Geotechnical Investigation Report

6.3.2 PILED FOUNDATIONS

Pile foundations can be used to support the proposed structures to transfer building loads to the more competent soils
at depth.

The piled foundation options which are suitable for the site include:
Continuous flight auger (CFA) concrete.

Driven steel tube, H pile or precast concrete.
Screw piles.

Bored piles can be designed using the ultimate skin friction and base bearing capacities in Table 3 below.

Table 3: Ultimate Geotechnical Design Parameters for Bored Piles

Material Type Ultimate Skin Friction (kPa) Ultimate Base Bearing (kPa)

Loose sand/silty sand Not Recommended Not Recommended
Medium dense sand/silty sand 2 500
Dense sand/silty sand 4 1000

The above parameters are only suitable where the excavation of piled foundations are supervised and at a minimum
inspected by a suitably qualified geotechnical engineer to confirm the founding conditions and materials encountered
in the side walls.

The design of piled foundations should be undertaken in accordance with:
AS 2159-2009: Piling - Design and installation;
Any other relevant design standards (Australian or international); and
Engineering Principles.

The piled foundation design should also consider:
Piles should be founded a minimum of two pile diameters into the nominated strata and have a minimum pile
length of four times the pile diameter;
The upper 1 m of pile length should be ignored due to pile load development and shrink swell effects; and
Settlement of properly designed and constructed footings should be less than 1% to 2% of the pile diameter.

To calculate the design geotechnical strength (Rd,g), the ultimate geotechnical strength should be multiplied by a
suitable geotechnical strength reduction factor (@g) as per Section 4 of AS 2159-2009. The geotechnical strength
reduction factor (@g) is based on:

gb = The basic geotechnical strength reduction factor, calculated using a risk assessment procedure;

otf = Intrinsic testing factor, based on the type of pile testing adopted; and

K = The testing benefit factor, based on a percentage of the piles that are tested.

Typically, where no pile testing is undertaken and the piles have a low redundancy, the assessed risk rating would be
high and a $g value of 0.45 would apply. It should be noted this assessment has been undertaken by PTG to assist
with the design of the foundations, however as assessment of each risk rating and factors must be undertaken by the
designer of the piles for the final design.

Specialist piling contractors should be consulted when selecting and installing piles.

6.4 SAFE TEMPORARY AND PERMANENT BATTER SLOPES

Excavations up to 1.5 m depth could generally be designed with a safe 1.5H:1V temporary batter slope. This assumes
that soils are not wet/saturated and groundwater is not present, and is subject to the below recommendations:
All temporary batters should not have construction loads (i.e. plant, equipment or soil stockpiles) or people at the
crest of the batters at a distance equal to the height of the slope;
Permanent batters (at a minimum) should be protected from erosion. Topsoiling and planting, or erosion matting
should be applied to the batter as soon as possible;
Suitable drainage and stormwater provisions must be installed for the batters. Surface and stormwater runover
must be directed away from batters and prevented from pooling at the base and crest. At a minimum, spoon
drained should be installed at the crest and base of permanent batters;
Temporary batters should not remain open for extended periods of time. Vertical excavations of up to 1.2 m are
usually formed near vertical for short periods of time to allow for the installation of services and construction of