Study On Hydro-mechanical Behavior Of Unsaturated Soil Over A Wide Suction Range

Unsaturated soils over a wide suction range or at high suction are widespread in nature,particularly in the arid and semi-arid regions,such as in northwestern China,where the unsaturated soils are always in a low relative humidity environment.That is,those soils encounter very high suction magnitudes,such as tens or hundreds of megapascal(MPa).Therefore,it is necessary to investigate the hydro-mechanical behavior of unsaturated soils over a wide suction range.In the dissertation,on the basis of understanding the existing literatures,hydromechanical behavior of unsaturated soils with different initial densities over a wide suction range was studied and the main conclusions are drawn out as follows:(1)The determination of basic parameters(such as air-entry value,residual suction,slope at transition zone,etc.)of soil-water characteristic curve(SWCC)is the most fundamental for predicting the strength,permeability and constitutive relation of unsaturated soils.A method is proposed to determine the basic parameters of unimodal and bimodal SWCCs based on the modified Fredlund and Xing's fitting equation.Finally,the proposed method is validated to be feasible by using the experimental data of unimodal and bimodal SWCCs.(2)To investigate the SWCC of compacted Pearl clay over a wide suction range,the suction is imposed or measured using the pressure plate method and vapor equilibrium technique with saturated salt solution.The experimental results show that the SWCCs in terms of gravimetric water content curve over a wide suction range are independent of the initial dry density or void ratio when the suction is higher than a specific value,which can be determined by results of the Mercury intrusion porosimetry(MIP)tests.When the SWCC is expressed in terms of the degree of saturation versus suction relation over a wide suction range,the influence of dry density or void ratio is highlighted.This behavior can be explained by the features of the pore-size distribution measured by the MIP tests.Finally,a method to describe the SWCCs of specimens with different initial void ratios during drying is proposed,and the predicted results are compared well with measured values.(3)To investigate hydro-mechanical behavior of compacted Pearl clay over a wide suction range,the suction is imposed using axis translation technique and vapor equilibrium technique with saturated salt solution.The experimental results show that the effect of void ratio or dry density on shear strength for unsaturated soils is notable over a wide suction range.Moreover,the effect of different specimen preparation methods(compaction and reconstitution)on the hydro-mechanical behavior was studied through a series of triaxial tests.The difference in the stress versus strain relationship and strength of two kinds of specimens can be attributed to the different soil structures.(4)A number of shear strength equations were based on regression analyses of experimental data at low suction range(almost below the suction of 500 kPa)from either direct shear or triaxial tests.The advantages and limitations associated with various shear strength equations are discussed at high suctions or over a wide suction range.And on this basis a strength equation is proposed to predict the shear strength of sand and silty soils over a wide suction range.Moreover,based on a comprehensive study on the effects of stress state and density on hydro-mechanical behavior of unsaturated soils,a new strength equation is proposed to predict the shear strength of unsaturated soils under different stress states.