Reference Document - Geotechnical Investigation Report - DBW24/0090 & RAP24/0025

                Rebecca Stead                                                                                   20248-001-Rev0
                                                                                                              3 September 2020

                4.0       RECOMMENDATIONS
                General recommendations to help maintain the stability of the site are given in the document Some
                Guidelines for Hillside Construction, which is attached.

                4.1     Site Layout and Development Limitations
                The plan extents of the proposed extensions shown in Text Figure 2 above are considered suitable from a
                slope stability viewpoint. Should any revisions be made to the proposed extent of the extensions, Tectonic
                must review these prior to construction to assess if there are any impacts on slope stability.

                It is anticipated that cut earthworks for the proposed extensions will be in the order of about 2 m depth. To
                maintain slope stability filling must not exceed 0.5 m in height above natural ground levels on the northern and
                eastern sides (i.e. downslope) of the residence, and up to 1 m depth on the western and southern sides. It is
                noted that a retaining wall is shown in the architectural design drawings (Section A) that is 1.5 m high with a
                floor slab adjacent. Due to the limits on fill heights, the structure will need to be designed as a suspended slab
                with void space behind the wall.
                The existing distressed retaining walls (to the northern and southern sides of the residence), should be
                removed and replaced with new engineer designed retaining walls.

                4.2     Earthworks
                4.2.1         Site Preparation and Trafficability
                In construction areas, any vegetation, tree stumps and large roots should be stripped and ‘grubbed out’. Any
                organic rich topsoil should also be stripped and removed from site. Organic topsoils and garden soils must
                not be re-used as structural fill; however, small volumes may be retained on site for later use across
                landscaping areas, if required.

                The natural subgrade following stripping will likely be in a loose condition. Where slab on ground
                construction is proposed (i.e. carport) the ground surface should be compacted by several passes of a
                vibrating plate compactor or small smooth drum roller (or similar), in conjunction with surface application of
                ample water.

                Due to the presence of loose sands trafficability will be poor for wheeled vehicles/plant. Placement of a
                minimum 150 mm thick granular working platform (roadbase/rock fill) may be required at access locations
                and across laydown areas.

                Care must be taken with the use of vibratory equipment to ensure that there are no adverse impacts on
                surrounding structures. The impacts from ground vibration must be closely monitored and managed during
                construction.

                4.2.2           Filling
                As noted above, fill heights must not exceed 0.5 m to 1 m above existing ground level. The existing sandy
                soils (excluding topsoil) may be reused as structural fill, subject to appropriate moisture adjustment. It should
                be noted that compaction of such relatively ‘clean’ sands as structural fill requires the addition of substantial
                volumes of water (‘flooding’) combined with the use of vibratory compaction equipment (typically a smooth
                drum roller or vibrating plate). Care must be taken with the use of vibratory equipment to ensure that there
                are no adverse impacts on surrounding structures. The impacts from ground vibration must be closely
                monitored and managed during construction.

                Should importation of fill be required, it is recommended that such fill comprise a clean sand (<10% fines).

                Any fill should be compacted at moisture contents within the range of ±2% of optimum moisture content for
                Standard Compaction. Fill material should be placed in 200 mm thick (loose thickness) layers and be
                compacted by repeated rolling using a roller and/or repeated passes with a vibrating plate compactor, with
                vibration on if safe to do so to achieve a Density Index of at least 70%.
                Confirmatory compaction testing should be carried out at regular intervals. A minimum of two tests per
                vertical lift is recommended where structural fill heights are to exceed 300 mm. Further details for control




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Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024