Technical Program-Tension Pile Study
Authors: Matlock Hudson , Bogard Dewaine , McClelland Fugro , Chan Jack H-C. 
Published: October 2008

The pile research program was designed for fundamental understanding of pile and soil behavior as a basis for rational design, primarily for deepwater piles in soft clay. Thus, the experimental program was planned to cover a wide range of static and cyclic loadings over a long time. For extrapolation to large prototypes, fully instrumented field tests were performed on a 30-inch diameter pipe pile and on 1.72 and 3-inch diameter pile-segment models, tested in situ. For the 30-inch pile, loading was applied by four large two-way hydraulic rams activated by a 50-hp servo-eontrolled hydraulic power system. The pile was assembled in the field from three pieces: a fully instrumented 180-ft test section and two 90-foot follower sections. After driving, the tip was 234 ft below the sea floor. The soil was a very uniform deposit of soft clay. All transducers, cables, and cable connections, plus the computer-based control and data recording system, were custom-designed for maximum stability and reliability over the full 2.5-year test program. Special strain modules were welded along the inside of the pile and were backed up with 60-inch long extensometers lowered to seats after the pile was driven. Lateral pressure cells were welded flush with the pile wall to measure total pressures. Commercial pressure transducers were similarly installed to measure pore pressures for deducing effective pressures and to monitor the progress of soil consolidation in the critical zone adjacent to the pile. Axial displacements at points along the pile were determined from integration of the measured strains. Shear transfer was deduced from differences in strain along the pile length. During driving of the pile, static lateral pressures were observed at all pressure stations as a function of pile penetration. Static tension and compression load tests were performed within two hours of completion of the installation to provide the immediate strength after driving. Residual stresses were observed at various times after driving. Effects of consolidation and setup were clearly shown by four complete series of static and cyclic load tests over 2.5 years. Extensive records were made of soil shear versus pile displacement along the pile. Analytical modeling and empirical correlations were done to characterize the behavior and furnish recommendations for design. One of the most revealing elements of the program was the detailed study of the thick soil layer adhering to the pile after extraction.

Introduction

About 1980, Conoco Inc. was designing the Hutton tension-leg platform for the North Sea and was considering a second platform for deep water off the Mississippi delta. The foundation conditions in the second case were quite different, with the expectation of very soft clay. The behavior and reliability of tension pile anchors in such material was of prime concern, particularly considering the superposition of cyclic storm loading on the steady-state tensile loading. Intuitively, degradation of soil resistance could result in catastrophic failure. The need for a high degree of reliability in design and the lack of an experience base for the expected soil and loading conditions led Conoco engineers to seek a test of performance that would support design of full-scale prototypes. To gain the maximum benefit, a fully instrumented pile test was recommended.