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

                              PROPOSED RESIDENCE
                              33 ROSS CRESCENT, SUNSHINE BEACH

Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed residential
development at 33 Ross Crescent, Sunshine Beach; carried out by Tectonic for 33 Ross Crescent Pty Ltd.
The results contained herein represent the first stage (Stage 1) of investigation, and further investigation is
recommended comprising deep borehole drilling to depths in the order of 20 m to 25 m below ground level
(BGL) to confirm the findings of this report.
Preliminary architectural drawings indicate that the development will consist of the construction of a three
storey building terraced into the slope. Terraced landscaping and an above ground pool will extend towards
the eastern boundary of the site (which is situated along a moderately steep dune crest, dipping down to
Sunshine Beach).
The boreholes and in situ test results indicate a subsurface profile of poorly graded dune sands to the
maximum investigation depth of the borehole drilling and testing (which ranged up to 5.6 m BGL). Very
loose and loose sands were deepest in boreholes BH2 and BH3, extending to about 2.7 m BGL.

Slope stability modelling was carried out considering short-term erosion from a 100 year average recurrence
interval extreme storm event (as nominated by the appointed coastal engineering experts, Haskoning
Australia); and estimated retrogression of the dune over time. It was indicated in the Haskoning Australia
assessment that virtually the entirety of the dune face would be affected by such a future erosion event,
extending approximately 5 m to 10 m laterally into the dune. The storm erosion cut alone would be of
sufficient magnitude to affect the proposed building footprint without engineered protection.
It is recommended that the eastern property boundary be protected by a shear pile structure. Given our
understanding of the ground conditions and the predicted future dune morphology, this would be founded
into the underlying dense (or denser) sands. Subject to detailed design, relatively ‘heavy’ construction (e.g.
secant or contiguous piles), with additional structural support (e.g. ground anchors) is likely to be required.
This would be in order to allow the anticipated ‘wall upstand’ of about 6.6 m height indicated by the stability
analyses to be achieved. The boundary piles may need a minimum installation length of about 12.5 m
depth. Protection against erosion and adverse interaction with adjacent properties shall also be installed.
This should consist of a grout curtain extending from the shear pile wall to an equivalent distance of 23 m
from the rear (eastern) boundary, and along the northern and southern boundaries, tapering from 8.5 m
depth in the east. Structural 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 design checks of global and internal stability.
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 (or denser) sands. In some
        areas of deeper cut, high level footings may be adopted. Elsewhere, piles will be required.
       Excavations close to site boundaries may necessitate the installation of temporary shoring.
       Ground vibrations during construction must 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 dune face downslope of the residence would also aid in restricting the
        development of slope instability.
       Collected roof water should be directed by pipes to the council stormwater system along Ross
        Crescent (possibly through the use of a ‘wet’ or ‘charged’ drainage system) or; where not physically
        possible, surface water should be directed to infiltrate into the ground around the edges of hard
        surfaces, with infiltration areas set back as far as possible from the rear boundary. Hydraulic design
        should be such that under no circumstances are uncontrolled stormwater flows permitted over the
        dune face; where channelised erosion and scour features could develop.




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