The Strength And Deformation Characteristics Of Unsaturated Soil At High Suction And Low Stress

Unsaturated soils are often encountered in geotechnical engineering practice.In the study of the mechanical properties of unsaturated soils,the key issue is concentrated on the influence of suction.Based on the physical mechanism of the soil water interaction,the suction can be divided into capillarity and adsorption.At present,the understanding and use of the suction is confusing.In the current study of unsaturated soils,the suction is often directly considered as capillarity,as if the suction corresponds to capillary mechanism in the whole range.Therefore,the adsorption in the suction is ignored.This directly leads to a deviation in the understanding of the suction effect when describing the properties and behaviors of unsaturated soils.Meanwhile,little research has focused on the strength and deformation characteristics of soils with high suction,where adsorption is dominant.At present,soil mechanics theory mostly focuses on the strength and defonnation characteristics under relatively large compressive stresses.Among them,the soil failure is mostly based on compression-shear mode,the description of stress and deformation during shearing is mostly limited to strain hardening and shrinkage.However,in practical engineering problems,the soil is often under low and tensile stress.The soil failure may be based on different modes,such as tensile-shear and compression-shear modes;Stress and deformation during shearing are mostly post-peak softening and dilatancy.Under these circumstances,the soil mechanics theory based on the larger compressive stress is difficult to apply.It is necessary to establish a reasonable failure envelope criterion and a stress-strain constitutive model,in order to describe the practical problems more accurately.The paper focuses on the strength and deformation characteristics of unsaturated soil at high suction and low stress,and mainly contains the following parts:(1)A new shear strength is proposed for unsaturated soils to account for the capillary and adsorptive mechanisms.Firstly,two ideal states are defined for unsaturated soils,i.e.,the ideal capillarity and adsorption states,which are only affected by capillary or adsorptive mechanisms,respectively.Then,the shear strength expressions of the two ideal states are proposed respectively.Based on the concept of the binary-medium model,the unsaturated soil is regarded as a media combined with the two ideal parts by a participation function.Then the new unsaturated soil shear strength with capillary and adsorptive mechanisms is established for unsaturated soils,which is applicable to a wide range of suctiop.Thus,the results have shown that the capillary and adsorptive mechanisms should be distinguished when considering the effect of suction on the mechanical property for unsaturated soils.(2)The current study of unsaturated soils is mostly limited to lower suction,in which capillarity is dominant.In practice,due to climatic changes,soils near the ground surface often experience cyclic wetting and drying,and being the low water content and high suction,where adsorption is dominant.Choosing the non-expansive kaolin-sand soil,through the vapor equilibrium technique on the saturated salt solution,the high suctions were controlled under drying and wetting paths respectively.The strength and deformation characteristics under high suction were tested,of which four small confining pressures of 0(close to unconfined).?25.?50.?100 kPa were selected.Based on the test results,the mechanism of suction effect on soil strength and deformation at high suction is analyzed.(3)The strength and dilatancy behavior of unsaturated compacted soils at low and tensile stresses was investigated via low-stress direct shear and direct tensile tests over a wide range of water content.Based on the test results and theoretical analysis,the variation characteristics of the strength envelope in tensile stress regime?the peak strength and dilatancy for unsaturated soils are clarified.Then,Combined with the triaxial compression test of four small confining pressures of 0?25?50?100 kPa,a method of deriving failure envelopes for unsaturated soil is discussed.Firstly,according to the mode of failure,the failure envelope can be approximated by a bilinear Mohr-Coulomb envelope.Secondly,the eff-ect of unsaturated state on peak strength can be described in terms of dilatancy and interparticle stress;then accounting for the primary components of unsaturated peak strength and its description,two curved failure envelopes with clear physical meaning are proposed,named after cohesion-dilatancy and interparticle stress-dilatancy envelope respectively.Based on the piecewise linear failure envelope,the method for determining the parameters of the curved failure envelope is discussed.Afterwards,the failure envelopes and its variation characteristics under different water content in unsaturated state are given and analyzed.Finally,the rationality of the proposed failure envelope is discussed by comparison with the experimental data and failure criterions in the literature.(4)For the stress-strain characteristics of unsaturated compacted soil under low stress and different water content,based on the experimental results,the critical state equation?the stress-dilatancy relationship and the prediction of the minimum dilatancy rate are discussed.Compared with the Nor-Sand model framework of saturated soil,the constitutive model for shear deformation characteristics of unsaturated compacted soil with different water content is obtained.Thus,the peak strength and dilatancy characteristics of unsaturated compacted soil with different water content are well captured.