A Numerical Study On Time-Dependent Behavior Of Interaction System Of Piles And Raft

In coastal area, there are always saturated layers of soft soil, of which the rheology characteristics are considerable and drainage conditions are poor. Therefore, it is necessary to consider time effects on interaction system including piles and raft. It is a challenging and applicable project to study on the time-dependent behavior of interactive system. Actually, numerical simulations based on mature and sound theoretical frameworks, are main approaches to study on time effects of interactive behavior, especially when boundary conditions are complicated and it is hard to put experimental methods into use. So far, many fruits were achieved in the domain of time-dependent behavior of interaction system. However, due to the limitations of computational hardware and theories, there are some characters and limitations in previous research as follows. Firstly, semi-numerical methods cannot be suitable for complicated boundary conditions such as discontinuous contact behavior, diverse structures, and multiple loading cases. Secondly, models simplified into plain strain state cannot reflect 3-D space effect and influence of superstructures. Thirdly, for the complexity of numerical computation and solution of consOlidation, complicated contact behavior often adopt simple mathematic description. Lastly, simple constitutive models are used such as linear elastic model and elements model ofrheology, which can not describe the stress-strain relationship realistically under high level loads.On the basis of previous research and fully considering most of factors such as contact behavior, elasto-plastic behavior of soil and effect of structure stiffness ere, more intensive and extensive works were conducted in the thesis by precise and proper numerical computations in order to discuss and analyze the mechanism of interactive behavior, which may provide valuable and referable experience for engineering practice and design. There are three faces as follows.1 In plane strain state, by coupling 2-D Biot consolidation theory and Mohr-Coulomb yield criterion, as well as using Coulomb contact pair theory to simulate the contact behavior of foundation and soil, time effect analyses due to elasto-plastic consolidation are conducted for incomplete interaction system of both raft and piled raft. It is shown that Mandel-Cryer effect which is related to the regulation of dissipation of excess pore water pressure has much close relationship with the time-dependent behavior of incomplete interaction system of both raft and piled raft. Typical time variation feature is generalized based on comparative analyses by changing piles length and previous research. And it also shows that, raft and piled raft with deep piles as special modes of piles-raft system display the extreme characters of the typical feature mentioned above. Furthermore, the limitation of simplified plane-strain interaction model of piled raft for conducting time effect analyses of consolidation is discussed, which when piles are disposed sparsely and drainage conditions are well. From another viewpoint, the necessity to conduct 3-D analysis is emphasized.2 Based on the analyses above, considering the effect of superstructure stiffness and soil foundation space, time effect analyses due to elasto-plastic consolidation are conducted for complete interaction system of both raft and piled raft, by developing 3-D numerical FEM model, coupling 2-D Biot consolidation theory and Mohr-Coulomb yield criterion, as well as using Coulomb contact pair theory to simulate the contact behavior of foundation and soil. Incorporating the dissipation and time variation regulation of excess pore water pressure in three-dimensional space, the mechanism of the time-dependent feature of complete interaction system of piles-raft foundation is analyzed. It is also shown that the redistribution of internal forces in the superstructure due to consolidation displays clear time variation characteristics and the stiffness of superstructure also has effects on the time-dependent behavior of entire interaction system, which exhibits a certain limitation.3 On the platform of ABAQUS program, an elastic visco-plastic constitutive model within the frameworks of Perzyna over-stress theory is developed by coding FORTRAN interface subroutine and is coupled with Biot consolidation theory numerically. Based on the elastic visco-plastic constitutive model firstly the dissipation and time variation regulation of excess pore water pressure due to both creep characteristics and consolidation of soil is discussed. It is examined that the phenomenon of increase of pore water pressure is closely associated with both visco effect and Mandel-Cryer effect when considering the coupled influence of creep characteristic and consolidation of soil foundation. Furthermore, through developing 3-D numerical FEM model of piles-raft system incompletely and completely, elastic visco-plastic consolidation comparative analyses are conducted, and the effect mechanism of creep characteristics of soil on time-dependent behavior of interaction system is discussed as well. It is shown that it shows similar time variation feature considering the coupled influence of creep characteristic and consolidation, to that when only soil consolidation is considered. The larger the creep index is, the more significant the coupled influence of creep characteristic and consolidation of soil foundation is, the more obvious the imbalance Of distribution and variation of excess pore water pressure in subsoil is, and then the more evident time variation features of interaction system of piles-raft display.