Probabilistic CPT Method for Estimating the Ultimate Capacity of Friction Piles

This paper presents a probabilistic CPT method to estimate the ultimate capacity of driven friction piles utilizing CPT data. A database of 35 square PPC piles that were loaded to failure and their adjacent CPT were used to develop this method. Backcalculation was carried out to calibrate the correlation factors needed to determine the unit end bearing (qb) and unit skin friction (f) of the pile from CPT data (qc, fs) based on the probability of having each soil type. The ultimate pile capacities (QP) of the investigated piles estimated by the proposed method, three other CPT methods, and static α and β methods were compared with the measured pile capacities obtained from pile load tests. Statistical analyses were carried out to evaluate the reliability of the probabilistic CPT method and the results showed that the proposed method can predict the ultimate capacity of the piles with good accuracy. The capability of the proposed method was further verified by predicting the capacities of an additional 17 pile load tests. The probabilistic CPT method, in addition to Schmertmann, α, and β methods were calibrated to evaluate the resistance factors needed for the design of single driven piles based on the LRFD method.