Abstract

Two nominally 24-in. diameter piles, instrumented for measuring bending moments, were driven into stiff clay and subjected to lateral loading. A nominally 6-in. diameter pile was also instrumented for measuring bending moments. It was driven at the same site, loaded, pulled, redriven, and reloaded. Short-term static and cyclic loading was employed on both 24-in. diameter and 6-in. diameter piles. The water table was maintained a few inches above the ground surface during the testing program. The results of the tests were analyzed to obtain families of curves showing soil resistance p as a function of pile deflection y. Based on the experimental p-y curves and on theory for the behavior of soil, procedures for predicting p-y curves for stiff clay were developed. The procedures were used and predictions of pile behavior at the site were made. The predictions compared favorably with actual behavior.

Foreword

The research described in this paper was sponsored by Amoco Production Company, Chevron Oil Field Research, Exxon Production Research Company, Mobil Oil Corporation, and Shell Development Company. Shell Development Company was the operator of the project.

Location of Test Site

The field studies on which this paper is based were made at a location five miles to the northeast of Austin, Texas adjacent to US Highway 290. The surface soils in this area consist of stiff, preconsolidated clays of marine origins. These clays are referred to as the Taylor group and studies showed that unconfined compressive strengths of 2 to 10 tons/ sq ft were not uncommon in zones near the surface. These clays have a secondary structure, such as fissures, joints or slickensides, that was desirable because of the wide occurrence of this type of soil in nature. The secondary structure was expected to result in a less favorable behavior of the clay; therefore, a possibly critical soil condition could be investigated.

Preparation of Test Site

On December 15, 1966, a pit 45 ft wide by 50 ft long by 3 ft deep was excavated at the test site. A plan of the test area is shown in Fig. 1 with the locations of all soil borings and test piles. After the excavations was completed the pit was flooded with water to saturate the near surface clays and to simulate conditions that would exist in clays on the ocean floor. This initial inundation was done some five months before the installation of the first test piles and some six months before lateral loading of test piles. In April 1967, two soil borings, borings 5 and 6, were made to investigate the influence of some four months of ponding on the water content of the clay. These borings indicated an increase in water content with an accompanying decrease in shear strength for clays in the first few feet below the pit bottom. After borings 5 and 6, the pit was lowered an additional 2-1/2 ft. This additional excavation was done on April 21, 1967 and the pit was again inundated.

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