Cyclic Degradation Mechanism Of Vertical Bearing Capacity Of Pile Foundation In Sand

In view of the insufficiency of the understanding of the degradation mechanism of pile foundation under cyclic loading,this paper studies the behavior of pile foundation under vertical cyclic loading by means of model test,theoretical analysis and numerical simulation,focusing on cyclic degradation effect.The main research work is as follows:Firstly,Study on shear behavior of pile-soil interface under cyclic loading.The effects of cyclic number,cumulative displacement and normal stiffness on the friction fatigue performance and monotonic shear performance after cyclic loading were studied.The test results show that the normal stress and shear stress decay rapidly with the increase of the number of cycles in the initial 10 cycles,and the decay rate increases with the increase of the normal stiffness,the accumulated cyclic displacement and the number of cycles are smaller,and the shear stress ratio of monotonic shear and normal stress recovery after cyclic loading is smaller than that of the first monotonic shear.The shear dilatancy of cyclic shear tests after normal stress recovery is smaller than that of shear tests.Secondly,load test is carried out for the vertical cyclic loading and aging of pile foundation.A series of vertical cyclic and monotonic loading tests were designed and carried out.The effects of cyclic displacement amplitude and loading rate on the bearing capacity of pile foundation were studied through model tests of pile foundation in sandy soil.The results show that the vertical bearing capacity of pile foundation decreases rapidly in the first few cycles with the increase of cyclic loading times,which indicates that the strength of soil degrades during cyclic loading process.The larger the cyclic loading rate,the more obvious the reduction amplitude of pile head load caused by cyclic loading.With the cyclic loading,the vertical bearing capacity tends to be stable gradually.When the cyclic displacement is larger,the pile strength decreases notablely and the compaction effect of the foundation soil is strengthened and the strength of the soil body recovers quickly.Thirdly,the numerical simulation is studied on cyclic mechanism of pile foundation bearing capacity under cyclic loading.Based on the cyclic shear test results of soil element and cyclic degradation characteristics of pile foundation during vertical loading and unloading,T-z model and Q-z model of cyclic degradation of shear stress at pile-soil interface are established and applied.The cyclic mechanism of pile foundation bearing capacity under cyclic loading is numerically simulated by load transfer method.The results show that the variation rule of the simulation results is basically consistent with the test results.With the accumulation of accumulated displacement,the maximum and minimum loads on the pile head tend to be stable gradually.The lateral modulus resistance values of piles at different locations are different.The smaller the h/R value means the closer the pile tip and the deeper of the soil,so the soil stiffness is larger and the cyclic degradation is more obvious.