Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                             PROPOSED RESIDENCE
                             33 ROSS CRESCENT, SUNSHINE BEACH

6.0      GEOTECHNICAL RECOMMENDATIONS
It is recommended that the eastern property boundary be protected by a shear pile structure. Alternatively,
depending on access for the shear pile installation, the wall may be brought back to protect the structure
alone, depending on the expectation of future maintenance and serviceability of the back garden area to the
east of the proposed building and pool. Given our understanding of the ground conditions and the predicted
future slope geomorphology, the shear pile wall would be founded into the underlying dense (or denser)
sands. Subject to detailed design, such a structure could comprise cased micro-piles, secant, or contiguous
piles.
The adopted structure should comprise piles structurally tied together at the surface using a steel-reinforced
concrete capping beam. An ‘A-frame’ structure may ultimately be required (i.e. vertical and raked piles) to
achieve the required shear force resistance, at least if micro-piles are adopted, especially where allowance is
given for the anticipated ‘upstand’ ranging up to about 6.6 m high indicated by the stability analyses.
A summary of the minimum piling requirements for the shear pile wall is given as follows:
Case 4a: Shear pile wall along eastern edge of building area
       Minimum Pile Length:                               10 m
       ‘Upstand’ Height:                                  3.7 m
       Unfactored resistance to normal shear forces:      200 kN/m*
Case 4b: Shear pile wall along eastern property boundary
       Minimum Pile Length:                               12.5 m
       ‘Upstand’ Height:                                  6.6 m
       Unfactored resistance to normal shear forces:      250 kN/m*
*The quoted resistance to normal shear forces is as utilised within the scope of the program Slope/W. It is
noted that this does not necessarily relate to actual design parameters, and designers must undertake finite
element analysis to assess soil/structure interactions and calculate both bending and shear stresses in the
piles. Retaining structures should also undergo rigorous checks of global and internal stability. The designer
must be satisfied that the geotechnical data presented is adequate to allow the analyses to be undertaken
with a high degree of confidence.
For a non-contiguous pile wall system such as micro-piles, a grout ‘curtain’ would need to be installed along
the length of the wall to prevent the flow of sand between the piles. Based on the highest modelled vertical
face of about 6.6 m height along the eastern boundary, such a grout ‘curtain’ should be installed down to at
least 8.5 m below the ground surface (existing). Where the pile wall was constructed along the eastern edge
of the building, this would be reduced slightly to about 5.5 m depth.
Whilst the proposed structure will prevent erosion along the eastern (rear) boundary of the property, there is
potential for the design erosion event to remove sand from the neighbouring properties to the north and
south 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 from
the estimated dune toe, following extreme storm erosion, and subsequent retrogressive failures. Erosion
protection is required to extend from the shear pile wall to an equivalent distance of 23 m from the eastern
property boundary, along the north and south boundaries, tapering from ground level to a depth to tie into the
shear pile wall along the eastern edge.
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. This should be considered as part of the detailed design.
General geotechnical recommendations for site development are provided in the following sections.




18 June 2024
Report No. 24096-001-Rev0                             13