A Method For Accounting For Pile Setup and Relaxation in Pile Design and Quality Assurance

Abstract

When piles are installed by jacking or driving, they cause substantial changes in the state of soil located near the pile. These changes result from the complex loading imposed on the soil by expansion of a cylindrical cavity to make room for the pile, by multiple cycles of shearing in the vertical direction as the pile gradually moves down into the ground, and by the slow drainage associated with clayey soils. If a pile is load-tested a short time after installation, it will develop an axial resistance that reflects the existence in the soil of the excess pore pressures caused by the installation process. After the excess pore pressures dissipate, the axial pile resistance will be different from that measured in the short term. This difference is referred to as pile setup (if the resistance increases) or relaxation (if the resistance drops). This report focuses on the pile setup observed in clayey soils, in which it can be quite significant. Pile setup in clays result primarily from shaft resistance gains with time after installation because the base resistance contributes proportionally much less in soft to medium stiff clays, which are the focus of the research. Accordingly, our focus has been on analyzing setup in shaft resistance, validating the equations resulting from these analyses and then proposing design and quality assurance procedures based on the results of the analyses. The analyses were done using the finite element method and an advanced constitutive model developed specifically for this project. The constitutive model captures all the key features required for these analyses, and the finite element analyses are 1D analyses of shaft resistance that can handle the large deformations and displacements involved in pile installation. The results of the analyses compare well with load test data from the literature. Design equations for the unit shaft resistance are proposed. Equations for unit shaft resistance in the short term (for comparison with load tests) are also proposed.