A Microstructure-Dependent Hydro-Mechanical Coupled Constitutive Model For Unsaturated Soils

For the double-pore structure soil,internal pores include macropores(intra-aggregate pores)and micropores(inter-aggregate pores).There are significant difference on mechanical and hydraulic behavior of different scale pores.Therefore,the influence of microstructure is not negligible.From micro perspective,the three aspects,constitutive relation,soil-water characteristic curve(SWCC)and evolution of the internal micro-structure,have been carried on the thorough research.The main achievements are listed as follows:(1)Due to the difference on mechanical and hydraulic behavior between macropores and micropores of double-pore structure soil,there are obvious difference on the evolution of the two pore scales.The bimodal pore-size distribution can be obtained using the superpositon of unimodal pore-size distribution with macro-scale and micro-scale.It is assumed that the pore size distribution in a deformed state can be transformed from initial state through following parameters:translation,scaling and dispersion degree.By establishing the relationship between the evolution parameters and the pore ratio,the evolution laws of microstructure is proposed and capable of describing the transformation of pore size distribution during drying process.Based on the established model and the existing MIP test data,the model parameter calibration and model prediction were carried out for the unsaturated soil with double-pore structure,and the feasibility of the model was verified.(2)Stress and hydraulic paths significantly affect the micro structure of unsaturated soil.On the other hand,different pore types and structures also have different impacts on mechanical and hydraulic behavior of soils with double-pore structures such as expansive soils and compacted soils.Based on the hydro-mechanical coupled constitutive model,GCM,the effective degree of saturation considering the different effects of macro and micro pores on the hydro-mechanical characteristics is introduced into the Bishop effective stress equation to take the influence of pore structure into account.On the basis of the effective degree of saturation,a microstructure-dependent hydro-mechanical coupled constitutive model for unsaturated soils is then proposed.The performance of the model is verified by various experimental data of isotropic compression tests and the predictions match the observations well,indicating the validity of the proposed model.(3)The existing models of soil-water characteristic curve(SWCC)are confined to single pore structure soil.In order to reflect the influence of internal structure and deformation on mechanical and hydraulic behavior of soil,a relevant study for SWCC is carried out.Choosing the inflection point of the bimodal SWCC as a cut-off point for different scale pores of double-pore structure unsaturated soil,the corresponding equations were established for SWCC in different region respectively.The relationship between parameters and initial porosity ratio is introduced and the soil water characteristic curve of double-pore structure soil considering effect of volume change is set up.Meanwhile the model describes main drying and wetting curves.The comparison with dry-wet cycle test results of single and double pore structure soil verifies the model and also reflects the different hydraulic behavior of soil samples with various initial void ratio and structure under the same hydraulic path.