Research On Mechanical And Deformation Characteristics Of Structural Unsaturated Soils

Structural soils is ubiquitous in nature.Under the influence of factors such as the rise and fall of groundwater level and rainfall-evaporation,structural soils is often in an unsaturated state,and the change of saturation will lead to irreversible transformation of the mechanical properties of structural soils.In engineering application,for example,the instability of natural roadbed and structural soils slope under the effect of complex factors such as load and rainfall is closely related to the structure and unsaturation of soils.At present,the research on considering both soil structure and unsaturation is rare and is in the stage of exploration.Therefore,to carry out the experimental study of structural unsaturated soils,fully grasp the influence of structure and unsaturation on soils mechanics and deformation characteristics,and put forward a constitutive model which can predict the mechanical and deformation characteristics of structural unsaturated soils.It is of great significance to predict the mechanical properties of natural subgrade and solidified soil subgrade and the wetting deformation under the coupling action of load and rainfall,and to ensure the safety of engineering.In this paper,based on the experiment,Wuxi sludge clay was solidified with Portland cement,and structural soils with different cement content were prepared.The consolidation drainage shear and one-dimensional consolidation test of structural saturated soils,structural unsaturated soils,remolded unsaturated soils,and wetting deformation test of structural unsaturated soils were carried out systematically.Based on the above test results and literature,the constitutive model of structural soils proposed by our research group is further improved,the concept of sub-loading surface is introduced,and a constitutive model which can describe the structural soils in the state of overconsolidation is established.Considering the influence of unsaturated factors on the mechanical properties of structural soils,the constitutive model of overconsolidated structural unsaturated soils is derived.Finally,the validity of the two models is verified by experimental data.The main research results of this paper are as follows:(1)The consolidation and triaxial drainage shear characteristics of structural saturated soils and remolded saturated soils with different cement content are studied by high pressure consolidation apparatus and stress-strain controlled triaxial apparatus.The results show that there are obvious differences in compression curves between structural saturated soils and remolded saturated soils.The compression curve of structural saturated soils changes linearly before yield and abruptly changes nonlinearly after yield.Due to structural damage,the compression curve of structural soils will gradually approach the compression curve of remolded soils after yield,and the smaller the structure is,the more obvious this phenomenon is.With the increase of structure,the shear stress ratio curve gradually changes from strain hardening to strain softening.The initial consolidation confining pressure will cause some damage to the soils structure.the greater the initial consolidation confining pressure,the more serious the structural damage.In addition,the critical stress ratio of structured soil increases with the increase of structure.(2)The one-dimensional consolidation test and triaxial drainage test of structural unsaturated soils under constant matric suction were carried out by using unsaturated consolidation and triaxial apparatus.The results show that the change of matric suction will cause soils shrinkage,which shows that the initial void ratio decreases to a certain extent,but this phenomenon is not obvious in high structural unsaturated soils.The change of matric suction will have a certain effect on the yield stress of structural unsaturated soils.the greater the matric suction,the greater the yield stress.With the increase of matric suction,the shear stress ratio of structural unsaturated soils increases accordingly,and the contribution of high matric suction is more obvious than that of low matric suction.The increase of matric suction will aggravate the strain softening phenomenon of structural soils,and the strain softening phenomenon is more obvious with the increase of matric suction.(3)The wetting deformation test of structural unsaturated soils was carried out under the condition of different cement content and consolidation stress.The effects of consolidation stress and matric suction on the wetting deformation characteristics were discussed.The results show that with the increase of structure,the time needed for wetting deformation stability increases and the change value of saturation decreases.The increase of structure will lead to the increase of deformation in the wetting deformation stage,and the stronger the structure is,the smaller the structural damage is,and the greater the wetting deformation is.However,the wetting deformation will not increase with the decrease of matric suction,but will increase at first and then stabilize.(4)Based on the constitutive model of structural soils proposed by the research group,the concept of sub-loading surface is introduced to describe the plastic change of soils in the state of overconsolidation,and the constitutive model of overconsolidated structural soils is deduced.The model can reasonably describe the smooth transition from the overconsolidated state to the normal consolidated state,and improve the prediction accuracy of the peak strength of structural soils in the overconsolidated state.In order to consider the effect of saturation and matric suction,based on the constitutive model of overconsolidated structural soils,the effective stress of structural unsaturated soils was modified by introducing saturation and matric suction into the effective stress of structural unsaturated soils with using Bishop suction parameter.Assuming that the compression index of structural soils in dry and saturated state is the linear difference of saturation,the relationship between modified effective stress and volume deformation is established,and the constitutive model of overconsolidated structural unsaturated soils is derived.The validity of the model is verified by the test data of structural soils and structural unsaturated soils,and the results show that the model can reasonably describe the mechanical properties of structural unsaturated soils.