Slope Stability Assessment

Paul & Janelle DeSouza                                                                                22198-001-Rev1
C/- Sarah Waller Architecture                                                                            25 July 2022

Confirmatory testing must be carried out at regular intervals and further details for control and testing of fill are
given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and Residential
Developments. Because of the depth of fill, testing under a Level 1 overview as per AS3798 is
recommended.

Select fill should have a maximum particle size of 100 mm for an uncompacted layer thickness of 200 mm
and shall be compacted by repeated rolling with a small compactor to achieve a dry density ratio of at least
95 % of the Standard Maximum Dry Density for cohesive soils, or 70 % Dry Density Index for any imported
cohesionless soils.

All fill must be ‘benched’ into the slope, which would apply whether the fill subgrade consists of soil or rock.
Fill should be over-built by 0.5 m and then trimmed back to the well compacted material.

Excavation through the natural soils should be achievable using conventional earthmoving plant (e.g. 5 to 8
tonne excavators with toothed buckets). For efficient excavations, below this, in weathered sandstone and
for the removal and handling of large corestones, larger plant (e.g. 20 to 30 tonne excavators) with ripper
attachments should be allowed for, particularly for any confined excavations. Due to the variable nature of
weathering within the rock, it is possible that higher strength zones could be encountered in deeper
excavations. If such materials are continuous with minimal defects (relatively massive ‘bedrock’), heavy
breaking with a hydraulic breaker may be required.

Permanent batter slopes should be constructed at a maximum grade of 1V:2H in medium dense (or
denser)/stiff (or stiffer) natural soils or controlled fill up to 1 m high. Batters in low strength or stronger
weathered sandstone may be steepened to 1V:1H, or steeper if assessed by an experienced geotechnical
engineer. Cuts above 2 m height will require further assessment as to permanent battering arrangements,
where not retained. Permanent soil or fill batters will require erosion protection (e.g. revegetation or surface
protection).

4.2    Retaining Structures
Retaining structures must be founded as described in Section 4.3 below. Retaining structures greater than
1 m high or supporting backslopes greater than 5% will require engineer design and certification of
construction. We suggest the parameters given in Table 3 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)


 Loose upper sands           17          28                0.40                   0.56                2.70



 Stiff (or stiffer)
 residual clay soils and     18          26                0.39                   0.56                2.56
 XW material



 Low strength (or
 stronger) weathered         21          36                0.26                   0.41                3.85
 sandstone



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

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.




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