Attachment 3 - Revised Slope Stability Assessment

Brett Hage                                                                                       22328-001-Rev1
                                                                                               17 November 2022

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.

Given the inclination of the slopes around the vicinity of the proposed building area, filling should be limited
to no more than 1.5 m height above the natural ground.
Fill materials should be compacted at moisture contents within the range of -2 % to +2 % of optimum
moisture content for Standard Compaction. Confirmatory compaction 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.

Fill (site won and imported) 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 roller 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. Topsoil must not be re-used as structural fill, but may be used for landscaping.

Any imported fill should comprise a good quality material meeting the following specifications:
          Soaked CBR ≥ 10 %
          Shrink-swell index (Iss) ≤1 %
          Maximum particle size ≤ 100 mm

Following stripping of topsoil and any organic laden surface soils, fill must be ‘benched’ into the natural soil,
with batters over filled by 0.5 m (horizontally) and then trimmed back to the well compacted material.

It is anticipated that site earthworks could generally be completed by medium size excavators (8 tonne to
10 tonne) with toothed bucket attachments. However, the ground profile indicated by the boreholes suggests
that harder digging conditions in very low strength (or stronger) rock could be expected within 2.3 m to 3 m of
the ground surface. Such materials may require the use of ripping attachments, under slow digging
conditions.

Temporary batters in natural soils of stiffer (or stiffer) consistency, and future controlled fill may be formed at
1V:1H; with permanent batters formed no steeper than 1V:2H. Flatter batters may be required if
groundwater seepage is encountered. Permanent batters will require erosion protection such as covering
with erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
batter slopes, cut or fill batters would need to be supported by retaining walls.

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

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

Ground surfaces 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,
especially given the presence of natural gullies within the site boundary downslope of the proposed
construction area.

4.3       Site Classification
Based on the laboratory test results, site classification strictly of Class P would apply to the site, given the
presence of historic uncontrolled fill. Were it not for this, the subsurface profile encountered in the
boreholes would be equivalent to Class M as per AS2870, providing an estimated surface movement due
to seasonal moisture variation (ys) of up to 40 mm.

Designers should therefore also take account of the potential surface movement due to tree induced
suction change (yt) additional to the normal design suction change. An understanding of the full mature
height of the trees in the vicinity of the proposed residence, and finalisation of which trees would be
retained and which be removed would be required. Appendix H of AS2870-2011 comprises a Guide to
the Design of Footings for Trees.




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