Cyclic Stability Diagram for Axially Loaded Piles

This paper develops the idea of a cyclic stability diagram in which the mean and cyclic axial loads on a pile are plotted and three regions are identified: (1) A stable zone in which cyclic loading has no effect on pile capacity; (2) a metastable zone in which cyclic loading causes some limited reduction of load capacity; and (3) an unstable zone in which cyclic loading will result in failure of the pile within a specified number of cycles. An analysis that can be used to generate the stability diagram is described, and two examples are presented to demonstrate the characteristics of the three zones. One example also examines the relative importance of cyclic degradation and degradation due to a strain‐softening shaft response. Finally, a comparison is made between theoretical predictions of the lower boundary of the unstable zone and the results of some field and laboratory tests.