RFI - 21370-001-Rev1 66 Seaview Tce

                             PROPOSED RESIDENCE
                             66 SEAVIEW TERRACE, SUNSHINE BEACH

Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 66 Seaview Terrace, Sunshine Beach (‘the subject site’). The investigation was carried out
by Tectonic for Catherine and Gavin Deane.
Haskoning Australia (RHDHV) has provided a coastal engineering report, detailing a review of the Erosion
Prone Area at the site (Ref. PA2906-RHD-ZZ-XX-RP-Z-0001, Status P01.01/S0, dated 12 April 2022).
The geotechnical investigation comprised drilling of boreholes at three locations; one using a tracked vehicle
mounted rotary rig; the other two by hand auger methods on the face and crest of the dune. Dynamic cone
penetrometer (DCP) testing was also conducted along with geotechnical laboratory testing of selected soil
samples. This report presents the results of the geotechnical investigation and slope stability modelling to
address possible slope failure following a hypothetical future sea surge/storm event nominated by RHDHV,
along with associated geotechnical recommendations and comments.
Preliminary architectural drawings supplied to us indicate that it is proposed to construct a building of three
storeys. The lowest storey would be completely in-ground to the western side of the structure, and the
uppermost level would be only partially enclosed. The centre storey would have a cantilevered frontage
extending around 3 m from the eastern edge of the building, mimicked above by the third storey. To
accommodate the lower level being ‘dug in’ to the dune, on the upslope side, bulk excavations against the
western boundary with Seaview Terrace are expected to reach around 3.5 m below ground level (BGL).
Excavations will extend close to some of the site boundaries, and temporary retention will thus be required.
Borehole BH3 was drilled within the proposed building envelope using a rotary drill rig and encountered:

        Natural Sands – comprising loose, dry to moist, fine to medium grained sands to 1.25 m BGL;
         then becoming medium dense to 4.2 m BGL; followed by
        Interlayered Indurated Sand & Sands – comprising dense materials to 9.3 m BGL, then
         becoming very dense 29.6 m BGL; and consisting of sands, occasionally being weakly cemented
         indurated sand (‘coffee rock’); underlain by
        Residual Soils and Extremely Weathered (XW) Material – comprising, hard, medium to high
         plasticity, sandy clays to the borehole termination depth of 31.45 m BGL.

Boreholes BH1 and BH2 were drilled using hand augering equipment at the eastern edge of the
development area, and at the toe of the dune, respectively. DCP4 was undertaken around the centre of the
main high dune face. These locations encountered/indicated a similar subsurface profile as follows:

        Natural Sands – comprising loose sand (occasionally silty) to between 1.1 m and 1.6 m BGL, then
         becoming medium dense to depths of between 2.3 m and 4.8 m BGL, and dense thereafter to the
         maximum depth of investigation (Ref. attached borehole logs and in situ test results). Borehole
         BH2 at the toe of the high dune encountered the deepest depth to dense (or denser) materials.

DCP3 was undertaken through a fill embankment to the western side boundary and indicated that the
materials are variable in density, from very loose to medium dense.

No signs of groundwater seepage (‘springs’) were observed at the ground surface during our walk-over.
Groundwater seepage was encountered in borehole BH2 (located at the toe of the high dune) at about 1.9 m
BGL, which is inferred to represent a ‘perched’, unconfined groundwater table that builds up and diminishes
in the sands above the denser materials below, in response to prevailing weather conditions. No
groundwater was present in the standpipe installed in borehole BH3 to 6 m BGL following completion of
drilling on 8 October 2021. Groundwater conditions are expected to be reflective of prevailing weather
conditions and seepage may develop, particularly following periods of prolonged or heavy rainfall.

Modelling has been carried out using Slope/W to assess the factor of safety (FOS) against instability of the
sand dune slope considering a hypothetical storm and wave scenario assessed by RHDHV. Each of the
cases analysed are summarised as given overleaf:




28 April 2022
Report No. 21370-001-Rev1                               ii