Mechanism Of Parameters Affecting Shear Strength Of Fiber-Reinforced Soil And Constitutive Modelling Of Fiber-Reinforced Soil

Ground improvement technique is a research emphasis in geotechnical engineering,because it plays a big part in structure safety and stability.As a relatively new ground improvement technique,fiber-reinforced soil technique has some advantages over other ground improvement techniques.The advantages are,for example,great improvement effect,low cost,convenient construction,and non-pollution.Because of this,fiber-reinfoced soil technique has been attracting great attention of researchers and civil engineers over the last few decades.Researchers has conducted numerous studies on this technique,and this technique has been applied successfully by civil engineers to many engineering practices such as embankments,retaining walls,foundations,and slope protection.Despite wealth of the studies on fiber-reinforced soil,the mechanism of parameters affecting shear strength of fiber-reinforced expansive soil still remains unclear.Additionally,the established constitutive models of fiber-reinforced soil take no account of the evolutive behavior of fiber-soil interaction.The objectives of this study are(1)to find out if the idea of improving the changeable shear strength behavior of Hefei expansive soil by introducing randomly distributed discrete short jute fibers is technically feasible,(2)to capture the respective effects of fiber content,fiber length,fiber distribution form,and specimen preparation parameters(i.e.dry density and moisture content)on shear strength behavior of jute fiber-reinforced Hefei expansive soil as well as the respective mechanisms of these factors,and(3)to propose a constitutive model of fiber-reinforced soil that can account for the evolutive behavior of fiber-soil interaction.To achieve the objectives,laboratory experiments including mainly direct shear and triaxial compression tests,modelling of constitutive behavior of fiber-reinforced soil,and verification of the proposed constitutive model are conducted.Analysis of loboratory experiments results indicates that it is feasible to improve the changeable shear strength behavior of Hefei expansive soil by introducing randomly distributed discrete short jute fibers.An optimum fiber content and an optimum fiber length corresponding to greatest improvement in soil shear strength behavior exist.The existence of these optima involves a mechanism that twisting and twining of fibers associated with fiber content greater than its optimum and fiber length longer than its optimum can restrain and even reduce mobilization,of fiber reinforcement effect.The mechanism by which fiber distribution form affects the improvement in soil shear strength behavior involves magnitude of tensile stress induced in the active fibers.Specimen preparation parameters(i.e.dry density and moisture content)exert a significant influence on fiber-soil interaction in terms of effective contact area and friction coefficient,which serves as the mechanism of specimen preparation parameters affecting fiber reinforcement effect.Development of the proposed constitutive model of fiber-reinforced soil is built on a combination of the stress superposition method and the energy-based homogenization concept.The two basic modelling assumptions are(1)the active fibers remain elastic during the entire loading process,and(2)partial slippage of the active fibers along fiber/soil interface controls the before-failure constitutive behavior of fiber-reinforced soil while the failure constitutive behavior is governed by pullout or full slippage of the active fibers.In the modelling process,a fiber-reinforced soil is regarded as a two-phase composite material composed of a soil phase and a fiber phase.The soil phase conforms to the Modified Cam-Clay model,while the constitutive behavior of the fiber phase is linear-elastic.Apply the principle where the strain increments are coordinated while the stress increments are superimposed to the two-phase composite material,an elastoplastic stiffness matrix describing the incremental relation between stress and strain for a fiber-reinforced soil before reaching failure is derived.A macroscopic failure criterion is established on the basis of an equilibrium between the dissipation rates of effective mean stress and deviatoric stress and the rates of work done by macroscopic stresses.Comparison between model prediction and experimental results indicates that the proposed constitutive model captures successfully the key characteristics of the stress-strain relation,shear strength,shear strength parameters and volumetric behavior of soil reinforced with fibers of various fiber contents.