Attachment 3 - Revised Slope Stability Assessment

Brett Hage                                                                                        22328-001-Rev1
                                                                                                17 November 2022

If any soil conditions encountered during construction are found to differ from those noted above, Tectonic
must be notified immediately and an inspection carried out to determine if changes to footing design are
required.

4.5       Retaining Structures
Any retaining walls must be founded, at a minimum, as described in Section 4.4 above. Engineer design
and certification of retaining walls is required where such walls:

          Exceed 1 m height.
          Are subjected to surcharge loads (e.g. from backslope, adjacent structures, etc.).
          Are located within close proximity to existing structures.

Retaining structures should be engineer designed in accordance with AS4678-2002 Earth-retaining
structures. We suggest the following parameters may be adopted for retaining wall design:
Table 4: 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 Natural Clays         18          25                0.41                   0.58                  2.46


 Very Stiff to Hard
                             19          27                0.38                   0.55                  2.66
 Natural Clays


 Very Low Strength,
                             23          35                0.27                   0.43                  3.69
 Weathered Rock



These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.

Lateral earth pressure coefficients and material properties for fill materials would be dependent on the type
of material used, and the quality of compaction achieved.
Retaining wall designers should include rigorous checks such that sufficient factors of safety to limit the risk
of bearing failure, internal instability and global instability to acceptable levels are allowed for in the design.

4.6       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
proposed construction area prior to commencement of Building Works (to intercept flows from Black
Mountain Range Road), 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.
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.




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