall on the neighbouring property to the south-west) 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.

Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).

All excess stormwater collected on site must be directed by pipes to the legal point of discharge, at least 5 m
down slope from the proposed or existing structures. 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 development. This advice is subject to implementation of the recommendations
given in this report.




                                                      13/14