RFI - 21327-001-Rev0 Plover St

                             PROPOSED RESIDENCE
                             34 PLOVER STREET, PEREGIAN BEACH




                        Text Figure 9: Footing depth required to prevent undermining

Site drainage should be designed to readily remove surface water and to prevent ponding of water on
subgrade areas during construction and also adjacent to foundations and perimeter slabs once construction
is completed.

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.

6.4      Temporary and Permanent Retention
Excavations are expected to range up to around 3 m BGL (existing ground levels) and, given that the
planned excavation extends close to some of the site boundaries and underground infrastructure (e.g.
sewer), some form of temporary shoring will likely be required to support the excavation.

Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, however Unitywater requirements for the sewer 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). However, the proximity to neighbouring properties 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.
Other shoring methods that could be considered include bored contiguous or secant piles. 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.

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 subsurface drainage system. It will be necessary to conduct numerical analysis



10 September 2021
Report No. 21327-001-Rev0                              12