Appendix H Geotechnical Investigation - 14-16 Mary Street, Noosaville - Bounce Hostels 15.6.2018

J000605-001-R-Rev1                                                                                   March 2018


6.2.1         Temporary support & dewatering
Sheet piles founded in the underlying clay are considered a suitable option for temporary support during
basement construction. Provided that the sheet piles are founded in the clay, sump pumps in the excavation
base should be a suitable dewatering method. Alternatively, spear-point dewatering around the basement
perimeter will be required.
The temporary basement support and dewatering system must be designed by a suitably qualified and
experienced engineer, 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. The
selected perimeter wall system will require lateral support during basement excavation (e.g. propping and/or
cantilever).
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 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
founded in the underlying clay 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 stormwater.
Shoring will 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       Filling
It is recommended that filling is carried out with Level 1 supervision and testing in accordance with Australian
Standard 3798-2007 Guidelines on Earthworks for Commercial and Residential Developments.
It is recommended that general fill should:

     be compacted at moisture contents within the range of ±2% of optimum moisture content for Standard
      Compaction

     have a maximum particle size of 100 mm

     be placed in loose layers not exceeding 300 mm loose thickness

     be compacted to not less than 98% of Standard Maximum Dry Density (MDD) for pavement/slab areas,
      or 95% for general fill and service trench backfill (70% density index for cohesionless soils).
Any imported fill should be free from deleterious material, oversize (>100 mm size) particles and organic
matter, and have a minimum CBR of 10% and a maximum plasticity index of 15%.
In accordance with good construction practice, fill embankments should be ‘over built’ then trimmed back to
the well compacted material.
Further details for control and testing of fill are given in Australian Standard AS 3798-2007.

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
ground level during ‘extreme’ events.




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