A Coupled Hydro-mechanical Constitutive Model For Unsaturated Soils With Double Pore Structure

To some extent,the mechanical and hydraulic behaviours of unsaturated soils are the macro-embodiments of their microstructure characteristics.For unsaturated soils with double pore structure,including compacted clay and some expansive soils,the mechanical and hydraulic behaviours are significantly affected by the inter-aggregate pores and intra-aggregate pores contained within the internal structure.Considering the microscopic characteristics of unsaturated soil with double pore structure,the influence of soil's microstructure on mechanics and hydraulic behaviours and the hydro-mechanical coupling constitutive relationship under three-dimensional stress state are deeply studied,and the following results are achieved:(1)Summarize the definition,classification,characteristics and testing methods of soil structure,and emphatically explain the difference on microstructure between single-pore structural soil and double-pore structural soil;Comb through the mechanical and hydraulic features of unsaturated soils affected by microstructure properties,including shear strength,swelling,collapsibility,sensitivity,and soil-water characteristic curve,and focus on features of the soil-water characteristic curve of the double-pore structural soil and its relationship with the double pore structure;The variables affected by the structure and the characteristics and application scope of each variable are given;the current research achievements of the coupled hydro-mechanical constitutive model for unsaturated soils are organized.(2)Select the effective degree of saturation as the index reflecting the influence of double pore structure on the mechanical and hydraulic behaviours,and substitute this index into the Bishop effective stress formula to establish the Bishop effective stress formula that can consider the effect of the double pore structure;Fully coupled hydro-mechanical constitutive model(GCM)and Modified Cambridge Model are used as the basic stress-strain framework.The generalized stress and strain tensor are selected,combined with the associated flow law,hardening law and consistency condition to derive and establish a fully coupled hydro-mechanical constitutive model for double-pore structural unsaturated soils,which can be used under three-dimensional stress state;(3)The application of the model is realized through MATLAB programming,the model response and prediction ability of the constitutive model under various stress paths are analysed,and the similarities and differences of stress-strain behaviours between the models that adopt the(effective)degree of saturation respectively are generalized under the same stress path;The influence of the parameters contained in the model on the model's predictability was studied.The predictive results show that the hydraulic and mechanical behaviours of the established model that adopts the effective degree of saturation are significantly different from those of the model that adopts the degree of saturation.There are distinct differences on the effective stress path,volumetric deformation and change of degree of saturation between the two types of models.The parameters included in the model,the effective degree of saturation,coupling parameters,the slope of the normal consolidation line of saturated soil,the slope of the main drying/wetting curve and the slope of scanning curve can have a great impact on the prediction results of the model,which can not only affect the yield mechanism activated under the same stress state but also significantly change the generalized strain state during loading.(4)Experimental data of isotropic compression,triaxial shearing and drying-wetting cycle tests on samples of Speswhite kaolin,Josigny silt,Pearl clay,Zaoyang expansive soil and Nanyang expansive soil are selected and compared with the model prediction for verification.All soil samples chosen have significant double pore structure.The results manifest that although the constitutive model of unsaturated soil with double pore structure established in this paper will overestimate the axial strain required to reach the critical state during shearing,generally it can accurately predict the generalized stress-strain behaviours under various stress paths.The simulative outcome is far superior to the constitutive model that only considers the degree of saturation and ignores the double pore structure,which proves the predictive ability and applicability of the model.