Attachment 4 - Geotech Report · MCU23/0030

                             50 NOOSA PARADE, NOOSA HEADS
                             PROPOSED RESIDENCE

details for control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks
for Commercial and Residential Developments.
Due to the close proximity of structures on surrounding blocks, care will need to be taken with any activities
which may cause excessive ground vibrations (e.g. fill compaction and excavations). It would be prudent to
conduct a dilapidation survey of nearby structures before commencement of construction, and to monitor
ground vibrations during construction works. When in proximity to settlement sensitive structures or
services, only static compaction rolling should be conducted, particularly if excessive vibrations are
encountered, and/or to prevent pumping of pore water leading to subgrade heaving/strength loss over the fill
platform generally.

6.2       Temporary and Permanent Retention
Excavations are expected to be up to about 3 m deep BGL; and given that the planned basement footprint
extends close the eastern and western site boundaries (where a two-storey structure on the neighbouring
allotment was located), some form of temporary shoring will be required to support the excavation. Shoring
methods for the ground conditions encountered could include sheet pile, secant (over-lapping) piles and
mud-diaphragm walls. From a cost-benefit perspective, based upon our experience with similar sites in the
Noosa coastal area, it is anticipated that sheet piles could be adopted.
Depending on the thickness of sheet piles, and other considerations, it may be possible to design for
cantilever such that no lateral restraint is required. For a 3 m deep basement excavation, if not designed for
cantilever, it is anticipated that lateral restraint may be restricted to internal props or struts in places (as
anchors may not be feasible given the proximity to adjacent structures/property boundaries). Along the
western boundary where the basement excavation will be close to a neighbouring house, it may be
necessary to install a structurally stiffer retention system such as secant piles to mitigate the risk from ground
settlement.
The advice of a specialist contractor must be sought regarding feasibility, design and installation of the
retention system for this site.
Sheet pile installation may require pre-drilling to the desired depth due to the presence of dense to very
dense sands (including indurated materials) from depths of about 5 m BGL. Care would need to be taken
with sheet pile installation to manage potential vibration impacts on nearby structures. It would be prudent to
conduct a dilapidation survey of such adjacent structures prior to commencement of the site works.
Considering the potential for adverse impacts on nearby structures due to settlement, and potential
environmental impacts due to significant dewatering of the groundwater table, external dewatering outside
the chosen retention system should be avoided if possible and otherwise managed through a site-specific
dewatering management plan prepared by the shoring/dewatering contractor, if necessary. The proposed
retention system should be designed to support full hydrostatic pressure. If sheet piles are adopted, subject
to detailed design of the basement support system, heavy gauge sheets with an efficient overlap/interlock
would be required.
To prevent ‘boiling’ of the basement subgrade, the retention system would need to be designed to penetrate
to an adequate depth below the basement excavation level such that groundwater flow paths are
lengthened, and inflows are minimised. Very dense indurated sand (about 2 m thick) was found at about
7 m BGL in borehole BH1. If this layer is continuous across the site, which is to be confirmed by future
investigation works (Ref. Section 6.5 below), and the retention system is toed into it, the indurated sand
should act as an aquitard to reduce the amount of groundwater inflow through the base of the excavation
such that the impact on the surrounding, external groundwater table would be negligible. Depending on the
efficiency of the perimeter cut-off wall, an internal dewatering system (e.g. a series of sand spears or wells
connected to an extraction pump) may be required to pick up any seepage that migrates up through the
sands across the basement excavation floor.
To minimise the volumes of extracted groundwater and the associated treatment requirements, it would be
prudent to adjust the depths of internal wells/spears and pumping rates such that the stabilised groundwater
level below the excavation floor is optimised (i.e. dewatering is not carried out to an excessive depth, say no
more than 1 m deep below the excavation level).


2 February 2023
Report No. 22500-001-Rev0                               9