PILE CAPACITY FOR AXIAL CYCLIC LOADING

The effects of cyclic loadings from storm waves on the axial capacity of piles driven in cohesive soils and supporting offshore platforms is described. An advanced analytical procedure is developed to estimate pile penetrations and to evaluate the pile performance when a platform is subjected to intense cyclic loadings. The procedure is illustrated with application to a platform located in a water depth of 1,000 ft (305 m) in the Gulf of Mexico. Loading rates, ratio of steady to cyclic load amplitudes, the sequencing and numbers of cycles of loading, and relative pile-soil stiffness are indicated to be important parameters that determine pile performance. Cyclic loadings and high rates of cyclic loadings have compensating effects on pile capacities. For the example location soils and piles, the limited numbers of cycles of intense design storm loadings result in significant strength degradation in the upper two-thirds of the pile length. The bottom one-third of the pile provides the margin of safety for loadings that could exceed those of the design storm.