Laboratory study of local clay-pile friction evolution for large numbers of cycles

Abstract

This work aims at investigating the evolution of local friction mobilized at the pile-soil interface, focusing on small amplitude and large number of displacement controlled cycles, corresponding to fatigue behaviour (10^5 cycles). An experimental program has been conducted in a calibration chamber with an instrumented pile-probe installed and loaded in reconstituted specimens of saturated clay. After describing the testing setup and the experimental procedure, a typical test is presented, carried out on a reference French kaolinite to evaluate the local friction mobilized upon different monotonic loadings and its evolution during the application of displacement-controlled cyclic loading. Quasi-static displacement-controlled compression tests are carried out before and after the cyclic sequence in order to evaluate the influence of the cyclic sequence on the mobilized static local friction after cycles, as compared to the initial friction mobilized. The displacement-controlled cyclic test allows to observe an initial phase of friction degradation which starts progressively from the very beginning of the cyclic sequence (cyclic softening), followed by a stabilization phase until the end of the test. The evolution of mobilized friction during the application of the cycles is quantified by introducing a coefficient of evolution which allows to clearly visualize the degradation and reinforcement phases. The local shaft friction evolution, upon the cyclic loading, is interpreted in terms of the combination of excess pore water pressure generation and dissipation which develops during the cyclic sequence and controls the evolution of effective normal stress acting on the soil-pile interface. A comparison is made between maximum static shear mobilized before the cycles and after the cycles, showing a clear influence of the cyclic sequence on this quantity. Finally, the repeatability of the test is checked, showing a fairly good level of repeatability.