Geotechnical Report (1) · MCU21/0212

                             PROPOSED RESIDENCE
                             66 SEAVIEW TERRACE, SUNSHINE BEACH

6.0      GEOTECHNICAL RECOMMENDATIONS
It is recommended that the eastern property boundary be protected by a micro-pile structure (or similar)
founded into the underlying natural medium dense (or denser) sands. An alternative option to a micro-pile
wall could be a relatively slender continuous flight auger (CFA) pile system. The adopted structure should
comprise piles structurally tied together at the surface using a steel-reinforced concrete capping beam. An
‘A-frame’ structure may be required (i.e. vertical and raked piles) to achieve the required shear force
resistance, should micro-piles be adopted.
It is noted that due to the relatively steep nature of the existing sand dune, access for construction plant
along the eastern boundary may be difficult; however the moderately sloping natural area across much of the
actual lot may provide an opportunity for progressive construction staging. An alternative option could be to
support the eastern edge of the proposed residence via a similar structure as illustrated in Case 5, shown in
Figure 7 (attached at the end of the report). A disadvantage of constructing the shear wall structure along
the eastern edge of the residence would be the loss of ground below the wall (east) following possible future
erosion events and retrogressive slope failure(s). Slightly longer piles would be required along the eastern
boundary compared to the eastern edge of the residence.
Based on the slope stability analyses the micro-piles (or similar) would need to be constructed as follows:
       Case 4 (along the eastern boundary) – piles should be at least 7 m long, with a 120 kN shear force
        resistance and vertical pile to pile spacing of 1 m.
       Case 5 (along the eastern edge of the residence) – piles should be at least 6 m long, with a
        120 kN shear force resistance and vertical pile to pile spacing of 1 m.
A grout ‘curtain’ must be installed along the length of the wall to prevent the flow of sand between the piles.
Based on an anticipated ‘worst case’ vertical face of about 1.5 m to 2 m high along the eastern boundary or
eastern edge of the residence, the grout ‘curtain’ should be installed down to at least 3 m below the ground
surface.
Whilst the proposed micro-pile (or similar) structure will prevent erosion along the eastern (downslope)
boundary of the property, there is potential for the design erosion event to remove sand from the
neighbouring property to the south, and the vicinity of the public access stairs to the north; exposing the side
boundaries of the subject property to instability and erosion. The extent of the side boundary erosion
protection required has been determined by a line drawn at 28° extending upwards to the west from the base
of the rear point of the erosion event. Erosion protection is required to extend from the rear boundary, or
rear of the residence, at least 15 m westward along the north and south boundaries, tapering from 3 m deep
at the eastern side.

Whether the pile wall will also need to extend around the side boundaries will depend on the dimensions and
strength of the grout curtain installed and its ability to provide at least emergency support to the retained
sand.
It should be noted that the above specifications are indicative of similar micro-pile and grout curtain designs
used for slope stabilisation projects and detailed design will be required by the piling contractor.
General geotechnical recommendations for site development are provided in the following sections.

6.1       Earthworks
Provided drawings indicate that excavations in the order of about 3.5 m deep (to the western boundary) will
be required, with filling anticipated to be for landscaping only less than 0.5 m high. Should proposed
cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability before construction.
Tectonic must review detailed design drawings for the residence prior to construction.

6.1.1       Site Preparation and Trafficability
Following clearing of existing vegetation, any tree stumps and large roots should be ‘grubbed out’. Any
organic rich topsoil or uncontrolled fill should then be stripped and set aside. Based on the boreholes and
site observations, stripping depths to remove existing root affected soils and uncontrolled fill (e.g. where the
road embankment impacts the building envelope to the western boundary) should generally range between


23 November 2021
Report No. 21370-001-Rev0                              15