Ultimate Lateral Capacity of Piles in Clay

Abstract

ABSTRACT This paper presents the results of centrifuge tests and nalyses performed to determine the influence of cyclic lateral loading on the ultimate lateral resistance of foundation piles in normally consolidated clay soil. The work was motivated by the need for an appropriatemethod for estimating lateral soil resistance in order to assess the foundation reserve strength ratio (RSR) of fixed offshore platforms. In the absence of test data, Industry practice has been to use the fully degraded cyclic resistance from the working stress design criteria recommended in API RP2AA1 (based on the recommendations of Matlock2 for cyclically loaded piles. It was suspected that this working stress approach substantially underestimates the ultimate lateral soil resistance available. The data reported herein demonstrate that while the cyclic criteria are appropriate for working stress analyses, they are extremely conservative for ultimate strength analyses. It is recommended that the non-degraded, static clay criteria be used to obtain soil resistances for use in RSR calculations. Since the data were obtained in kaolin, caution should be used when dealing with highly sensitive or fissured soils; in those cases additional study may be warranted. The data reported herein show that when the pile displaces into undisturbed soil, the ultimate resistance that can be mobilized is, to a large extent, regained. The most significant implication of this finding is for platforms whose foundation reserve strength is controlled by lateral pile resistance. Platforms most favorably affected by the conclusions of this study are older, shallow water References and figures at the end of the paper. template-type jackets with unbattered piles. Using thecriteria recommended herein, the computed foundation RSR of such platforms is raised about 30 percent over that computed using cyclic criteria. INTRODUCTION The foundation reserve strength of steel piled jackets depends to varying degrees upon the ultimate lateral resistance exerted by the soil on the piles. Fig. 1 is a chematic representation of the failure mechanism of a steel pile jacket (SPJ) in which the lateral (environmental) load causes the platform to translate horizontally as pilesfail in lateral bearing. At failure, plastic hinges (dots) form in the piles, and the full lateral soil resistance is mobilized between the hinges. Depending on the degree to which they are battered, the lateral bearing capacity of the piles (including pile moment capacity) comprises most of thelateral strength of the foundation system. Shallower water platforms tend to exhibit this shear type foundation failure mechanism most prominently, and their reserve strength can be governed by lateral (rather than axial) pilecapacity. A detailed description of SPJ foundation failure mechanisms is given by Murff and Wesselinks3. Since offshore platforms are subjected to cyclic wave loading, there is a potential for the ultimate lateral resistance of the foundation piles to become cyclically degraded during the buildup phase of a storm. The lateral soil resistance on a pile is usually characterized by a series of uncoupled, nonlinear soil springs whoseresponse is defined by a load per unit length (p) per unit of local lateral displacement (y).