Geotechnical Report (1) · MCU21/0212

                             PROPOSED RESIDENCE
                             66 SEAVIEW TERRACE, SUNSHINE BEACH

       Case 1 – Vertical cut into the toe of the sand dune slope caused by erosion scouring, and allowing
        for a 500 year average recurrence interval (ARI) storm event.
       Case 2 – Slope failure following reprofiling of the slope due to the failure/erosion from Case 1.
       Case 3 – Retrogressive upper slope failure following reprofiling of the slope due to the Case 2
        failure.
       Case 4 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern property boundary protected by a micro-pile ‘A frame’ structure (piles min 7 m in length and
        with 120 kN shear force resistance).
       Case 5 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern edge of the residence protected by a micro-pile (or similar) structure (piles min 6 m in length
        and with 120 kN shear force resistance).
The modelled ground surface profile for the initial Case 1 was based on provided survey information across
the subject site, with surface profiles beyond the eastern property boundary augmented with slope angles
and measurements in the field by Tectonic. The horizontal extent of erosion adopted in Case 1 was based
on information provided in the referenced RHDHV report, which included a combination of short-term erosion
from a 500 year (ARI) storm event and shoreline recession due to sea level rise for the 2100 climate extreme
event scenario.

Whilst the proposed micro-pile (or similar) structure would prevent erosion along the eastern (rear) boundary
of the property or residence, there is potential for the design erosion event to remove sand from the
neighbouring properties to the north and south, therefore 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. Grout curtain erosion protection is required to extend from the rear boundary at least 10 m
westward along the north and south boundaries, tapering from 3 m deep at the eastern end.

It should be noted that this report gives advice on the use of a piled wall, based on slope stabilisation factors
only, and detailed design will be required by the piling contractor.
The following additional recommendations are provided for the proposed development:
       The proposed residence and any ancillary structures (including retaining walls and pools) must be
        supported by footings founded at least 1 m into natural medium dense to dense (or denser) sands.
        Based on the results of the investigation founding depths (excluding the proposed pile wall) are
        anticipated to range up to about 3 m BGL (existing ground levels). In some areas of deeper cut, high
        level footings may be adopted. Elsewhere, piles would be required.
       Cutting for the lower level up to about 3.5 m BGL along the western site boundary will necessitate
        the installation of temporary shoring prior to excavation. Based upon our experience with similar
        projects in proximity, shoring could comprise sheet piles or contiguous piles.
       Landscaping fill to the rear of the property should be limited to 0.5 m high.
       Ground vibrations during construction should be minimised and monitored. Where excessive
        vibrations occur, alterations to construction techniques may be required. It is recommended that a
        dilapidation survey of adjoining structures be completed prior to construction.
       Revegetation of bare slopes around the development area should be undertaken as soon as
        possible following construction. Similarly, improvements to the vegetation (i.e. with site appropriate
        natural species) on the natural high dune face downslope of the residence would also aid in
        restricting the development of initial surficial failures.
       All collected roof water should be directed by pipes to the council stormwater system along Seaview
        Terrace or, where not possible, surface water should be directed to infiltrate into the ground around
        the edges of hard surfaces, with infiltration areas setback as far as possible from the rear boundary.




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