Effect of cyclic loading on the mobilization of unit base resistance of model piles jacked in sand

Abstract

This paper reports the results of a series of monotonic compressive and cyclic load tests performed on a closed-ended jacked model pile installed in a half-cylindrical calibration chamber with image analysis capabilities. The monotonic compressive load tests were carried out before and after performing a displacement-controlled cyclic load test to determine the impact of cycling on unit base resistance. Digital images of the sand and the model pile were taken during cyclic loading and processed using the digital image correlation (DIC) technique to obtain the cumulative displacement and strain fields in the sand domain. The results show that the ultimate unit base resistance can drop significantly after cycling. The magnitude of the drop in ultimate unit base resistance depends on both the magnitude of the cyclic displacement amplitude and the number of cycles. However, the unit base resistance at plunging increases after large-displacement half-amplitude cycling. The increase in unit base resistance at plunging after cycling is linked to the flow of sand particles to the zone below the conical base, the occurrence of sand particle crushing, and the dilative behavior of the sand outside a bulb of crushed particles formed during cyclic loading. The processed DIC data show that for cyclic displacement half-amplitudes ∆wcyclic less than or equal to 0.25 mm (0.0007B), the cumulative radial and vertical displacements in the soil domain normalized by ∆wcyclic are negligible. Values of ∆wcyclic greater than 0.25 mm produce normalized displacements in the soil domain that increase as the cyclic displacement amplitude increases.