20220109 - Geotech · OPW23/0182

                             PROPOSED RESIDENCE
                             21 MAHER TERRACE, SUNSHINE BEACH

Executive Summary
This report details the results of a geotechnical investigation undertaken for the proposed multi-storey
residence at 21 Maher Terrace, Sunshine Beach. The investigation was performed by Tectonic for Libby &
Greg Baynton. Kelder Architects (Kelder) are the consulting architects for the project.

The investigation comprised drilling of boreholes at two locations across accessible areas of the
development footprint, together with in-situ strength and laboratory testing. This report summarises the
results of the investigation and provides geotechnical recommendations regarding earthworks and footings,
together with temporary and permanent basement retention.
Preliminary architectural drawings indicate that development of the site will comprise the demolition of the
existing residence and pool and construction of a two-storey building, over a single level basement carpark.
No earthworks details were provided at the time of reporting; however, provided plans indicate that
excavations for the basement would likely be in the order of up to about 4 m deep in the rear daylighting at
the front, with filling less than 1 m high anticipated. Excavations will extend close to the site boundaries and
therefore, some form of temporary retention will be required.
Ground conditions in this area were expected to comprise dune sands; however, borehole drilling revealed
a sedimentary profile of residual soil over weathered sandstone. The subsurface conditions encountered in
the boreholes are summarised as follows:
        Existing Fill (presumably uncontrolled) – comprising loose clayey sand to depths between
         0.6 m and 0.8 m below ground level (BGL); then
        Residual Soils – comprising mostly medium dense clayey sand and stiff to very stiff sandy/silty
         clay of medium to high plasticity to depths between 1.2 m and 2.5 m BGL (borehole BH2
         termination depth).
        Extremely Weathered (XW) Materials/Sandstone – very dense clayey sand tending towards very
         low strength sandstone was encountered beneath the residual soils in borehole BH1 to 4 m BGL.
         The XW materials were underlain by very low strength sandstone, with auger refusal on inferred
         low strength (or stronger) sandstone at 5.4 m BGL.

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 structural engineer,
there would be a Low Risk to property from slope instability.
Excavations for the basement are anticipated to encounter some loose to medium dense sands then mostly
stiff to very stiff clays, and XW materials at depth. Excavations in such materials should be achievable using
conventional earthmoving plant (e.g. medium to large tracked excavators) with a toothed bucket, possibly
with the assistance of ripper attachments at depth or in confined areas. Temporary unsurcharged cuts in the
existing soils and weathered rock could be formed at 1V:1H, for batters up to 3 m in height, provided
groundwater seepage is not encountered. It is expected that space restrictions will limit battering of temporary
cuts on the site and so some form of temporary retention system will be required such as sheet piles.

Given the excavation depths and based on the results of the borehole drilling, stiff/medium dense (or
stronger) residual soils and possibly XW materials are expected to be exposed across the basement floor
level. In areas of shallower proposed excavations, weaker foundations may be present at subgrade level. It
is recommended that a uniform foundation is adopted across the entire building footprint to reduce
differential settlements. Depending on the design loads, high level strip and pad footings, or a stiffened raft
slab, may be adopted for the proposed basement and development. An allowable bearing pressure of
100 kPa may be adopted for high level footing design where footings are founded in stiff/medium dense (or
stronger) residual soils. Should excessive column loads preclude the use high level footings, or in areas
where unsuitable materials are exposed at the design level, deeper piled footings would be required. Please
note that as outlined in Section 6.1 an additional borehole is required following removal of the existing
residence to confirm the required foundation design parameters.



16 July 2021
Report No. 21236-001-Rev0                               ii