In-Situ Model Pile Experiments at West Delta 58A
Authors: Bogard D. , Matlock H. , Chan J.H.C. 
Published: October 2008

During the years 1981 through 1986, Conoco, Inc. sponsored a comprehensive program of research aimed at improving the understanding of pile-soilinteraction along driven piles subjected to static and cyclic tension loading. The goal of the research was the development of guidelines and procedures for the design and analysis of foundation piles for Tension Leg Platforms. The experimental work was done at a decommissioned CAGC platform at Block 58A of the West Delta Area of the Gulf- of Mexico. Included were tests on a fully instrumented 30-inch-diameter pile driven to a penetration of 234 f t and eighteen experiments with instrumented probes. This paper contains the results of the experiments with the instrumented probes, with comparisons made to similar experiments at other sites.

INTRODUCTION

In 1981, Conoco, Inc. initiated a five-year study of axial pile-soil interaction along driven piles subjected to static and cyclic loading. The study was performed by The Earth Technology Corporation, acting as a designated subcontractor to Conoco Norway, Inc. through A.S. Veritec. The work included field experiments observing the behavior of fully-instrumented piles and pile models, along with parallel analytical work aimed at increasing the understanding of the fundamental behavior of axial pile-soil interaction in normally consolidated clays, with emphasis on applications for Tension Leg Platforms. In order to ensure a high degree of confidence in the extrapolation of the results to a site in Green Canyon Block 184, a search was made for a site having homogeneous soft-to-stiff normally consolidated clays with engineering properties very similar to the deepwater site. Such a site was located in Block 58 of the West Delta Area of the Gulf of Mexico. A decommissioned production platform located at the site, West Delta 58A, was refurbished for use as a testing platform. The major part of the work involved static and cyclic load tests on a fully instrumented 30-inch-diameter pile which was driven to a penetration of 234 ft below the seafloor. Load tests were performed at times ranging from 2 hours to 29 months after driving, times which encompassed the complete consolidation history of the near-pile clay. Early in the work, it was considered to be extremely important to collect parallel data from in-situ experiments using small-diameter probes for comparison with the results of the large-diameter pile tests. The use of small-diameter probes allowed a much wider variety of test conditions to be employed than was possible with a single large-diameter pile, particularly including variations of consolidation time and loading history. Two types of instrumented probes were employed, one 3.00 inches in diameter and fitted with an opened cutting shoe and the other 1.72 inches in diameter that had a solid conical point. The probes were installed beyond the bottoms of boreholes and provided continuous measurement of pore pressure, total lateral pressure, and axial shear resistance, all as a function of axial displacement. In the constant temperature environment of deep soil embedment, the instruments exhibited excellent stability and repeatability. They have been described in detail previously (Bogard, et a1 1985).