Design, Construction and Design Validation through High Strength Testing (PDA) of CFA and DD Foundations-Paper

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The paper describes the design, construction and testing of more than 800 CFA and drilled displacement (DD) piles for high-rise developments in western Sydney. The sites were underlain by fill, deep alluvial and estuarine soils, and sandstone bedrock. Because conventional bored piles were uneconomic in these conditions, CFA and DD methods were selected.<br /><br />CFA piles remove soil and generate spoil, whereas DD piles displace and compact it, greatly reducing spoil and associated costs. However, standard DD augers could not reliably penetrate the site’s stiff clay layer and achieve the required sandstone sockets. A modified DD auger, combining features of Omega and De Waal systems with an extended flight, enabled DD installation through up to six metres of stiff clay and into medium-strength sandstone. The final design used 55% CFA and 45% DD piles, with 600 mm and 750 mm diameters and sockets up to one metre.<br /><br />Design followed Australian Standard AS2159-2009 and accounted for settlement, structural capacity and negative skin friction. Construction quality was monitored using real-time rig data, including penetration rates, torque and concrete pressure. CPTu testing found that pore-pressure increases after DD installation were generally modest and below the concrete pressure, indicating low risk to adjacent pile integrity.<br /><br />Dynamic testing was conducted on 18 production piles. PDA measurements and CAPWAP analyses were used to assess load–settlement performance and validate design predictions. Some piles did not fully mobilise their capacity, consistent with the high energy needed to develop resistance in rock sockets. The project was completed on time and within budget.

Keywords

CFA piles

drilled displacement piles

high-rise foundations

Western Sydney

stiff clay

sandstone sockets

pile design

dynamic testing

CAPWAP analysis

pore pressure monitoring