Study On Hydro-mechanical Coupling Constitutive Model For Structural Unsaturated Soils

Most natural soils have structural characteristics and structure of soils include fabric and cementation.During the process of loading and unloading,structural soils have significantly different characteristics from that of non-structural soils.At the same time,most of the soils in nature have both structural and unsaturated characteristics,and the coupling effect generated by shrinkage film in unsaturated soils and the cementitious action produced by interparticle cementitious bodies are structural reasons for the macroscopic appearance of unsaturated soils,and both change with the change of water volume in soil.Therefore,in order to better describe the mechanical characteristics of natural soils,it is important to establish an constitutive model that considers both the structural and unsaturated properties of soils.In order to better describe the mechanical properties of structural unsaturated soils,the constitutive model of boundary surfaces of structural unsaturated soils and the hydro-mechanical coupling constitutive model were studied,and the following research results were obtained:（1）The volume change equation of saturated structural soil is extended to the unsaturated state,and the boundary surface constitutive model of structural unsaturated soil is established.According to the experimental results of the one-dimensional compression curve of structural soil,it is believed that the void ratio of structural soil is the sum of the void ratio and the additional void ratio of the remodeled soil under the same stress state,and the deconstruction index b and the initial additional void ratio indicate the structural decay rate and the size of the initial structure,respectively.The LC yield surface in the BBM model is introduced,the yield stress and the slope of the compression line in the volume change equation are considered to be functions of suction s,and the structural strength is introduced to reflect the enhancement effect of soil structure on peak strength during shearing.Combined with boundary surface equations based on modified Cam-clay models,the constitutive model is extended to a three-dimensional state.The prediction of the model was realized by writing the MATLAB program,and the experimental data of the unsaturated loess in northeast Iran and the Q₂unsaturated loess in Xi’an were used to calibrate and verify the parameters of the model,and the results showed that the established model could effectively predict the deformation characteristics of the structural unsaturated soil.（2）By extending the constant suction state to hydro-mechanical coupling,saturation is selected as the state variable,and an elastoplastic constitutive model that can consider both structural and hydro-mechanical coupling characteristics for unsaturated soils is established.The Bishop effective stress and effective degree of saturation were selected as state variables,and the influence of structure on volume change was also considered by introducing the additional void ratio,and the volume change equation of structural unsaturated soil was established inv-p‘-S_e space,and the compression index was regarded as a function of effective degree saturation;The SWCC equation and the nonlinear scan line equation considering the effect of deformation are introduced,and it is believed that the increase in effective degree saturation is caused by a combination of two parts:a change in suction and a change in volume;A hydro-mechanical coupling unsaturated constitutive model considering structure is derived by combining the yield surface equation,the hardening law,and the associated flow rule,of which can effectively reflect the influence of structure on soil mechanics and hydraulic characteristics.（3）Based on the existing experimental data,verification study of the established structural hydro-mechanical coupling constitutive model for unsaturated soils was carried out.Three different types of structural unsaturated soils were selected,including the loess of Gorgan in the northeast of Iran,bentonite-kaolin mixed soil and kaolin soil,to achieve the prediction function of the established hydro-mechanical coupling constitutive model under isotropic compression,drying and wetting cycle and triaxial drainage shearing path;the results showed that the model could reasonably reflect the saturation change due to loading,the volume deformation due to drying and wetting cycle,and the hydraulic hysteresis characteristics of structural unsaturated soil;by comparing the calculation results with the structural constitutive model without considering the structural constitutive model,the rationality of the proposed model in reflecting the coupling characteristics of structural soil and hydraulics is verified.