Stress Analysis Of Neutral Surface Of Pile Bottom Subjected To Vertical Pulse Load

The rigid pile has the characteristics of high bearing capacity and uniform settlement,and is widely used in various building foundations.When the surrounding soil of the pile is consolidated or the external factors such as surface loading cause the settlement of the surrounding soil of the pile to be larger than the pile foundation,the negative frictional resistance will be generated to the pile foundation.With the negative frictional resistance,the maximum stress of the pile foundation is not at the top of the pile,but at the neutral plane.The near-field earthquake has significant vertical acceleration,when the vertical earthquake propagates up the pile foundation,an additional dynamic stress is generated.When the additional dynamic stress is transmitted to the neutral plane,The stress at the neutral plane is the superposition of static stress and additional dynamic stress,and will be greater,and the safety factor will be reduced.In response to the above issues,the main research contents include:(1)The rationality of pile foundation with negative frictional resistance is simplified,and the one-dimensional strain wave equation considering pile-soil damping is established.The finite-difference method is used to solve the axial stress characteristics of pile foundation at the bottom subjected to pulse load.The results show that the additional dynamic stress is approximately linearly attenuated with the propagation distance.The additional dynamic stress at the neutral plane increases linearly with the increase of the elastic modulus of the pile foundation,and increases nonlinearly with the increase of the pile foundation radius.The value of the damping coefficient of the pile-soil has less effect on the additional dynamic stress at the neutral plane.(2)The factors affecting the negative skin friction of the pile foundation are analyzed by numerical simulation.The results show that the mechanical properties of pile foundation are mainly affected by the uniform distribution load on the surface within 1.3 times of pile length.The greater the friction coefficient of the pile soil,the greater the axial force of the pile foundation,and the neutral plane moves toward the pile top.The greater the elastic modulus of the pile-bottom soil,the greater the axial force of the neutral surface,and the neutral plane moves toward the pile end.(3)The pile foundation with negative frictional resistance is applied with pulse load at the pile end,and the numerical simulation results are compared with the theoretical calculations to verify the rationality of the theoretical calculation.The numerical simulation of the pile foundation under the action of vertical seismic load indicates that the additional dynamic stress at the neutral plane of the pile increases linearly with the increase of the acceleration peak,with the same peak value of seismic acceleration,the longer the strong action time,the greater the additional dynamic stress on the neutral plane.