CPT-based design of pile foundations in sand and clay: Perspectives

Purdue University, Lyles School of Civil Engineering, West Lafayette, IN, USA

^* Corresponding author: vsaklesh@purdue.edu

Abstract

This paper provides some perspectives and guidance for the design of pile foundations in sand and clay using cone penetration test (CPT) results. A key variable in the estimation of the limit unit shaft resistance q_sL of piles in sand is the critical-state interface friction angle δ_c, which is a function of the critical-state friction angle ϕ_c of the sand. In the absence of direct shear or triaxial compression test results, which is often the case for routine infrastructure projects, engineers typically assume a conservative value for ϕ_c in pile design. In addition, effective stress-based methods for estimation of q_sL of driven piles in clay rely on the residual interface friction angle δ_r, among other variables; however, in these methods, δ_r does not vary with the normal effective stress on the pile operative at the time of shearing. In this paper, we present relationships and approaches to address these issues. Finally, a relationship between the cone resistance q_c and the corrected standard penetration test (SPT) blow count N₆₀ was also developed so that engineers may obtain an estimate of q_c for use in a CPT-based design method when only SPT blow counts are available for a site.