Slope Stability Assessment

Jane & Cameron Blake                                                                             22178-001-Rev0
                                                                                                    26 May 2022

The subgrade materials exposed after site stripping is expected to comprise stiff (or stiffer) clays, possibly
with some firm zones. The clayey soils could be subject to strength loss from repetitive vehicle loading
(particularly from rubber tyred plant) or if they become wet. Due to the potential for poor trafficability during
and after wet periods, placement of a minimum 150 mm thick granular working platform may be required,
particularly along access ways and any laydown areas.

The site should be graded to shed surface water runoff away from construction areas. It should be noted
that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion and
sediment control measures must be installed prior to commencement of earthworks at this site.

4.2.2 Excavations
Excavations within the existing soils, should be achievable using small to medium sized conventional
earthmoving plant (e.g. 5 to 14 tonne excavators) with toothed buckets. Should excavations extend into any
very low strength (or stronger) weathered rock, the use of larger earthmoving plant with ripper/hydraulic
breaking attachments may be required. Should any high strength (or stronger) rock require excavation, the
use of other specialised excavation techniques (e.g. pre-splitting the rock with expansive grout, etc.) may be
required.

Temporary non-surcharged batter slopes may be formed at 1V:1H in stiff/medium dense (or stronger)
materials, provided groundwater seepage is not encountered. Flatter batters may be required if groundwater
seepage is encountered, or if earthworks are undertaken during a period of heavy rainfall. Permanent
batters in stiff/medium dense (or stronger) natural soils and weathered phyllite up to 1 m high must be formed
no steeper than 1V:2.5H. Steeper temporary and permanent batters may be permissible in low strength (or
stronger) weathered rock, subject to an assessment by a geotechnical RPEQ.

Any excavation required adjacent to existing structures will need to be specifically designed to ensure that
the structures are not undermined.

The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).

4.2.3 Filling
Site soils where free of deleterious materials (e.g. vegetation, oversize etc.) are expected to be suitable for
re-use as structural fill, subject to appropriate moisture adjustment. It should be noted that placement of
highly reactive clays could be difficult and impacts on site classification and pavement design should be
considered.

Should importation of fill be required, it is recommended that such fill should comply with the following:

       Minimum CBR of 10%
       Maximum plasticity index of 15%
       Shrink-swell index ≤ 1%
       Maximum particle size of 100 mm
Controlled fill materials should be compacted at moisture contents within the range of -2% to +2% of OMC for
Standard Compaction. 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. 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.

We recommend that any future fill be placed as a minimum under Level 2 testing, with density testing
conducted at a Type 2 frequency. Fill must be ‘benched’ into the natural soil, over-filled by 0.5 m
(horizontally) and then trimmed back to the well compacted material.




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