Elastoplastic Model For Unsaturated Soil-Structure Interfaces And Its Numerical Implementation

Interaction between soil and structure are commonly encountered in the fields of civil,traffic and hydraulic engineering and other infrastructure constructions,such as interaction problems of pile-soil,retaining wall-backfill,earth and rockfill dam-subsoil,tunnel liningsurrounding rock,etc.,studies on which have always been important content of geotechnical engineering.The key for solving the problem is to establish a mechanical model of the contacting interface between two materials which can capture the mechanical behavior of interaction between soil and structure reasonably.However,the present constitutive models of interface are mainly designed for saturated/dry soil-structure interfaces,hence they are not applicable for interaction problems between unsaturated soils and structure.Therefore,based on state-dependent concept and critical state theory,this thesis firstly proposed an elastoplastic model for unsaturated soil-structure interfaces with the adoption of net normal stress and suction as stress state variables.Next,the model is validated by self-conducted loess-concrete interface shear tests and reported sand-structure interface shear tests,and the comparison between current model and existing unsaturated interface models is made at the same time.Finally,the finite element calculation programs of the current model are developed based on zero-thickness element and thin-layer element,then effectiveness,advantages and disadvantages as well as applicability of both numerical implementing methods are analyzed and discussed.The main achievement of current study is shown as follows:(1)An elastoplastic model describing the mechanical behavior of unsaturated soilstructure is presented.The model considers influence of suction on elastic stiffness matrix and plastic constituents(e.g.,yielding function,hardening law,flow rule,etc.),and is capable of capturing the mechanical behaviors of interface under different initial void ratios,suctions and stress levels,including strain hardening/softening and contraction/dilation.The agreement between model results and experimental results of direct shear tests of both loess-concrete and sand-structure interface shows that,the model has good applicability in different suctions,initial conditions and boundary conditions.(2)Compared to the net stress model,The new model can consider the influence of net normal stress on the normal deformation,can describe the phase transformation behavior better,and can reflect the non-linear increase of shear stress at the beginning of shearing;Compared to the existing effective stress models,the model here has a clearer concept by adopting net normal stress and suction in the formulation,and the calculated peak shear strength and critical stress are closer to the measured values,the decrease of shear displacement corresponding to peak shear strength with suction is also well captured by the model,indicating that the model can consider the suction effects on interface’s peak shear strength,critical state and hardening behavior better.(3)The calculation programs of the proposed interface model are developed based on widely used zero-thickness element and thin-layer element in finite element method(FEM),then numerical experiments of friction test and single-element shear test are carried out,and the accuracy and reliability of both numerical implementation methods is checked by comparing the results of numerical tests with the theoretical calculation results of the proposed model.(4)Based on the developed calculation programs of current model by methods of zerothickness element and thin-layer element,the numerical slide-block test is further carried out to simulate the soil-structure shearing tests,and advantages,disadvantages and applicability of both methods are discussed through a comparison between their calculation results.The results show that thin-layer element method is advantageous to zero-thickness element method: for one thing,the calculated stress on interface from the former method is homogeneous,and the numerical results highly agree with the theoretical results,but for the latter one,the stress concentration phenomenon is occurred in both ends of the interface element,leading to the discrepancy of calculation results;for another,the zero-thickness element method can be only used to simulate constant normal load condition,while the another is applicable to all boundary conditions(constant normal load,constant normal stiffness and constant volume)due to its capacity in the simulation of normal interfacial behavior.