20220109 - Geotech · OPW23/0182

                             PROPOSED RESIDENCE
                             21 MAHER TERRACE, SUNSHINE BEACH

prior to placement of steel reinforcement and pouring of concrete to confirm allowable bearing pressures and
cleanliness of excavations.
If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic should be notified immediately and an inspection carried out to determine if changes
to footing design are required.

6.5       Temporary and Permanent Retention
Excavations are expected to be around 4 m BGL (existing ground levels) and, given that the planned extent
of the excavations extends close to some of the site boundaries, some form of temporary shoring will likely
be required to support the basement excavation.

Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles or a ‘soldier’ pile wall system. Such shoring techniques may require lateral restraint, and this is
conventionally achieved by anchoring. However, the proximity to neighbouring properties would require a
number of permissions to be obtained for subterranean access. Therefore, it is suggested that internal
struts/propping be considered to provide lateral restraint for the shoring system.

It is recommended that the excavation of a test pit(s) closer to the time of construction is undertaken to
enable further assessment of potential impacts on the shoring system from groundwater and subsurface
conditions.

Based on the results of the investigation, the following temporary retention options are considered potentially
feasible for the proposed development:

       Anchored or propped sheet pile walls (limited to soils and very low strength sandstone). This
        system would require pre-drilling to prevent adverse vibrations during sheet pile installation and
        prevent penetration refusal on stiff residual soils or weathered sandstone. Temporary lateral
        support can be provided by installation of helical steel anchors (similar to screw piles), dowels
        (similar to soil nails) or internal props/struts connected to walers. As a permanent basement wall is
        constructed inside the temporary structure, the sheet pile wall becomes sacrificial.

       Anchored or propped ‘solider’ pile walls. Back-to-back steel C-channels (welded together with a
        central spacer) would be installed in bored holes at about 2.5 m centre-to-centre spacing. These
        soldier piles are typically ‘toed in’ at least 2 m below the bulk excavation level, then backfilled with
        concrete to just below that level. Steel mesh or fibre reinforced shotcrete is then installed in panels
        between the soldiers and may be used as the permanent basement wall (with cosmetic finishes
        applied). It is anticipated that conventional auger/boring equipment would be suitable for soldier pile
        installation; however, should any medium strength (or stronger) weathered sandstone be
        encountered more specialised techniques may be required (e.g. pneumatic hammer drilling
        equipment). Furthermore, temporary stability of the soil panels between the soldier piles may be
        difficult to maintain if substantial groundwater seepage occurs during construction. A significant
        advantage of the soldier pile system, as compared to sheet piles, is that it forms part of the
        permanent basement structure.

Other shoring methods that could be considered include contiguous or non-contiguous piles and secant
piles. The advice of a specialist shoring contractor must be sought regarding feasibility, design and
installation of the proposed retention and 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.

Where sheet pile installation proposes pre-drilling, care would need to be taken with this, and eventual pile
driving, to manage potential vibration impacts on nearby structures, including underground service pipes
such as the sewer and stormwater to the rear. It would be prudent to conduct a dilapidation survey of such
adjacent structures prior to commencement of the site works.

Although not encountered in the preliminary investigation, groundwater may be encountered in the additional
investigation or during construction. For design purposes it would be prudent to consider the impacts of


16 July 2021
Report No. 21236-001-Rev0                               15