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

J000605-001-R-Rev1                                                                                      March 2018


6.5       Foundations
Due to the presence of uncontrolled fill up to 1.1 m thick, as well as loose natural sand to about 4 m depth,
the use of high-level footings is not recommended.
The proposed at-grade buildings (i.e. without basement) could be supported on screw piers founded in the
dense sand or stiff clay. Reinforced concrete driven piles, timber push piles or continuous flight auger (CFA)
piles could also be considered. Bored piles will likely encounter construction difficulties due to the loose sand
and high groundwater.
The proposed basement could be supported on a raft, pad footings or screw/driven piles in the dense sand
or stiff clay. If a raft or pad footings are adopted, founding conditions must be confirmed following basement
excavation through additional DCP testing by a geotechnical engineer from Core. If loose sands are
encountered, over-excavation and compacted replacement may be required (possibly up to 1 m depth or
more) to support a raft or pad footings in the sand. A design allowable bearing pressure of 400 kPa may be
adopted for a raft or pad footings in the dense natural sand.
Steel screw piles with a 0.6 m diameter helix, founding in the dense sand below 4 m depth could be
designed using an allowable end bearing pressure of 650 kPa. For uplift, the capacity of screw piles should
also be checked against the weight of the enclosing cylinder of soil above the helix together with friction
developed on the walls of this cylinder. An average soil density of 18 kN/m3 should be used for the sands
above the water table and an average buoyant soil density of 8 kN/m3 should be used for the sands below
the water table, with a friction angle of 32o.
The actual capacity of steel screw piles depends not only on the soil conditions, but also on structural
considerations of the piles such as the strength of the helix and the helix/shaft joint. It is considered that the
structural section capacity as well as geotechnical capacity will need to be considered where the required
load carrying capacity of individual steel screw piles is greater than (say) 600 kN. Measurement of
installation torque should not be relied on to indicate pile capacity, as it has been documented that
significantly misleading results can be obtained. For this reason, piling contractors must be responsible for
assessment of actual pile capacities for their piles. Structural capacity of the steel screw pile should be
checked, and due allowance made for inclined or eccentric loads, and possible corrosion effects.
Lateral capacity of steel screw piles could be increased by constructing concrete pile caps or by using
proprietary head attachments which are dragged into the soil providing additional lateral resistance at the
pile head. The lateral support is generally limited and is generally suited to non-critical structures that can
accommodate some lateral movement such as light poles, signs and small towers.
Where limit state methods are used to design the piles, the ultimate geotechnical strength (Rd,ug) can be
calculated by multiplying the allowable values by the adopted factor of safety of 3 and then by a suitable
geotechnical strength reduction factor (Фg) to obtain the design geotechnical strength (Rd,g). The design
geotechnical strength Rd,g of a single pile is determined by multiplying Rd,ug by a suitable geotechnical
strength reduction factor Φg. Where no load testing is proposed, the Φgb value may be adopted for Φg. After
assessing the overall design average risk rating (ARR) in accordance with the guidelines presented in AS
2159-2009, a Φgb value of 0.45 is suggested for the site, design and installation risk factors anticipated.
Specialist piling contractors should be consulted with respect to selection of suitable pile types, detailed
design of piles, the potential vibration effects caused by pile driving, and suitable dynamic pile testing
methods and testing frequency.

6.5.1 General
All footings should found such that they are not adversely affected by any adjacent excavations batter
slopes, trenches (e.g. sewer), or retaining walls that are not designed to support building loads. To minimise
the potential for any adverse interaction effects, footings should found at least below a plane extending 1 m
horizontally from the base of trenches/batter slopes/excavations/retaining walls, then rising up at 1V:1H.
This requirement is illustrated in the diagram below.




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