Comparison of Axial Pile Capacities in Clay at Offshore Sites Investigated by Both Early and Recent Sampling Techniques
Authors: Quiros Gerardo W. , Aramburu Jose , Peters Patricia 
Published: October 2008

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:for the percussion-sample profiles, used the design method and soil strength parameters as selected by the original professionals;again, for the percussion-sample profiles, used the current API (2000) design method with the originally selected design soil parameters; andfor 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.

Introduction

The empirical procedures for computing the axial compressive capacity of driven pipe piles in clay are directly influenced by the undrained shear strength values selected for the analyses. The selected undrained shear strength, in turn, is influenced by the sampling technique (Hvorslev, 1949). Early in offshore geotechnical site investigations, a percussion-sampling technique was employed that resulted in lower undrained shear strength measurements in comparison to those typically obtained with current sampling techniques, such as push sampling. A comparison is made of shear strength data collated at eleven sites (Sites D through N in Table 1), most located in the Gulf of Mexico (GOM) where both percussion-sampling and push-sampling techniques were used. Additionally, the authors compare axial compressive pile capacities developed using both the percussion-sample and the push-sample shear strength profiles. These results were assessed to gain insight on the degree of conservatism in the foundation design of early offshore structures. Sets of data from three other sites are also presented (Sites A through C in Table 1), which reveal various aspects of marine geotechnical site investigations that researchers should consider when evaluating the foundation design of early offshore structures.