Collaborative Research: Effect of Point Loading on Lateral Resistance of Rigid Piles

University Of Florida, Gainesville FL

Investigators

Abstract

The lateral resistance of short/rigid piles depends on the point where the lateral load is applied. Experiments have shown that lateral resistance can be increased significantly by shifting the point of the load application downward to where the resulting mode of the pile movement becomes purely translational, i.e., no rotational movement. Through experimental studies, this point has been identified near the mid-height of the pile for cohesive soils, whereas it is toward the bottom of the pile for cohesionless soils. Analytical studies have been tried to identify this optimum point of the lateral load application, but they have been mostly in two-dimensions without considering the significant effects of three-dimensional development of stresses and movements. The PI has developed a preliminary analytical formulation for the estimation of the lateral resistance of rigid piles considering the truly three-dimensional effects. A limiting equilibrium approach was used incorporating a tetrahedron-shaped failure wedge. The formulation considers circumferential and radial normal and shear stresses around and along the pile as functions of the depth and the magnitude of the lateral movement. The solution method considers an approach of progressive failure. For a given point of lateral load application, the thickness of the soil layer experiencing failure gradually increases from the ground surface (or from the bottom of the pile in case when the lateral load is applied near the bottom of the pile). At a given thickness of the soil layer experiencing failure, the corresponding pile lateral resistance and the point of the pile rotation are calculated from equilibrium conditions through iterative search methods. As the thickness of the soil layer experiencing failure increases, new values of the pile lateral resistance and the point of the rotation are calculated. The process continues until the maximum lateral resistance is obtained. This preliminary analytical solution indicates that the ultimate lateral resistance can be increased by up to 170 % and 340 % for piles embedded in clay and sand, respectively, when the point of the lateral load application moves from the top of the pile to the optimum point. The solution has also been used to compare with the centrifuge model test results reported in the literature. Results indicate that the analytical solution, though preliminary in nature due to numerous assumptions introduced, is capable of estimating the measured pile lateral resistances as a function of the point of the lateral load application very closely However, additional in-depth study is necessary to improve the solution method before it can be used with confidence in accurately estimating the lateral resistance of rigid piles. Centrifuge model tests will be conducted in both sand and clay soils. The effect of pile insertion method and point of loading will be studied. Pile rotation and normal stress measurements will be made during testing. Digital photography will be used to verify pile rotation and document the tests. This is a collaborative research proposal. The South Dakota School of Mines and Technology will be the lead institution, conducting the analytical portion of the study including the solution development and verification. The University of Florida will conduct the experimental centrifuge model tests.