Mobilization of Component Interface Stresses Between Soil and Pile Under Lateral Loading

The inelastic deformations for pile foundations under lateral loading occur below grade; therefore, the overall lateral load behavior of the system is influenced by the interaction between the shaft and the surrounding soil. This complex interaction is commonly modeled using p-y curves. The standard p-y method does not explicitly take into account all the key aspects of soil-pile interaction, namely the nonlinear response of the soil, frictional interface stresses between the soil and the pile, the generation of gaps between the soil and pile, damping under dynamic loads, and the structural response of the pile. Past research efforts have been aimed at developing unidirectional p-y interaction curves comprised of an assembly of distinct sub-elements; the sub-elements are each in charge of modeling a specific component of soil-pile resistance under lateral loading. These assembled soil-pile interaction curves are then compared to a backbone curve based on the existing standard p-y curves. However the current models for p-y curves are calibrated primarily from lateral load tests and lump all components of lateral resistance into one curve. Thus the contribution of each sub-element to the overall assembled behavior of the interaction "p-y" curve becomes somewhat arbitrary. This study examines the effects of nonlinear soil resistance to relative displacements between a model pile and soil, including the effects of gapping, cyclic degradation, and interfacial slip through laboratory testing. Measurements are presented for the normal compressive soil stress on the leading face of the pile, soil/pile interface stress along the sides of the pile, and active normal soil stresses on the pile. Using current sensor technology, normal stresses in soil are measured and side-frictional interface stresses are inferred. Professor Michael O'Neill stated in his Thirty-Fourth Karl Terzaghi Lecture that "the design of deep foundations is a complex matter that should be addressed in a design context by engineers who are experienced in the observation of pile behavior, theoretical modeling, and the appropriate use of design methods" — this paper attempts to stay true to Professor O'Neill's words by applying both observations from experimental measurements and theoretical modeling to a complex topic in foundation engineering.