Research Of Strengthening Mechmism And Effect Evaluation Of Foundation By Dynamic Compation

As a foundation strengthening technique,dynamic compaction(DC)is characterized by its many advantages,such as obvious reinforcement effect,high efficiency,simple process,and low cost.It has been widely used in the foundation reinforcement of the gravel soil,sand soil,low saturation silt and clay soil,miscellaneous filling soil,plain filling soil,collapsible loess,and interbedding silt and clay,in roads,railways,industrial and civil construction and other projects.However,given that DC process is related to the large deformation,there is a lack of completed theoretical methods of soil improvement,especially a soil constitutive model suitable for the reinforcement effect analysis of DC.As a result,the reinforcement mechanism of foundation has not been clear,and both technological parameters and effect evaluation of DC are mainly experiential.Therefore,the research on the strengthening mechanism and effect evaluation of DC reinforcement on foundation is of great theoretical significance and engineering value in the improvement of analysis methods,rationalization of scheme design and the accuracy of the reinforcement effect evaluation of DC.In this paper,based on the classical soil constitutive models(Mohr Coulomb and Drucker Prager models),a dynamic compaction constitutive model for unsaturated soil with large deformation is proposed.The hydro-mechanical model of dynamic consolidation of saturated soil is established by integrating the extended Biot dynamic equations.Further,the reinforcement mechanism of dynamic compaction and dynamic consolidation are revealed with the finite element method(FEM).Finally,the calculation method of reinforcement effect is established and the design method of DC for foundation reinforcement is put forward.The main research contents and achievements are as follows:(1)Based on the classical soil constitutive model,an improved cap model adapted to the unsaturated soil with large deformation is proposed,considering the modulus change with drop numbers of DC,and the corresponding VUMAT subprogram is developed.A FEM,in which the ABAQUS overlapping meshes and connector elements are used to characterize the coupling effect of soil and water,has been developed based on generalized Biot's u-U formulation.The corresponding VUMAT subroutine has been developed to realize the numerical simulation of dynamic fluid-solid coupled method of saturated soil.The accuracy and reliability of the method are validated with the analytical solution of one-dimensional wave,which provides the theory foundation for the subsequent study and analysis.(2)Based on the improved cap model for unsaturated soil,FEM of DC by single-point tamping is built and the reliability of the numerical model is verified by comparison with the centrifugal experimental results.Then,the soil stress path in p'-q plane and the stress-strain in both vertical and horizontal directions are obtained during the DC process,which reveals the characteristics of the soil stress-strain and reinforcement mechanism.The predicting formula of soil effective reinforcement range by single-point tamping is put forward on the basis of numerical analysis results.The soil reinforcement effects of DC with different drop energy,tamper radiuses,tamping distances,and different combinations are investigated by multi-point tamping FEM.The effect of the tamping sequence on the reinforcement between adjacent impact points is especially revealed.The evaluation method for estimating the effect of compaction between adjacent impact points is proposed considering above-mentioned influence factors.The performance of the formula is assessed by comparison of two engineering cases.(3)Based on the improved fluid-solid coupling cap model of saturated soil,the development law of excess pore water pressure and effective stress of saturated soil of dynamic consolidation is obtained by establishing the FEM of homogeneous saturated soil,and reinforcement mechanism of dynamic consolidation is also revealed.The effect of groundwater table and soil permeability on reinforcement of saturated soil is studied,the prediction model for the reinforcement effect of saturated soil with high groundwater table is presented.The adaptation of the proposed procedure is illustrated by comparing its prediction with two cases in the engineering fields.From the perspective of practical engineering,the influences of the different thickness and location of soft interlayer,and drop energy on the development of excess pore water pressure and reinforcement effect during DC process are studied through the FEM of nonhomogeneous saturated soil layers.(4)Based on Han's(2014)construction parameter estimation method of DC,the improved design method of DC construction parameters for saturated/unsaturated soil is put forward,and the construction scheme is optimized through a soft soil foundation treatment by DC.Results reveal the rationality and practicability of the presented method in the design of the parameters in DC construction.