Effects of Cyclic Loading and Pile Flexibility on Axial Pile Capacities in Clay

Abstract

ABSTRACT The paper investigates the degree to which relative pile-soil stiffness, rate of pile loading, and severity of axial load cycling influence the behavior of piles in clays that are typical of those found in the Gulf of Mexico (GOM). It is confirmed that each factor can impact axial pile behavior substantially. Results of analyses of typical piles subjected to design storm load patterns also are presented. These show that some pile foundations in GOM-type clays can have greater calculated axial capacity when cyclic loading effects are considered explicitly than when static procedures are used. This improvement was demonstrated for a deepwater and a shallow water steel jacket platform, a compliant tower, and a TLP. INTRODUCTION Piles for offshore structures usually are designed to have an axial capacity at least equal to the greater of twice the applied operating loads and 1.5 times the load during extreme environmental conditions [1]. Axial capacities commonly are calculated using methods that are based on tests in which piles were loaded to failure slowly. The short rise times and cyclic nature of storm loads generally are not considered explicitly, but experience indicates present practice generally produces safe pile foundations. Explicitly incorporating these factors and other known effects into the design recipe presumably would result in more rationally designed foundations. This paper explores how cyclic axial pile loading impacts pile capacities in normally consolidated clays and the degree to which considering such loading can affect pile capacity estimates. The analyses that have been performed assume the soils that are modeled do not soften during monotonic straining, but can lose strength during cyclic loading. We first discuss the role of cyclic axial pile analyses in foundation design practice and how such analyses are performed. This is followed by a review of available data that shows how pile behavior in clays is influenced by the nature and magnitude of cyclic loads, the rate of loading, and relative pile-soil stiffness; Finally, example analysis results are presented to illustrate the impact of cyclic loading effects on the calculated capacities of the foundations of two steel jacket platforms, a compliant tower, and a tension leg platform (TLP) CYCLIC ANALYSES IN FOUNDATION DESIGN PRACTICE Interaction with Other Design Elements. Designers of offshore platforms and piled foundations deal with numerous uncertainties, including imperfect knowledge of: the loads to which the superstructure is subjected; the behavior of the superstructure under those loads; and how the founding soils respond when those loads are transmitted to them via the foundation piles. With respect to the foundation, several simplifying assumptions regarding pile-soil interaction commonly are made in order to keep the analyses tractable. These assumptions include:The capacities of offshore piles can be calculated using methods based primarily upon tests of relatively short onshore piles that were loaded slowly to failure;Pile capacity reductions due to the degradation of soil resistance by cyclically applied storm loads do not need to be considered explicitly; andPile capacity increases that can occur in clay soils during rapid loading also do not need to be considered explicitly.