Theoretical And Experimental Study On The Behavior Of Vertically Loaded Piles

With continuously development of the Western China and strategic opportunity of central rising, the extra-long large-diameter pile is being more and more applied in construction of highways and bridges. For the deep bury-depth and complex geologic conditions around pile, especially the large compressing of pile shaft, the load transfer mechanism of the extra-long large-diameter pile is quite different from used one, and it's better to limit the vertical bearing capacity design by settlement on pile top. Besides, when pile is surrounded by weak soil or of large unburied length, it usually failed buckling. So the calculating and design of extra-long large-diameter pile should take into account the effect of load transfer mechanism, geometrical size and bury ratio, besides the soil condition, construction method, the particularity of it bring us new challenge. Recently, the vertical bearing behavior of extra-long large-diameter pile is still need to be studied, so systematically work is done in this paper on the basis of national natural science foundation of China"Study on the buckling mechanism of pile foundation with elevated caps for bridge by meshfree method", Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP)"Theoretical and experimental study on buckling mechanism of pile foundation for bridge"and Project from Hunan communication department"Study on the bearing mechanism and optimization design of pile foundation under extra-high pier in mountainous area".⑴Based on lucubration on the differential equation and load transfer function of pile, the double-broken-line model and hyperbola model were introduced to deduce the analytical solution of load transfer of pile, which can take the character of layered stratum and nonlinear behavior of side resistance into account. Then, the boundary conditions adjustment method was put forward to calculate the load-settlement relationship of pile and carried out the design theory that limit the vertical bearing capacity of pile by settlement on top. Applied it to analyze the test pile of Mao-cao-jie bridge in Hunan province, and the result indicates that the mehtod could describe the load transfer behavior and load-settlement relationship well.⑵Based on the load transfer method, the two-broken-line model was improved to fit the exertion character of negative skin friction, and the piecewise analytical solution of negative skin friction along pile shaft was deduced, which can take the character of layered stratum into account. Then, an engineering example was analyzed by the method in combination with the Boussinesq solution, the results fits good with the spot measured data.⑶For the restriction of the mesh in finit element method (FEM), the meshfree local Petrov-Galerkin method (MLPG), which based on the local weak form of differential equation, was introduced into the fields of pile foundation for the first time. Then, the calculation format was deduced by weighted residual method and moving least squares approximation, and the related problems, such as the choice of weight functions, treatment of essential boundary conditions were studied. Based on these, the elastic-plastic model of pile and soil, the treatment of discontinous on the interface between different materials were introduced, and relevant program was also worked out. Analysis showed that the MLPG is much better in accuracy than FEM.⑷The orthogonal experiment design theory was introduced into the theoretical analysis of pile foundation, the orthogonal table was constructed by orthogonal Latin matrix to permuted and combined the four main effect factors on vertical bearing behavior of pile with their levels. Then, the effects of the factors, rules of the load-settlement curve on pile top and behavior of vertical bearing capacity were analyzed by the meshless program to deepen the understanding of vertical bearing behavior of extra-long large-diameter pile.⑸The long slender pile can fail in buckling mode, especially in soft soil. For this, experience methods, energy method and power series method for solving the buckling load of pile were firstly discussed, then, in order to analyze the character of the large displacement occurred on pile shaft when buckling, the non-linear large deflection differential equation for entirely and partial buried pile was established respectively by energy approach and variation method. The perturbation theory was introduced into buckling problem of pile for the first time, and through transforming the load on pile top and maximun deflection of pile shaft into power series by perturbation parameters, the higher-order asymptotic solution of load-deflection was founded. And effect factors of the buckling load and the behavior of the pile after flexural buckling was analyzed, some qualitative conclusions were got.⑹A series of model tests in lab were designed and performed by revised model test apparatus, in which the model piles were of various pile length, diameter, constraint conditions at pile top and surrounding by various soil materials. Then the test result was analyzed by various theory according to the failure mode of the model pile to validate the rationality of the methods in this paper.