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 q_c. 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 q_c 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.