4895-Geotech Assessment

Mr Max Richardson                                                                                 20362-001-Rev0
C/- Long View Consulting Engineers Pty Ltd                                                           28 July 2021

4.4       Retaining Structures
Any retaining walls must be founded as described in Section 4.3 above. Any proposed fill on slopes
exceeding 10° (18%) shall be fully supported by an RPEQ engineer designed and certified retaining wall.
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 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
                           23           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 (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.

4.5       Drainage
During construction temporary swales and diversion drainage may be required to divert seepage and/or
runoff away from the building and driveway areas, to prevent saturation of subgrades.
Surface diversion drainage should be constructed above the crest of any cut or fill embankment (e.g.
grassed or landscaped swales, diversion mounds or graded surfaces). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building and retaining wall footings (e.g.
positive gradients and/or channel drains), and also not allowed to flow over fill embankments.

Uncontrolled flow of stormwater over fill batters (e.g. from gutter overflow during high intensity storms) has
the potential to saturate and weaken the fill, potentially leading to slumping failure of the fill. Future building
and landscaping must be carried out to maintain good drainage.

Along the driveway, the finished surface should grade into a ‘table drain’ cut into the up slope side, with
collected waters directed to discharge at regular intervals via culverts under the alignment. If an unsealed
gravel pavement is adopted, it would be prudent to install water bars at regular intervals on steeper sections
to intercept long stormwater flow paths and prevent erosion of the driveway surface.
Subsurface drainage must be installed behind the retaining wall (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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