Estimation of friction fatigue for preformed piles based on constant normal stiffness shear test

A series of constant normal stiffness direct shear tests are performed to investigate the friction fatigue for preformed piles. The results show that the shear stress (friction resistance) displays a exponential decay trend with the increasing shear cycle number, and most of the reduction occurs during the first few cycles and reaches a steady value after about 25 cycles. Cyclic shears induce a contraction of the narrow shear zone, leading to a release of normal stress, which is one of the causes for the friction fatigue. The interface friction angle decreases exponentially with the shear cycles within the first few cycles and subsequently approaches a residual value, which is another cause for the friction fatigue. The normal stiffness controls the degradation rates and degradation extents of the shear stress, normal stress and interface friction angle. A larger normal stiffness causes a faster degradation and a smaller residual value. The above results indicate that the pile-soil friction fatigue is closely related to the stiffness of the surrounding soil, and the stiffer soil will lead to a more evident friction fatigue effect.