A laboratory model study of tension piles subjected to simulatedseismic loading through the soil was conducted. The objective of the study was to assess the magnitude of biased (static) tension load that can be sustained by displacement-type piles driven into medium dense, saturated fine sand during seismic events typical of those inSouthern California. The prototype pile characteristics modeled in this study consisted of a close-end, or plugged, impact-driven pipe pile, 20 to 40 inch in diameter, 20 to 40 ft long (or top 20 to 40 ft of a longer pile). The model test pile was an instrumented, steel, closed-end tube, 1 inch in diameter and 16 inch long, that was loaded through a spring-mass system to simulate feedback from a simple superstructure with a known natural period. Pile-head movements, pileload versus depth, and pore water pressures in the soil were measured during the experiments. Both the simulated seismic record and soil permeability were scaled to model the effect of drainage distance and its effect on pore water pressure generation and dissipation. Contour plots of stability conditions (sustained tension resistance and small pile movements), mobility conditions (sustained resistance associated with substantial pile movements), and failure conditions (total loss of pile capacity) for the model pile were developed from the tests. The effect of distance between the pile and event epicenter on stability was considered analytically.
Abstract