Six-phase Model And Generalized Stress Framework For Unsaturated Soils

For discrete geomaterials with diverse ingredients,stress concepts used in traditional single-phase continuum mechanics are meaningless.Therefore.Terzaghi proposed the principle of effective stress for saturated soils,which is considered as the keystone of modern soil mechanics.However,due to complex interphase interactions,the principle of effective stress is widely challenged in unsaturated soil mechanics,which is still an attractive research topic at present.For establishing a reasonable stress framework with specific physical meanings,it is started form defining a six-phase model which separates unsaturated soil into six phases including soil particles,cement,solid water,liquid water,contractile skin and pore air.Based on the new model,the bearing structures of saturated and unsaturated soils are defined as generalized soil skeleton and generalized soil structure respectively,and then it is clear that the intrinsic differences between saturated and unsatrrated soils are different bearing structures and pore fluids.Then,a set of variables are defined as generalized structure properties to indicate the common characteristics of bearing structures of soils.Moreover,the mechanical effects on particle skeleton due to pore fluids,solid water,cement and contractile skin are clearly analyzed for establishing a new stress framework.Based on the six-phase model,unsaturated soil could be seperated into bearing structure and pore fluids to analyze each phase independently via equilibrium analysis method.It is proved that the essence of Terzaghi's and Bishop's equations of effective stress is stress relation equation which indicates the corresponding relation between two series of stress variables used in mixed and multiphase continuum models,respectively.While the physical meanings of effective and neutral stresses are the soil skeleton stress caused by loads and a synthetical stress concept due to pore-pressure,respectively.If just the external force is considered,the stress relation equation could be interpreted as the sharing relation of external force by bearing structure and pore fluids.Because neutral stress has specific physical meaning,the effective stress equation is always only the relation among total stress,effective stress and neutral stress whether soils are saturated or not.which does not include net stress,matric suction,suction stress and so on.However,stress relation equation is only associated with the effects due to external loads,so the soil skeleton stress caused by loads is not the sole stress controlling the strength and deformation of soil.Because stress relation equation could only indicate the difference of pore fluids in saturated and unsaturated soils,the inter-particle prestress is defined to represent the bonding effects due to solid water,cement and contractile skin.Then a generalized stress framework is constructed by associating stress relation equation with inter-particle prestress.which could decouple the complex mechanisms enhancing the interparticle connections out from the properties of bearing sturcutre of soil.After demonstrating the concept and effects of neutral stress,the total soil skeleton stress is searched out,which is the predominant stress controlling the strength and deformation of soil.For the static problems with only mechanical loads,the total soil skeleton stress includes the soil skeleton stress caused by gravity force,the soil skeleton stress caused by external force,the soil skeleton stress caused by pore fluid pressure and the inter-particle prestress.Finally,a generalized stress principle is proposed based on the generalized stress framework,and the relation between interphase interactions and the parameters in Mohr-Coulomb criterion is analyzed to obtain a shear strength equation based on the generalized stress framework.