Generalized Effective Stress Principle And Shear Strength Model For Unsaturated Soil Considering The Occurrence Characteristics Of Pore Water

The effective stress equation is the keystone of soil mechanics.However,both Terzaghi’s effective stress equation and Bishop’s effective stress equation lacked solid theoretical bases when they were proposed.For this reason,many scholars have tried to lay theoretical bases for them through different methods,but these methods have failed to account properly for the macro-and micro-occurrence characteristics of the pore water in unsaturated soils.At the same time,the physical meanings of the neutral stress and effective stress in effective stress equation have not been reasonably elucidated,which leads to the failure of the present principle of effective stress for unsaturated soils to distinguish the mechanical effects of various stresses existing in unsaturated soils,and further the difficulty of solving the problems of the deformation and shear strength of unsaturated soils,especially the problem of the shear strength in the high suction range that needs to be solved urgently.In response to the problems mentioned above,through macro and micro mechanism exploration,theoretical derivation and analysis,model establishment and verification,we establish the generalized effective stress principle considering the occurrence characteristics of pore water and propose a shear strength model applicable to a wide suction range.The main research results obtained are as follows:(1)By analyzing the existing test data,it is found that there is a gradient in the pore water content macroscopically.Through investigating the micro-occurrence of pore water and its mechanical properties,an extended three-phase porous medium model is established,i.e.,pore air,capillary water,and generalized soil skeleton(including mineral particle skeleton,adsorbed water,and contractile skin).Based on the above findings and extended model,the force balance equations for each independent phase considering the macro-and micro-occurrence characteristics of pore water are formulated and then a generalized Terzaghi’s effective stress equation(the effective stress=total stress-neutral stress)is derived by using an independent phase balance approach,which provides a relatively solid theoretical basis for the effective stress equation for unsaturated soils.After argumentation,the effective stress equation for unsaturated soils should be expressed as the generalized Terzaghi’s effective stress equation rather than Bishop’s effective stress equation that is commonly used.A smooth transition between the former and Terzaghi’s effective stress equation can be achieved not only mathematically but also physically when a soil is changed from unsaturated to saturated conditions,whereas the latter can only achieve a transition mathematically because of neglecting the existence of neutral stress.(2)According to the analysis of the mechanisms of various stresses in unsaturated soils,it is found that the neutral stress consists of three components:(1)the equivalent pore-fluid pressure within fluid(i.e.,air and capillary water);(2)the stress acting on the cross section of generalized soil particles induced by the equivalent pore-fluid pressure;and(3)the stress acting on the inter-particle contacts induced by the equivalent pore-fluid pressure.The second component only causes the volumetric deformation of the generalized soil particle,and the third component only influences the sliding of the generalized soil particle.The generalized Terzaghi’s effective stress is the stress that is transmitted through the generalized soil skeleton,and the other stresses acting on the generalized soil particles but not transmitted through the generalized soil skeleton should not be added into the effective stress.The seepage force per unit volume of unsaturated soil includes not only the tangential force but also the normal force acting on the generalized soil particles exerted by the pore fluids,which makes the seepage force equation derived in this paper more reasonable.The influence of the seepage force on the mechanical behavior of soils is similar to that of the neutral stress.The existing theory of unsaturated soil mechanics ignores the effect of the neutral stress and seepage force,but the results of this paper show that for fine-grained soils(especially in the case of low saturation),the contributions of the neutral stress and seepage force to shear strength and volumetric strain need to be considered.(3)Based on the physical meanings of the above stresses and their mechanical effects,a generalized effective stress principle is established:(1)the generalized Terzaghi’s effective stress is equal to the total stress minus the neutral stress;(2)the shear strength and deformation of the generalized soil skeleton are governed by not only the generalized Terzaghi’s effective stress but also the mechanical parameters of the generalized soil skeleton when the mechanical effects of the neutral stress and seepage force can be ignored.Compared with the existing effective stress principle of unsaturated soils,the generalized effective stress principle associates the local stresses induced by surface tension,van der Waals attraction,and diffuse double-layer repulsion that are not transmitted through the generalized soil skeleton with the mechanical parameters of the generalized soil skeleton,which avoids mixing various stresses with different transfer mechanisms into a single effective stress equation.Furthermore,the generalized effective stress principle provides a unified framework to describe the mechanical effects of various stresses in unsaturated soils.(4)By studying the experimental results of the stress-strain curve and failure behavior of unsaturated soils over a wide suction range,we find that the capillary mechanism changes the shear strength by mainly changing the generalized Terzaghi’s effective stress,whereas the adsorptive mechanism changes the shear strength by mainly changing the cohesion of the generalized soil skeleton.According to the found mechanisms,combined with the generalized effective stress principle established above,a shear strength model considering the capillary and adsorptive mechanisms is proposed.The proposed model agrees well with the shear strength data of five representative soils not only in the low suction range but also in the high suction range.In comparison with the existing Bishop-type shear strength model,the proposed model has two advantages:(1)the proposed model separately quantifies the contributions of the capillary and adsorptive mechanisms to the shear strength,which makes the physical basis of the proposed model clearer;(2)the proposed model only introduces an additional parameter representing the cohesion of the generalized soil skeleton in the drying state,but its predictive performance in the high suction range has been significantly improved.