Reference Document - OPW25 0085 (AS Published)

ratio of at least 95% of the maximum Dry Density Ratio (DDR) for cohesive soils, or 100% DDR based
upon the Standard compaction method within 0.3 m of pavement subgrades.
Confirmatory compaction testing must be carried out at regular intervals. A minimum of two tests per
vertical lift is recommended where structural fill heights are to exceed 300 mm. Further details for
control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for
Commercial and Residential Developments. It is recommended that Level 1 geotechnical inspection and
testing be conducted for all Operational Works filling, and for future building pad construction where fill
heights exceed 0.5 m.
Due to the sloping nature of the site, all fill must be ‘benched’ into the natural slope, over-filled by 0.5 m
(horizontally) and then trimmed back to the well compacted material.


6.3          Building Foundations
6.3.1        Site Classification
Based on the results of the investigation, site classifications for the proposed allotments are estimated to
be typically Class H1 in accordance with AS 2870-2011 Residential Slabs and Footings. It should be noted
that the predicted orders of movement are 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. In areas of deeper clayey sands or shallower weathered rock, site classifications may reduce to
Class M.
Reactive soil movements and site classification should be assessed by the footing design engineer at
Building Works stage to consider the extent of earthworks proposed on each lot, together with any
retained trees (trees located near footings can influence the potential ground movement, and result in
more severe site classifications). A site classification ‘soil test’ would be required prior to building works
to confirm the site classification and footing design parameters.
It should be noted that the placement of highly reactive clay fill may have an adverse impact on the site
classifications.

6.3.2        Footing Design
Footings supporting any structures (including retaining walls) are to penetrate through any uncontrolled
fill or unsuitable natural soils to be founded as follows:
    On slopes of 10° (18%), or less – at least 0.5 m into stiff/medium dense (or stronger) residual soils, or
     future controlled fill (Level 1).
    On slopes of more than 10° – at least 0.5 m into very low strength (or stronger) rock, which is
     typically anticipated to be about 1 m to 2 m BGL (existing ground levels) on the steeper slopes.
Due to the depths of foundations recommended, conventional high-level footings (e.g. slab on ground)
should be suitable for some areas, whereas bored piles (or similar) would likely be required for sloping
building sites. High-level footings on slopes greater than 10° could be possible in areas of future cut
where the target foundation stratum is exposed at the subgrade level.




Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                             19