Static and Cyclic Load Testing of a 30-inch-Diameter Pile Over a 2.5-year Period
Authors: Bogard Dewaine , Matlock Hudson 
Published: October 2008

Static load tests were performed on the pile two hours after driving and after four, sixteen, and twenty-nine months of consolidation and setup. The process of consolidation was essentially completed about twelve months after driving, so that the load tests encompass the complete life of a friction pile in highly plastic clays. Upon completion of the static load tests, the pile was subjected to a number of load variations that included cyclic loading in tension above and below a constant bias until failure was indicated by progressive pullout to large displacements. The pile was then subjected to alternate loading to failure in tension and compression to observe the maximum degree of cyclic degradation. Additional variations included studies of the effect of variations in the loading rate and large-displacement loading in compression to evaluate the end-bearing resistance, At the end of the field test program, load, pressure, and displacement data were recorded during pullout of the pile, Data were recorded for over fifty inches of axial displacement as the pile was removed, No significant losses in capacity were observed during pullout, indicating that temporary overload conditions on tension piles may not result in catastrophic failure particularly if redundancy is provided.

Introduction

At the time the study was initiated. little information was available regarding the setup behavior of long piles in normally consolidated highly plastic clays, The available load test data covering long periods of setup were limited to clays of low plasticity (Refs 1,2,3,4,5). Analytical methods that included consideration of reconsolidation and setup were limited to the empirical methods proposed by Soderberg (Ref 6) and the newly developed and untested finite-element solutions proposed by Carter, et at (Ref 7), Experimental data regarding the behavior of piles under cyclic axial loading in similar soils were limited to laboratory tests on model piles in a remolded soil (Refs 8, 9, 10). Computer programs had been developed to model the hysteretic nature of pile-soil interaction under cyclic loading. including the effects of cyclic degradation (Refs 11, 12), However. the programs had not been calibrated against actual load test data from prototype-size piles, The test program was therefore centered around a fully instrumented pile driven to a deep penetration in a soil deposit having a geologic history similar to Conoco's Green Canyon site in the deepwater Gulf of Mexico (Ref 13), In addition to measuring the load distribution along the pile the instrumentation was designed to obtain all the experimental data necessary to develop or verify evolving effective stress design procedures (Refs l4, 15), The load tests covered the full range of consolidation history and included static and cyclic loading in tension and two-way loading to failure in both tension and compression to provide a baseline for comparison of the effects of degradation during the cyclic tension tests.

Load Test Programs

While the load tests on the large-diameter instrumented pile provided invaluable information regarding the behavior of a long. elastic pile. the available variations in loading histories were very limited, Furthermore, the data were limited to only one pile of one diameter at one site.