Hydro-Mechanical Characteristics And Constitutive Modeling Of Unsaturated Soils Over A Wide Suction Range

The experimental study on hydro-mechanial characteristics,water-retention curve model,shear strength criterion and constitutive theory of soil over a wide range of suction are the most important topics in unsaturated soil mechanics.The change of suction over a wide range will induce different physico-chemical water-holding characteristics,the nonlinear evolution of shear strength,the transformation of soil deformation from strain hardening to strain softening and so on.However,existing experimental and theoretical studies have not been fully clarified the water-holding behavior,deformation and strength characteristics of soil over a wide range of suction,and the framework of the constitutive model is relatively under-developed.In this paper,water-retention and triaxial shear tests over a wide range of suction are carried out to systematically reveal the water-holding capacity,strength and deformation characteristics of unsaturated soil.Based on experimental results,unified waterretention model and strength criterion suitable for soils with unimodal and bimodal water-retention curve are established.Also,a novel constitutive model is constructed based on the critical state theory to describe stress-strain behaviors of soils over a wide suction range.The main contents and achievements of this thesis are as follows:(1)This work carries out water holding tests over a wide suction range to reveal the water-retention characteristics.Combined with axis translation technique,filter paper method,and vapor equilibrium technique,drying tests are carried out over a wide suction range for reconsolidated and compacted samples with different initial densities.The relationship between water holding characteristics and pore structure are studied through mercury injection tests.The mercury injection tests show that the reconsolidated sample has a single pore structure,while the compacted sample has dual porosity.The water holding tests show that the pore structure and dense state have significant effects on the soil water holding capacity of and shrinkage behavior.For reconsolidated sample,the water holding capacity is stronger,and the shrinkage deformation is lager than that of compacted sample.The water-retention curve of compacted soil shows bimodal patterns,i.e.,two stages of rapid drop of degree of saturation as the suction increases(double S-shape)with two air entry values.As the initial dry density mainly affects the macropores,the change of the water retention curve is mainly reflected in the low suction section,that is,the higher the initial dry density,the higher the saturation of the soil sample in the low suction section is and the stronger the water holding capacity is.(2)The unsaturated triaxial shear tests of compacted samples are carried out,and the mechanical properties over a wide range of suction are revealed.The experimental results show that under medium and low suction,the stress-strain behavior of soil shows strain hardening with shear induced contraction.Under high suction,the soils show strain softening with dilatancy after 1% to 3% contraction.The dense state has a significant effect on the mechanical behavior of unsaturated soil,that is,the soil samples with higher density have smaller critical suction value of post-peak softening,and the greater the peak strength.The brittleness of the sample increases with the increase of suction,which increases the amplitude of post-peak softening,that is,the difference between peak and residual strength increases.The growth of brittleness in specimen with increasing suction is clearly visualized from pictures taken at the end of the tests that indicate gradual transition in failure patterns(i.e.,ductile failure mode with barrel shape under low to medium suctions and sliding surface failure mode with strain localization high suction).(3)Based on the experimental study of water-retention characteristics,a unified unimodal and bimodal soil water-retention curve(SWRC)model is constructed.Firstly,the capillary water-retention equation is established based on pore structure,while the capillary condensation effect is explicitly considered in the adsorbed water-retention part to decouple the capillary and adsorptive saturation.Secondly,based on the effect of void ratio on capillary water holding behavior,the water holding curve model is extended to the soil water-retwntion surface(SWRS)model.Thirdly,considering the influence of hydraulic hysteresis,the distance function between the current state point and the mapping point is constructed to derive the equation of scanning water-retention curve.Fourth,the bimodal capillary water-retention function is established based on the distribution of large and small pore sizes,so that,the unimodal water-retention model is extended to a unified unimodal and bimodal model.Finally,different experimental results are used to verify the unified water retention curve model.The results show that the unified model can reproduce the hydro-mechanical coupling behaviors of soilswith unimodal or bimodal SWRCs.(4)Based on the results of unsaturated triaxial shear tests,the dependence of shear strength on different stress variables is studied,and a unified strength criteria of soils with unimodal and bimodal SWRC are established.First of all,the dependence of shear strength of soils with bimodal SWRC on stress variables(suction,suction stress,capillary suction stress)is studied.The results show that capillary suction stress is more reasonable to consider the contribution of suction to shear strength.In the capillary suction stress-strength space,coarse-and fine-grained soils with bimodal SWRCs show bi-linear strength characteristics.Secondly,the dependence of capillary suction stress and shear strength is verified in soils with unimodal SWRC.The results show that the critical peak strength state can be characterized by a unique envelope.Finally,a bilinear unified strength criterion suitable for different soils over a wide suction is proposed,which is verified by test results.(5)Based on the soil water holding test and unsaturated triaxial shear test,a hydromechanical coupling sub-loading model for unsaturated soils is established,which depends on suction level and compacted state.Firstly,the improved Bishop effective stress dependent on capillary suction stress is adopted.The normal compression line function over full suction range is constructed.Secondly,the loading-collapse yield surface model is established.The hydro-mechanical coupling equations of soils with unimodal and bimodal SWRCs are derived.Thirdly,the sub-loading plasticity theory is extended to unsaturated soil,while the effect of dense state on the yield strength of unsaturated soil is modelled.Finally,the constitutive model is verified by the results of isotropic compression,wetting test and triaxial shear test.The results show that the new model can reasonably reproduce the hydro-mechanical behavior of unsaturated soils over a wide range of suction.