Computation of Interaction Diagrams for Cyclic Axial Loads in Offshore Piles Based on an Effective Stress Method-Paper

Paper

Pdf Summary

The paper presents a method for predicting how cyclic axial loading reduces the capacity of offshore piles. It combines the CPT-based effective-stress ICP method with a numerical model of the pile and surrounding soil. The pile shaft is divided into elements with degrading t–z curves, while the pile tip is represented by a non-degrading Q–z element. Laboratory cyclic direct simple shear tests are used to estimate the reduction in effective stress and hence shaft resistance.<br /><br />Because cyclic shear stresses vary along the pile and change as resistance degrades, the load history is processed in blocks of cycles. At each stage, local stress amplitudes are calculated and degradation is accumulated using an equivalent-cycles approach. The procedure produces post-cyclic resistance estimates and interaction diagrams showing combinations of average and cyclic load that lead to failure for a specified number of cycles.<br /><br />The method is applied to a Gulf of Mexico soil profile containing alternating clays and sands. In this example, degradation in the sands is substantially greater than in the clays, and much of the capacity loss occurs during the first 15 cycles. Increasing the number of cycles from 15 to 1,000 further reduces available strength. The calculated normalized interaction diagrams show little sensitivity to pile diameter in the cases studied, but are sensitive to the assumed sand-degradation model.<br /><br />The authors emphasize that reliable results depend on representative laboratory testing and appropriate boundary conditions, particularly the choice between constant-volume and constant-normal-stiffness tests for sands. The approach can also represent variable storm load histories and is proposed as a practical screening and communication tool for geotechnical and structural designers. The example results are case-specific and should not be generalized without further validation.

Keywords

cyclic axial loading

offshore piles

pile capacity reduction

CPT-based ICP method

effective stress

t-z curves

Q-z element

cyclic direct simple shear

load history degradation

interaction diagrams