Reference Document - MCU25 0060 (As Published)

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 for the basement may extend to depths of around 3 m BGL (existing ground levels) and the
semi-basement footprint will extend close to the site boundaries, particularly on the northern side.
Therefore, some form of temporary shoring will be required to support the basement excavation in that
area at least. Isolated excavations elsewhere may also require temporary shoring.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by
use of sheet piles, however Unitywater requirements for sewer/water 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) in sands, although the proximity to neighbouring
properties may 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 of dense sands (generally anticipated to be below about 3.5 m to 4 m
depth BGL) 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
the sands would ‘flow’ through gaps between the piles before application of facing.
An advantage of contiguous or secant pile wall retention systems is that they are ultimately used for the
permanent walls.Sheet piles can provide temporary to retention during construction only, therefore
requiring a permanent structural wall to be constructed. Sheet pile walls can also be designed to provide
permanent support. Bored pile walls generally require greater width (horizontally) for installation as
compared to sheet piles and are often more expensive.
Advice of a specialist piling 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.
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 trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type and based on the
parameters listed in Table 4. These values do not include allowance for hydrostatic pressures nor
transient (e.g. construction) or adjacent footing loadings.




Proposed Residential Development
25 August 2025 | 25038-001-Rev0                                                                             17