CFA Pile Design Parameters in Sand-Paper

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The paper reviews the design and testing of continuous flight auger (CFA) and enlarged-base bored piles in sand, drawing on research and more than 1,200 commercial projects. It emphasizes that pile capacity depends not only on soil properties but also on installation quality. Poor construction can disturb or relax soil near the pile base, substantially reducing its contribution to capacity. Monitoring concrete or grout pressure and volume, and over-drilling the base after pressurized delivery begins, can help achieve a clean, well-constructed pile–soil interface.<br /><br />The author advocates using cone penetration test (CPT) tip resistance, \(q_c\), to estimate pile base resistance. A design method developed by Lehane and calibrated against centrifuge research and full-scale load tests has produced capacity predictions typically within about 10% of measured results. CPT is considered more reliable for sands than SPT, whose results can be affected by drilling disturbance and operator variability.<br /><br />High-strain dynamic tests can be useful and economical for verification, but the paper argues they should not be used alone to establish CFA pile base capacity. Fast loading can overstate base stiffness, and the test may not mobilize enough resistance to reliably extrapolate capacity at the specified displacement. Static load tests are needed to validate design parameters and assess settlement at working loads. The paper also warns that overestimating base resistance can lead to much larger-than-predicted pile movements.<br /><br />A Perth case study demonstrates the importance of accounting for stress relief caused by basement excavation. Excavation reduced CPT resistance; using pre-excavation values would have overestimated pile stiffness and capacity. Measurements showed that CPT-based predictions were most reliable over depths comparable to the excavation height.<br /><br />The paper concludes that sound installation, CPT-based design, explicit settlement assessment, and static load-test validation are essential for dependable CFA pile design in sand.

Keywords

continuous flight auger

CFA piles

enlarged-base bored piles

sand pile capacity

cone penetration test

CPT tip resistance

pile installation quality

static load tests

dynamic testing

basement excavation stress relief