Estimation of nonlinear static skin friction on multiple pile segments using the measured hammer impact response at the top and bottom of the pile

A technique is presented to estimate the nonlinear skin friction of a driven pile using monitored strains and accelerations at the top and bottom of a pile during a hammer blow. The scheme is based on a numerical solution of the 1-D wave equation with nonlinear static skin friction and viscous stiffness proportional damping. The soil–pile system is divided into pile segments based on pile length and the frequency content of the measured signal. Each segment is characterized with independent multilinear (bilinear loading with independent unloading) soil skin friction. The measured strains at the top and bottom of the pile are used to solve for the unknowns, including proportional damping and the loading and unloading stiffness of each pile segment from a least square error comparison of measured and computed particle velocities at the top and bottom of the pile. The technique was applied to four full-scale piles driven into layered sand and clay deposits with measured ultimate static skin frictions varying from 700 to 2700 kN. The analysis was carried out on five separate beginning of restrike blows within 1 week of the static load tests. The approach was shown to give consistent (within 20%) predicted skin frictions vs. displacements of the measured static load test results.