Comparison of Axial Pile Capacities in Clay at Offshore Sites Investigated by both 'Early' and 'Recent' Sampling Techniques

摘要

Research on structures lost during Hurricanes Ivan and Katrina has revealed that the structures' foundations performed well and that foundation failure was not the cause of the structures' loss. Geotechnical site investigations for most of these structures were performed using the percussion-sampling technique, which was the state-of-the-art practice early in the offshore geotechnical profession. At the time, geotechnical practitioners were aware of the disturbance effects of percussion sampling in clay, and tried to compensate for the disturbance by applying adjustment factors to the measured undrained shear strength values, and thus reduce conservatism in the foundation design. In time, geotechnical practice evolved to the use of in situ testing tools and push-sampling techniques to better evaluate the in situ mechanical properties of clay. However, at hurricane-damaged structure sites where the geotechnical site investigations involved percussion sampling and the foundations performed well, how does one assess conservatism in the foundation design? The authors reviewed geotechnical data from various sites, most located in the Gulf of Mexico, which contained two sets of soil data: one set from samples acquired by percussion sampling and another set from samples acquired by push sampling. The selected sites had stratigraphies predominantly of clay. Subsequently, the authors calculated axial pile capacities for each site using the following approach: (1) for the percussion-sample profiles, used the design method and soil strength parameters as selected by the original professionals; (2) again, for the percussion-sample profiles, used the current API (2000) design method with the originally selected design soil parameters; and (3) for the higher quality, push-sample profile, used the API (2000) design method. Results of this study can be used to evaluate the likely range in conservatism in the foundation design due to early geotechnical site investigation techniques. The authors also discuss factors that researchers should consider when reevaluating the foundations of sites investigated using early geotechnical sampling techniques.