Geotech Report Reference Document OPW19-0239 · OPW19/0239

Gympie Terrace Noosaville Pty Ltd



faces as a result of relaxation of these faces. Such movement has the potential to damage adjacent in-
ground services and buildings.
Provided that the sheet piles are founded in the stiff clay, sump pumps in the excavation base should be a
suitable dewatering method
Sheet pile walls can also be used as permanent basement walls thus reducing ‘lost’ space. The walls can
be sprayed with shotcrete and trowelled to provide an acceptable finish or covered by prefabricated panels
or blockwork.
The temporary basement support and dewatering system must be designed by a suitably qualified and
experienced engineer (RPEQ), and advice from specialist contractors should be sought regarding the design
and construction. The temporary support system adopted would need to prevent the loss of support to
adjacent buildings and other structures, whilst also acting to limit groundwater seepage into the excavation.
Upward infiltration of groundwater through the excavation base could significantly disturb the subgrade
resulting in loosening of the sands (‘boiling’ or liquefaction). The temporary support and dewatering system
must take this into account.
Consideration must also be given to the effects of dewatering during construction and the effect of
groundwater drawdown on neighbouring properties and in-ground services. Drawdown will likely be less for
sheet piles combined with sump pumps in the excavation base, rather than spear-point dewatering. To
reduce groundwater drawdown, consideration could be given to the re-injection of groundwater outside the
temporary shoring system (provided that the shoring system is suitably designed for hydrostatic pressure).
Groundwater must be tested and treated in accordance with relevant Council/State guidelines prior to
discharge to the stormwater system.
Shoring will also be required to enable safe personnel entry into trenches (with vertical sides) deeper than
1 m. The above recommendations do not supersede any existing safety regulations or legislation applicable
to excavations (e.g. limits on personnel entry into trenches).

  6.3 Material Suitability for Re-use
Based on laboratory results and our field classifications, site won materials from excavations would generally
be suitable for re-use as fill (excluding materials containing organic matter or deleterious substances) and
over-size particles.

  6.4 Basement Design
The basement structure is anticipated to be a fully waterproofed (‘tanked’) design. The basement should
therefore be designed to withstand a hydrostatic pressure that assumes groundwater could rise to the
surface during ‘extreme’ events.

  6.5 Foundations
    6.5.1 High Level Footings
Considering the foundation materials expected at the proposed basement excavation level and the likely
order of building column loads, either a pad footing system or raft slab could be considered.
Anticipated building wall and column loads (up to 2500 kN) could be supported by 3 m square pad footings
founded in the natural sands. In order to achieve the necessary allowable bearing pressure to support the
nominated pad footings (i.e. 2500 kN / 9m2 ~ 300 kPa), the natural sand subgrade at the basement
excavation must be compacted with a smooth drum vibrating roller with water applied if necessary. Further
geotechnical assessment (i.e. DCP tests) must be carried out following compaction to assess the density of
the sands following compaction. Total settlements of up to 30 mm and differential settlements of up to 15
mm are anticipated below 3 m square pad footings.
A suitably designed stiffened raft slab could also be used. Anticipated total settlements below a raft should
be less than 50 mm with differential settlements less than 25 mm.



J000799-002-R-Rev0                                                                                  Page 6