Measured End Resistance of CFA and Drilled Displacement Piles in San Francisco Area Alluvial Clay-Article

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The study evaluates fully mobilised end resistance in continuous flight auger (CFA) and drilled displacement (DD) piles founded in Pleistocene alluvial clay in the San Francisco Bay Area. These soils contain thin, variable sand seams, which can make designs that rely on sand layers uncertain; the authors therefore focus on resistance provided by the clay.<br /><br />The researchers analysed 15 high-quality axial compression load tests on cast-in-place piles, tested 7–14 days after installation. End resistance was interpreted from strain-gauge measurements at geotechnical failure and compared with the average net cone penetration test (CPT) resistance over one pile diameter below each tip. Pile tip area was estimated from grout volume and pile length.<br /><br />Measured end resistance generally increased with net CPT resistance. The data showed no clear evidence that DD piles developed greater end resistance than CFA piles, or that thin sand seams near the tips increased resistance. The findings also indicate that the conventional approach—estimating undrained shear strength from CPT data and applying a bearing factor of 9—tends to underpredict fully mobilised end resistance in these local clays. Local pile load tests may therefore support higher design estimates than conventional methods.<br /><br />The authors caution that the database is limited and that larger studies could reveal trends not apparent here. They also note that substantial sand seams near a pile tip may increase resistance, although the test sites were selected to represent conditions with negligible interbedded sand near the tips.

Keywords

continuous flight auger piles

drilled displacement piles

end resistance

Pleistocene alluvial clay

San Francisco Bay Area

cone penetration test

axial compression load tests

pile tip resistance

undrained shear strength

geotechnical design