Reference Document - MCU25 0060 (As Published)

Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 22/512 David Low Way, within The Dunes estate at Castaways Beach. The investigation
comprised drilling of boreholes at three locations across accessible areas of the development footprint.
This report summarises the results of our investigation and provides geotechnical recommendations
regarding earthworks and footings, together with temporary and permanent basement retention.
Architectural design drawings provided indicate that the development will comprise a three-storey
building. Bulk excavations will be in the order of up to 3 m to 3.5 m deep and localised filling up to
about 1.5 m high is anticipated. Excavations are expected to extend close to the site boundaries and
therefore, some form of retention will be required.
The subsurface conditions encountered in the boreholes are summarised as follows:
▪   Topsoil – comprising very loose to loose sand to depths of 0.1 m to 0.2 m below ground level (BGL);
    then
▪   Natural Sands – comprising poorly graded sands mostly being loose to medium dense to depths of
    between 1.1 m and 1.3 m BGL; then becoming medium dense and increasing to dense below depths
    of about 3.1 m to 3.8 m BGL in boreholes BH1 and BH2.
No signs of groundwater seepage (‘springs’) were observed during our walk-over. Groundwater was not
encountered in the boreholes at the time of investigation.
Provided the recommendations within this report are adhered to, and subject to detailed design and
construction of the footing and retention system by a suitably qualified and experienced RPEQ structural
engineer, there should be a Low Risk to property from slope instability.
Excavations through the existing soils should be achievable using conventional earthmoving plant (e.g.
small or larger excavators with toothed buckets). Temporary unsurcharged cuts could be formed at
1V:2H in the very loose to loose sands, and at 1V:1H in medium dense (or denser) sand, for batters up to
3 m in height, provided groundwater seepage is not encountered and sands do not slough from the
batter face as they dry out. It is expected that space restrictions will limit battering of temporary cuts on
the site and so some form of retention system, such as sheet piles or contiguous reinforced concrete piles,
will be required. These retention systems, which may have to be temporarily braced during construction,
may also be designed to form the permanent basement walls.
Footings for the proposed development and any ancillary structures (e.g. retaining walls) must penetrate
through any unsuitable natural soils (i.e. very loose to loose sands) to found at least 0.5 m into medium
dense (or denser) natural sands. Such materials were encountered in the boreholes below depths of
between about 1.1 m and 1.3 m BGL (existing ground levels). High-level footings would be suitable in
excavation areas where medium dense (or denser) natural sands are exposed at the subgrade level. Piled
footings would be required if loose sand materials are found at or near foundation subgrade level. Given
the sandy soil profile, where deepened footings are required, steel screw piles are considered to be a
suitable foundation type. Due to the presence of loose sands, together with the close proximity of
adjacent buildings, open bored piers or driven piles are not considered to be suitable for this site. Cast in
place continuous flight auger (CFA) piles may also be suitable for both foundation and shoring piles
provided a small rig capable of traversing the site can be sourced.




Proposed Residential Development
25 August 2025 | 25038-001-Rev0                                                                            iii