Model Test And Theoretical Study On Deformation Behavior Of Single Piles To Long-term Cyclic Axial Loading

In China, there are many energy facilities and high-speed transportation systems, like wind turbine, offshore platform and high-speed railway etc., are being built as the nation’s economy grows rapidly. In these projects, piles that support the superstructures are exposed not only to the dead load from superstructure self-weight, but also to the cyclic loading induced by winds, waves or trains in their service. Pile capacity and accumulative deformation are the two major aspects of the behavior that are of great interest to the foundation designer. Especially for the structures which are very sensitive to the uneven settlement, controlling of the long-term settlement caused by cyclic loading has become one of the key issues. At present, performance of the pile under cyclic loading is still not well understood and there appears to be few guidelines proposed yet. Therefore, it is of great practical significance to study the performance of the pile to axial cyclic loading.Large-scale model tests and numerical simulations were conducted to study the issues. The research works in this paper are based on the projects funded by the National Science Foundation of China (No.50878193, No.51225804and No. U1234204). The serviceability performance concept is developed and guidelines for the design of pile foundation under cyclic axial loading are proposed. The main research works and conclusions are as follow:(1) The response of axially loaded pile under loading-unloading was investigated with the FE method. A particular attention was laid to the load transfer mechanism and deformation behavior. It is found that rebound of pile due to unloading results in negative friction in the top of the pile. Unloading amplitude, length to diameter ratio, relative pile-soil stiffness, and Young’s modulus of the bearing stratum at the pile tip are indicated to be important parameters that determine pile performance. The range of negative friction increases with increasing unloading magnitude. Under a specified unloading magnitude, the range of negative friction increases with increasing pile length and relative pile-soil stiffness, but with decreasing Young’s modulus of the bearing stratum at the pile tip.(2) The design and building of the heavily instrumented model pile were described. The instrumentation includes axial load cells, pore water transducers and earth pressure cells. Techniques were developed to calibrate the pore pressure transducers and the earth pressure cells that are installed on model piles. A total of25cyclic load tests on piles were conducted to investigate the capacity and deformation response of stiff pile under long-term cyclic loading. The maximum number of loading cycles was up to50,000in single cyclic load test.(3) The effects of cyclic loadings on the axial capacity and deformation of single pile in silts under various cyclic loading conditions were described. The measurements of stresses at the soil-pile interface and axial force along pile were also presented. The mechanisms of capacity degradation and accumulation of permanent displacement were analyzed and discussed. A simple method for predicting the long-term accumulative settlement of piles with consideration of the influences of the characteristics of cyclic loads was proposed. It is found that the accumulated displacement increases with increasing number of loading cycles, and also with the cyclic load amplitude. The cyclic shearing results in a significant increase in excess pore water pressure and a decrease in effective stress at the interface, indicating that soil at the interface contracted under the cyclic loading. The stress variations are found to be highly dependent on shearing amplitude. The degradation of shaft resistance of piles subjected to cyclic loading is found to be attributed to the reduction of normal effective stress with cycles.(4) A user subroutine UINTER was written to incorporate the constitutive model for the cyclic behavior of the interface in the finite element program ABAQUS. The formulation of the constitutive model and the integration procedure of the constitutive equation were described. The UINTER was validated by comparing the numerical simulation results with the results of cyclic simple shear tests on sand-steel interfaces. Numerical calculations were carried out to study the cyclic shear interface behavior and main factors influencing that behavior. A simply cyclic model for the cyclic behavior of the soil-pile interface was proposed.(5) A linear hysteretic cyclic model for pile-soil interface behavior under cyclic loading was proposed. The degradation of normal stress at interface with cycles was described with a power function. The response of the pile tip was presented by a hysteretic hyperbolic soil model. A cycle-by-cycle load transfer approach for analyzing the behavior of single pile in silts under cyclic axial loading. The accumulation of permanent displacements and the dependence of loading amplitude were considered. A computer program ACLPAP was written. Numerical calculations were carried out to examine the various aspects of pile behavior under cyclic axial loading. The influences of length to diameter ratio, relative pile-soil stiffness, and on the pile behavior under cyclic loading were also investigated. The results show that the pile behavior mainly depends on the cyclic loading amplitude. A threshold exists, below which the pile behavior would be little affected and did not accumulate any deformation. Under a given cyclic loading amplitude, the cyclic effects increased with increasing relative pile-soil stiffness, but decreased with increasing pile length and Young’s modulus of the bearing stratum at the pile tip.(6) The aspects of the behavior of pile to cyclic loading that are of interest to the pile foundation designers were discussed. The concept of controlling the limit cyclic load ratios was developed. The idea of a cyclic deformation diagram was developed. Methodology and procedure for design of pile foundation under cyclic axial loading were proposed. Finally, a case was presented to validate the approach. Reasonable agreement was observed between the measured and computed results.In this research, both experimental and theoretical works have been done to study the response of single pile in silt under long-term axial cyclic loading. The results provide a better understanding of the axial behavior of the pile to cyclic loading.