Laboratory Scale Modeling of Pullout Behavior of Bias-Loaded Tension Piles During Earthquakes
Authors: O&apos;Neill M.W. , Ochoa Mauricio 
Published: October 2008

A laboratory model study of tension piles subjected to simulated seismic loading was conducted. The objective 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 offshore Southern California. The prototype pile characteristics modeled in this study consisted of a closedend, or plugged, impact-driven pipe pile, 20 to 40 in. in diameter, 20 to 40 ft in length (or top 20 to 40 ft of a longer pile). An acceleration record for a Magnitude 5.8 seismic event was scaled to higher magnitudes to simulate severe earthquake conditions in the laboratory.

A pressure chamber was used to simulate isotropic effective soil stresses and drainage conditions and to produce programmed soil motion. The model test pile was an instrumented, steel, closed-end tube, 1 in. in diameter and 16 in. long, loaded through a spring-mass system to simulate feedback from a simple superstructure with a known natural period.

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.

INTRODUCTION

Biased tension loads may exist on piles at the time of seismic events in tension-leg offshore platforms or other structures. Pore water pressure generation and superstructure feedback during the event can cause such piles to extract or deform appreciably. The problem was studied by means of scaled laboratory tests.

The prototype pile characteristics modeled in this study consisted of closed-end, impact-driven piles, 20 to 40 in. in diameter, 20 to 40 feet in length (or top 20 to 40 feet of a longer pile), and driven in a fine, uniform, clean siliceous sand at relative densities ranging from 55% to 70%. The lower limit relative density was chosen so as to account for aging in prototype sands of relative densities of 40 - 50 %. The soil-pile model system was excited so as to simulate the vertical, horizontal and combined vertical and horizontal components of an earthquake at a deep, underwater soil site.

EVENT SELECTION

The phenomenon was studied by selecting a seismic event in which acceleration time histories had been measured at an offshore deep soil site. The event chosen for study was the Oceanside, California, event of July 13, 1986, a Magnitude 5.8 earthquake whose epicenter was 45 mi. (74 km) southeast of the instrumentation site 1. Acceleration time histories were measured on the shallow sea floor, 250 ft below sea level. The measured horizontal acceleration time history, shown in Fig. 1, was not adequate to produce distress; therefore, the earthquake was scaled to higher magnitudes, Le., Richter Magnitudes (M) of 7.0, 7.5 and 8.0 for the resultant component of horizontal motion. The Oceanside event of 1986 appears to fit the distance attenuation pattern for alluvial sites in the 1989 Loma Prieta earthquake and similar events (Fig. 22).