Effect of Cyclic Load Sequence on the Accumulation of Deformations and Post Cyclic Strength of Sand-Structure Interfaces-Paper

Paper

Pdf Summary

The study examines how the amplitude and sequence of cyclic loading affect deformation, normal-stress degradation, and post-cyclic strength at sand–structure interfaces—factors that govern pile skin friction. Researchers conducted load-controlled direct-shear tests on dry Ottawa sand against a rough, sand-coated surface under constant normal stiffness conditions, simulating the restraint of surrounding soil. Tests applied cyclic packets at three amplitudes (30%, 50%, or 70% of monotonic interface strength), individually or in sequence, followed by monotonic shearing.<br /><br />For single packets, higher-amplitude cycling caused greater contraction and normal-stress loss, increased accumulated displacement, and reduced post-cyclic shear strength. The order of small and medium packets had little effect. However, sequence mattered when a large-amplitude packet was involved: applying small or medium cycles after large cycles caused greater stiffness degradation and further normal-stress loss, producing lower post-cyclic strength than when large cycles came last. In some cases, subsequent small-cycle displacements were up to 30 times those during a small-amplitude packet applied alone, and post-cyclic strength was reduced by as much as half. As few as five large-amplitude cycles substantially changed interface behavior.<br /><br />The results suggest that design methods that convert different cyclic load packets into an equivalent number of uniform cycles may underestimate deformation and capacity loss because they do not account for loading order. For offshore piles, a severe storm may not cause immediate failure but could weaken the interface enough for later routine loading to drive progressive failure. Further research using local measurements is needed to characterize changes within the interface shear band.

Keywords

cyclic loading

sand-structure interface

pile skin friction

direct-shear tests

normal-stress degradation

post-cyclic strength

amplitude sequence

offshore piles

interface shear band

load-controlled testing