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

                             PROPOSED RESIDENCE
                             33 ROSS CRESCENT, SUNSHINE BEACH




                       Text Figure 11: Footing depth required to mimimise risk of undermining

It is recommended that footings be inspected by a suitably qualified and experienced engineer 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.

6.3      Temporary and Permanent Retention
Excavations are expected to range up to around 3.5 m BGL (existing ground levels). Preliminary design and
layout information indicates that construction utilising temporary battering should generally be achievable,
however if spatial constraints do not allow this (possibly along the western boundary adjacent Ross
Crescent), some form of temporary shoring may be required.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, however Unitywater requirements for their infrastructure 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); and the proximity to the road corridor would 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.

Anchored or propped sheet piles may require pre-drilling to prevent adverse vibrations during sheet pile
installation (where penetration of the deeper dense sands is required).

Other shoring methods that could be considered include bored contiguous or secant piles. Conventional
non-contiguous or soldier pile walls are not feasible for the proposed development due to the likelihood that
loose sands would ‘flow’ through gaps between the piles. Their use may however be possible where
additional grout panels are included in the design.
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 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.



18 June 2024
Report No. 24096-001-Rev0                               17