Research On Nonlinear Transient Torsional Vibration Characteristics Of Pipe Pile In Saturated Soil

With the outstanding advantages of simple process,high bearing capacity,good durability,economy and environmental protection,pipe pile has become one of the mainstream engineering pile foundations in recent years.When the pile foundation bears eccentric load,torsional vibration will occur under the action of torsional force.When the pile-soil strain is large,the relationship between stress and strain is nonlinear.In most practical working conditions,pile foundation often bears transient impact load.Compared with static load and simple harmonic vibration,its dynamic characteristics are more complex,instantaneous,unpredictable and sudden.However,at present,the research on the nonlinear torsional vibration characteristics of pile foundation is only limited to the frequency domain,and the research on the time domain response has not been reported.Therefore,it is necessary to study the nonlinear transient torsional vibration characteristics of pipe pile in saturated soil.In this paper,the equivalent shear modulus and equivalent damping ratio in Hardin-Drnevich model are used to reflect the nonlinear characteristics of soil.The main research work is as follows:(1)Hardin-Drnevich model is used to simulate the radial nonlinear characteristics of soil,and the mathematical model of coupled torsional vibration of saturated viscoelastic soil and pipe pile under rigid support and elastic support at pile end is established.Based on the three-dimensional Biot wave theory and considering the viscous damping characteristics of soil,the dynamic control equation of pile-soil system is established.Using the variable separation method and Laplace transformation technology,combined with the pile-soil solution conditions,the circumferential displacement and stress of the soil around the pile,the soil in the pile core and the pile body are obtained.When the equivalent shear modulus is used to reflect the nonlinear characteristics of soil along the radial direction,the soil shear modulus participates in the iteration until the numerical difference obtained from the previous and subsequent cycles meets the accuracy requirements.Furthermore,the analytical solution of pile-soil torsional vibration response in time domain is obtained by using Laplace Inverse transform technology.The obtained solution is reduced to steady-state nonlinear solution and transient linear solution,which are compared with the solutions in relevant literature to verify the rationality of the results,and the influence of relevant pile-soil parameters is analyzed.It is concluded that the influence of soil radial nonlinearity on the torsional angle of pile top is mainly in the late stage of load and after unloading.(2)The compaction in the process of pile sinking,the interference and remodeling in the construction operation will weaken or improve the soil stiffness,making the properties of the soil around the pile show heterogeneous characteristics along the radial direction.Considering the radial heterogeneous distribution of soil shear modulus and the nonlinear relationship between stress and strain,the torsional vibration control equation of heterogeneous saturated viscoelastic soil and pipe pile system is established,and the circumferential displacement and circumferential shear stress of soil around the pile and in the pile core are obtained.Combined with the boundary conditions of elastic support at the pile bottom,the time-domain response of pile-soil torsional vibration is obtained.The time domain solution is reduced to the frequency domain solution.Considering the hysteretic damping characteristics of soil,the influence of relevant pile-soil parameters in the frequency domain is analyzed.It is concluded that considering the radial nonlinear characteristics of soil under the steady-state vibration of pile foundation will reduce the real stiffness of pile top and increase the dynamic damping.