Investigation On The Effect Of Change In Void Ratio On The Mechanical Behavior Of Unsaturated Soils

Unsaturated soils are widely distributed in most countries and regions.A sound understanding of the mechanical behavior of this type of soil is thus required to provide safe and cost-effective solutions to related engineering problems.Although great efforts have been done and a number of constitutive models have been proposed to describe the mechanical behavior of unsaturated soils,many of which however still need further investigated.Many mechanical properties of unsaturated soils are closely related to the soil-water characteristic curve(SWCC),and void ratio affects the SWCC.The principle objective of this dissertation is to investigate effect of change in void ratio on the fundamental properties of unsaturated soils such as shear strength,volumetric behavior and permeability coefficient.A triaxial apparatus that incorporates the digital image measurement system(DIMS)is developed for relevant testing on unsaturated soils.By assuming the lateral profile of standard triaxial sample is parabolic,a non-contact method is developed to accurately reconstruct the shape of the sample during deformation according to the strain field obtained from the DIMS.The volume change measurement is compared with drained water volume from the balance in saturated tests.A good agreement shows that the proposed method provides a satisfactory measurement of volume change,and it can be directly applied to unsaturated samples.The volume change measurement together with drained water volume enables all the physical and mechanical variables to be directly or indirectly obtained in unsaturated triaxial tests.In addition to triaxial tests,other experiments carried out includes a series of SWCC tests and suction-controlled compression tests on a compacted mixture of tailings sand and speswhite china kaolin,in order to gain a more complete understanding of the behaviors of the soil mixture Besides conventional testing results,three distinct stages in the volumetric behavior of initially saturated samples during suction equilibrium have been observed,namely the primary compression stage,stable seepage stage and secondary compression stage.When a unique SWCC is used to describe the water retention behavior of a soil,the conventional shear strength equation is validated against the testing data from the literatures.It shows that conventional shear strength equation can provide satisfactory prediction for most unsaturated soils.However,the prediction with a single SWCC is relatively poor for some sandy soils,especially for those loose or low density sands.The influence of SWCC on shear strength by differing void ratios is characterized by relating the state surface expression(SSE),then a void ratio-dependent shear strength criterion is proposed by introducing the SSE into the conventional shear strength equation.According to the proposed shear strength criterion,it's necessary to introduce the shrinkage curves under different net normal stresses and isotropic compression curves under different suctions into the SSE to accurately identify the suction-strength curves and net stress-strength curves,respectively.The new shear strength criterion is validated against the testing data on the soil mixture,showing that it can provide a more accurate prediction of shear strength than that of conventional prediction using a unique SWCC.Moreover,the proposed model is capable of reproducing several special shear strength behaviors of unsaturated soils,including the suction changes the slope of net stress-strength curves and the net normal stress affects the shape of suction-strength curves.The net normal stress and suction affect the shear behavior of unsaturated soil mixture in a coupled manner.Then,the new shear strength criterion is further validated against the testing data from the literatures.A series of drying shrinkage tests are conducted on the soil mixture and a standard kaolin to investigate the volumetric behavior of a soil.The suction stress compression curve(SSCC)can be obtained by relating shrinkage curve and the corresponding SWCC of the soil A perfect agreement of the SSCC and saturated isotropic compression curve indicates that the suction stress and net mean stress have the same effect on the contraction deformation of a soil under isotropic stress state.Then a virtual isotropic compression surface is established in the effective stress,effective degree of saturation and specific volume space to represent the volumetric behavior of unsaturated soils.During wetting,the volume change of an unsaturated soil is the result of both unloading of effective stress and a reduction of yield stress.Hence the wetting-induced collapse happens when the reduction of yield stress dominates the wetting process.Since the SSE determines the stable soil state after wetting,a combination of the SSE and isotropic compression surface enable calculation of the volumetric strain for a soil in wetting.For rigid soils,the effect of void ratio on permeability coefficient is mainly determined by initial void ratio.Considering the change of initial void ratios,the relative and saturated permeability functions are proposed respectively.Then the proposed permeability functions are validated against the testing data,demonstrating that both functions can provide reasonably accurate predictions of corresponding permeability coefficient(i.e.permeability coefficient of unsaturated soils).The prediction shows that the relative permeability coefficient with suction varies in an approximately linear fashion in double logarithm coordinates beyond the air entry value,and the slope of the straight line is considered as being approximately identical at different void ratios.For deformable soils,considering the effect of change in suction on both void ratio and degree of saturation,a combination of SSE and shrinkage curve can determines the void ratio and soil state at different suctions.The relative and saturated permeability coefficient at specific initial void ratio can also be calculated by numerical method.