ECM_22343520_v1_Geotechnical Investigation

                                           PROPOSED RENOVATIONS
                                           2 ARAKOON CRESCENT, SUNSHINE BEACH

            Executive Summary
            This report details the results of a geotechnical assessment undertaken for the proposed renovations to an
            existing residence at 2 Arakoon Crescent, Sunshine Beach. The investigation was carried out by Tectonic
            for Christopher Kelly. Adapt Planning are the consulting town planners for the project.
            Royal Haskoning Australia DHV (RHDHV) has provided a coastal engineering report, detailing a review of
            the Erosion Prone Area associated with the coastal building line at the subject site (Ref. PA3303, Status
            S0/P01.01, Rev R1, dated 20 February 2023). A site investigation ‘soil test’ report was completed by
            Australian Soil Investigations (ASI, Ref. Report No. 220343(S), dated 22 September 2022). Tectonic
            conducted three additional standalone dynamic cone penetrometer (DCP) tests down the main high dune
            face and near the dune toe for an assessment of the density of the underlying sands.
            This report presents the results of the geotechnical assessment 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 indicate that the proposed development will comprise partial demolition of
            the existing residence and pool, although some of the structural elements will be retained for future utilisation
            (subject to an assessment by a structural RPEQ). The proposed renovations would be superimposed over
            the existing footprint, extending to two storeys height. The northern edge of the building would be excavated
            into the ground for a garage level and cellar room, which will have a pool positioned directly above partially
            cantilevered towards the dune slope to the east. Provided sections indicate that excavations up to about 2 m
            deep will be required, which will be retained.

            Although the proposed footprint does not extend significantly beyond the existing building envelope,
            following demolition, it is anticipated that some form of temporary retention may be required, especially
            where excavations extend close to site boundaries or existing structures/elements (to remain). Filling is
            anticipated to be minor and less than 1 m high.

            The ASI investigation comprised two boreholes (designated BH1 and BH2) along the eastern edge of the
            existing residence. These boreholes encountered consistent subsurface conditions, summarised as
            follows:

                     Fill – comprising loose gravelly sand and silty sand, with deleterious materials (i.e. rubbish) from
                      the ground surface to depths between 0.9 m and 2.1 m below ground level (BGL), then
                     Natural Sand – comprising loose silty sand to the borehole termination depths of 3 m BGL.

            ASI continued DCP testing below the borehole termination depths which indicated that the natural sands
            became medium dense below depths between 3 m and 3.6 m BGL to the DCP termination depths.

            Standalone DCP testing by Tectonic along the high dune face and near the dune toe indicated the
            following:

                     Very loose/loose sands to depths between 1.1 m and 2.3 m BGL; then
                     Medium dense sands to depths between 1.4 m and 4 m BGL; followed by
                     Dense to very dense sands with practical DCP refusal at depths between 1.6 m and 5.8 m BGL.

            Our slope stability modelling considered a hypothetical storm and wave scenario nominated by coastal
            engineering experts RHDHV, which included a combination of short-term erosion from a 500-year average
            recurrence interval storm event and shoreline recession due to sea level rise for the 2100 climate extreme
            event scenario. Section 7 of the coastal erosion assessment indicates that the existing house site and
            proposed development is not likely to be impacted by beach erosion under this scenario. In accordance with
            previously adopted practices along the Sunshine Beach coastal strip a 28° line was modelled from the
            current dune toe which does not intersect the eastern boundary or proposed development area.
            Although the extreme storm case does not appear to impact the subject site, to assess the factor of safety
            (FOS) against instability for the proposed development, numerical modelling has been undertaken
            considering the existing dune slope and subsurface conditions from the investigation results.


             24 February 2023
             Report No. 22484-001-Rev0                               ii
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023