Study On Modified Load Transfer Method For Calculation Of Pile Foundation Settlement

Nowadays, pile foundations are needed in almost every large infrastructure construction, but the calculation of pile foundation settlement is always a difficult problem. A great many in-situ and centrifuge model tests of pile foundation indicate the skin resistance of pile represents obvious nonlinearity at different stress level; therefore, it is necessary to incorporate the nonlinearity into calculation of pile foundation settlement. Based on the variety of skin resistance nonlinearity, including material stiffening, softening and elasto-plastic model, a modified load transfer method with unified trilinear model is established to study on the settlement of single pile and pile group analytically and numerically.Comparing some concepts easily to be understood in the load transfer method, it is firstly found that the relationship of skin resistance and relative displacement of pile and soil is just the same as the one of skin resistance and shaft displacement, because the relative displacement and the pile shaft displacement at he interface of pile shaft are actually the same. Secondly, it is found that the hyperbolic model of load transfer can be divided into empirical and theoretical curves and how the critical displacement of stiffening model is determined.Through setting up the unified trilinear model, which can express the stiffening, softening and elasto-plastic properties of skin resistance, the analytical solution of load transfer of single pile is deduced, considering the limit skin resistance increasing linearly along the depth. Then the way to calculate the length of plastic and sliding segment is also suggested. With the analytical solution in this paper, the cases in publications and the centrifuge model test of pile are used to calculate the settlement of single pile, including the stiffening, softening and elasto-plastic model. The predicted values are in agreement with those of publications and results of the test. Then the analytical model is employed to study the influence of geometric parameters of pile, the load level at pile top and the properties of soil around pile on the behavior of load-deformation of single pile and the changing regularities of bearing ratio with pile parameters.Coupling the unified trilinear model with rod element in Finite Element Method (FEM), a convenient Displacement Matrix Method is established, which enlarge the application of unified trilinear model to the settlement calculation of single pile in layered soil. In this model, the skin resistance is transformed into equivalent nonlinear spring, and the pile shaft is divided into many rod elements, then the initial stress method areemployed to solve the nonlinear equations. Meanwhile the elasto-plasticity of pile concrete and the effect of skin resistance on pile end settlement can also be considered. The case in publications, centrifuge tests and case history are studied with this model, it shows the calculated values are well in agreement with measured values and analytical solution. Then the elasto-plasticity of pile concrete under heavy load and application scope of the influence of skin resistance on pile-end settlement are also studied.With concept of hybrid method for pile group, the linear elastic load transfer relationship in trilinear model is introduced to pile group settlement. By analogy with Randolph model, the theoretical expression of stiffness of linear elasticity is obtained. Then the expression is taken into pile group and carried on parametric study. The changing regularities of the load settlement ratio of each pile in different pile group and load distribution of each pile with slenderness ratio, space ratio and stiffness ratio in elastic half-infinite space are studied. Then the influence of finite thick layer and ratio of the modulus of pile-end soil to that of pile side soil on the settlement of pile group and load distribution are analyzed. Finally, the effect of configuration of layered soil on the load distribution of each pile in group is also investigated.The unified trilinear model is introduced to the calculation of pile group settlement through the hybrid method, and then the nonlinearity of skin resistance can be taken into pile group settlement. The centrifuge test and case history are analyzed by this method, and the calculated values of pile groups are compared with measured values, which show well agreement, hence the rationality of this method is also verified. Meanwhile, by this method, the changing regularities of the end bearing ratio of pile group and end settlement with loads at cap and the effect of softening degree to the behavior of pile group load-deformation are studied.