OPW052025 Report - Part 2

Further details regarding filling (including re-use of existing materials, and imported select 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.
Following stripping of topsoil and any unsuitable soils, fill must be ‘benched’ into the natural soil, over
filled by 0.5 m (horizontally) and then trimmed back to the well compacted material.

6.2.3         Excavations & Batter Slopes
Excavations within the residual soils to the test pit termination depths should be achievable using small
to medium sized conventional earthmoving plant (e.g. 5 to 14 tonne excavators) with toothed buckets.
Ripping attachments will be required in confined excavations to penetrate the phyllite ranging to low
strength and the upper weathered basalt layers, with localised hydraulic breaking possible where larger
high strength (or stronger) basalt corestones are encountered within the site for the proposed residence.
Temporary, non-surcharged batter slopes up to 1 m high may be formed at 1V:1.H in stiff (or stiffer) clays
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. Deeper
unsupported temporary batters should be assessed by a geotechnical RPEQ during construction.
Permanent batter slopes less than 1 m high should be constructed at a maximum grade of 1V:2H in stiff
(or stiffer) clays and future controlled fill (on slopes < 10°). Permanent soil or fill batters will require
erosion protection (e.g. revegetation or surface protection). Where such battering is not possible,
retention will be required. Existing slightly over-steep batters between the building platforms should be
trimmed back to the permissible grades as outlined above or retained.
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).


6.3           Building Foundations
6.3.1         Indicative Site Classification
In accordance with AS2870-2011, the existing subsurface profile would have a characteristic surface
movement due to normal seasonal moisture variation (ys) of up to 75 mm and therefore would be
equivalent to Class H2.
It should be noted that the predicted order of movement is based on the existing natural soil profiles (i.e.
no additional cutting or filling), and moisture content changes within soils due to normal seasonal
moisture variations only. Site classification would need to be assessed by a suitably qualified and
experienced person following completion of building plans and finalisation of earthworks details.
Due to the reactive nature of the site soils, it would be prudent to implement the following practices to
prevent changes in soil moisture (given overleaf):




Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                 20