Evolution of local friction along a model pile shaft in a calibration chamber for a large number of loading cycles

This Note presents the results of axial loading tests carried out on an instrumented "pile-probe" jacked into sand, in a calibration chamber, aimed at studying the evolution of local friction, up to very large numbers of cycles (10(5) cycles). After an initial phase of friction degradation (cyclic softening), a subsequent phase of mobilized friction reinforcement (cyclic hardening) is observed, which continues to develop up to very large numbers of cycles. The complete mechanism of shear degradation followed by the reinforcement phase is interpreted based on the competition between two mechanisms which are the mean normal effective stress decrease due to cyclic contractancy phenomenon, responsible for cyclic softening, and the progressive densification of the sand within the interface zone around the probe, which becomes predominant after a certain number of cycles, resulting in a cyclic hardening mechanism due to partially constrained dilatancy phenomenon. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.