Reference Document - MCU26 0001 & OPW26 0002 - AS PUBLISHED · MCU26/0001

        6.4             Temporary and Permanent Retention
        Excavations are expected to range up to around 5 m BGL (existing ground levels) and, given that the
        planned excavations extend close to some of the site boundaries and underground infrastructure (e.g.
        stormwater to north-west), some form of temporary excavation shoring will likely be required.
        Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by
        use of sheet piles, however Unitywater and NC requirements for existing infrastructure, and the
        sensitivity of neighbouring structures, may restrict the use of sheet piles. Shoring of deeper cuts would
        likely require lateral restraint, and this is conventionally achieved by helical steel anchors (similar to screw
        piles) in sands. However, the proximity to neighbouring properties would likely require permissions to
        be obtained for subterranean access. Therefore, where lateral support by cantilever only is not feasible, it
        is suggested that internal struts/propping be considered to provide lateral restraint for the shoring
        system.
        Sheet piles may require pre-drilling to loosen sands to the design foundations levels to prevent adverse
        vibrations during sheet pile installation. Ground vibration monitoring and management would also be
        necessary.
        Other shoring methods that could be considered include bored contiguous or secant piles, micropiles or
        CFA piles. Non-contiguous or soldier pile walls are not feasible for the proposed development due to
        the likelihood that the sands would ‘flow’ through gaps between the piles before application of structural
        facing.
        An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the
        permanent walls, whereas sheet piles would be temporary to provide retention during construction only,
        therefore requiring a permanent structural wall to be constructed. Bored pile walls generally require
        greater width (horizontally) for installation as compared to sheet piles and are often more expensive.
        The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation
        of the proposed retention and associated subsurface drainage system. It will be necessary to conduct
        numerical analysis to optimise the retention system. Detailed design will also need to account for
        relevant factors of safety to maintain the temporary and long-term stability of the retention system.
        Retaining structures greater than 1 m high, or those required to support surcharge loads or sloping
        backfill, will require engineer design and certification of construction. Retaining wall design should be
        carried out in accordance with AS4678-2002 Earth retaining structures.
        Design for temporary and permanent retaining structures could be carried out using a pressure
        distribution applicable to the wall type (e.g. trapezoidal for propped or anchored walls) and based on the
        parameters listed in Table 4 overleaf. These values do not include allowance for hydrostatic pressures
        nor transient (e.g. construction) or adjacent footing loadings.




        Geotechnical Investigation
        30 January 2026 | 25408-001-Rev0                                                                             15
Document Set ID: 25158026
                  24528612
Version: 1,
         2, Version Date: 10/09/2026
                          13/02/2026