Comparing CPT and pile base resistance in sand

The comprehensive database of load tests on closed-ended piles in sand has been re-examined to study the relationship between CPT resistance, q(c), and ultimate base capacity, q(b). The aim is to establish the origin of low reported values of q(b)/q(c) which contrast with continuum models that suggest q(b) = q(c) for steady deep penetration. Partial embedment of the pile tip into a hard layer underlying weak material has been accounted for by weighting qc. Partial mobilisation has been accounted for by defining failure according to a plunging criterion. When these two mechanisms are considered, the resulting values of q(b)/q(c) have a mean value of 0.90 and show no trend with pile diameter. The remaining slight underprediction of the 'continuum' model (q(b) = q(c)) could be attributed to the underestimation of plunging load in pile tests for which steady penetration is not reached. This exercise makes two contributions: first, it is suggested that any reduction of qc when estimating the end bearing capacity of closed-ended piles in sand should be linked to partial embedment and partial mobilisation, rather than absolute diameter; second, the dearth of high-quality pile load test data in the public domain is highlighted.