An Advanced UH Model For Unsaturated Soils With Coupling The Hydro-Mechanical Behaviour

According to the existing research,many natural disasters and engineering accidents are related to unsaturated soils,such as the high fill slope landslide caused by rainfall,the settlement and collapse of roadbed,and the airport runway fracture caused by uneven settlement of foundation.Due to the limitation of test conditions,the understanding of the mechanical properties and hydraulic properties of unsaturated soils needs to be improved.Moreover,the theory of unsaturated soil mechanics is in the developing stage.Therefore,it has important theoretical value and engineering significance to increase the understanding of the hydraulic and mechanical characteristics of unsaturated soils,to improve the unsaturated soil mechanics theory,to guide the actual engineering design,construction as well as the safety evaluation based on the unsaturated soils that take systematic unsaturated soil tests and establish a simple and practical constitutive model for over-consolidated unsaturated soils coupling the hydraulic and mechanical behaviour.Under the above background,the following researchs are carried out.(1)The hydraulic and mechanical tests of an unsaturated soil were carried out.The hydraulic and mechanical properties of unsaturated soil were tested with a modified unsaturated soil consolidation apparatus and the GDS triaxial test system for unsaturated soils,including the soil-water characteristic curve tests of the samples with different initial dry densities under lateral stress state and isotropic stress state,the one dimensional compression tests and isotropic compression tests of the samples with different initial dry densities under the controlled suctions,the triaxial compression tests of the samples under different suctions and different net confining pressure.The tests results show that,the linetype of soil-water characteristic curves of the samples with the same initial dry densitie under lateral stress state and isotropic stress state are basically the same.The degree of over-consolidation increases with the increasing of suction Before the yield stress is reached,the slope of the compression curve decreases with the increasing of suction,and after the yield stress is reached,the slopes of the compression curves of the samples under different suctions are almost the same.The phenomenon of softening and dilatation in the shear process of over-consolidated unsaturated soil is related to the suction and the net confining pressure(2)An advanced mechanical constitutive model for over-consolidated unsaturated soils is established.Basing on the test results of mechanical properties of an unsaturated soil,a constitutive model for normally consolidated unsaturated soils is established.Then an advanced mechanical constitutive model for over-consolidated unsaturated soils is established combining with the UH model,which is verified by lots of test results of the unsaturated soils under different stress pathsFirstly,basing on the triaxial compression tests of an unsaturated soil,a new suction stress formula is proposed.Then,the suction stress is introduced,and based on the results of one-dimensional and isotropic compression tests of the unsaturated soil,an isotropic compression formula of normally consolidated unsaturated soils is proposed,which can describe the compression behaviors of unsaturated soils from low to high stress levels and is more realistic.A new loading-collapse(LC)yield equation is derived then,and a constitutive model for normally consolidated unsaturated soils is established.Finally,without adding any parameters,the uniform hardening parameter H is introduced,and an UH model for over-consolidated unsaturated soils is established by combining with the form of elliptic yield surface of modified Cambridge model The proposed model can describe the stress-strain behaviors of unsaturated soils under normally consolidated or over-consolidated conditions and constant or variable suction conditions,including isotropic compression characteristics,shear contraction and dilatation,strain hardening and softening,wetting-collapse,influence of stress path and critical state.There are nine parameters in the proposed model,all of which have clear physical meaning and can be obtained by laboratory experiments.The correctness of the proposed model is verified via comparing with the Isotropic compression-wetting-isotropic compression test results,the one dimensional compression test results of the samples with different initial dry densities under different suctions,the triaxial compression test results of the samples under different suctions and different net confining pressure,the wetting test results of the samples with different degree of over-consolidation,and the triaxial compression-wetting-triaxial compression test results(3)An advanced UH model for over-consolidated unsaturated soils with coupling the hydraulic and mechanical behaviour is established.The suction stress is extended to the suction-saturation coupled stress,and the equation of soil-water characteristic curve in piecewise function is modified to a continuous smooth nonlinear form based on the soil-water characteristic curve test results.The mechanical constitutive model for over-consolidated unsaturated soils is extend to the UH model for over-consolidated unsaturated soils coupling the hydraulic and mechanical behaviour.Without adding any parameters,the proposed model is generalized to the 3-D stress state by means of transformed stress method combined with SMP criterion,which shows the broad application prospects.Based on the working condition of constant water content of unsaturated soil,the calculation steps of the UH model for over-consolidated unsaturated soils coupling the hydraulic and mechanical behaviour under constant water content are introduced in detail.The hydraulic and mechanical properties of over-consolidated unsaturated soils are analyzed In the case of isotropic compression under constant water content and In the case of triaxial compression under constant water content.The relationships between the water retention and strength are discussed,and then,the differences of the shear stiffness between the over-consolidated saturated and unsaturated soils are analyzed.There are eleven parameters in the proposed model,all of which have clear physical meaning and can be obtained by laboratory experiments.It shows that the UH model for over-consolidated unsaturated soils coupling the hydraulic and mechanical behaviour can reasonably described the hydraulic and mechanical behavior of over-consolidated unsaturated soils via comparing with the triaxial compression test results of the samples with different Initial saturations under the constant p condition,the triaxial compression test results of the samples with different Initial water contents under the constant net confining pressure condition,the triaxial compression test results of the samples with different Initial suctions and different Initial saturations under the constant water content condition.