Experimental And Theoretical Analysis Of Membrane Model Constitutive Relation For Multi-scale Soil

Soil is a granular material formed by the continuous weathering of rocks.Physical weathering makes the soil from block stones to clay particles span about 5 orders of magnitude,which shows an obvious geometrical mutil-scale characteristic.While,chemical weathering makes the soil from primary minerals to secondary minerals vary hundreds of thousands,which shows obvious diversity of mineral composition.The particles with different shapes,different sizes,and different compositions accumulate into natural soils with different shapes of pores and complex spatial structures.Therefore,the soil has the characteristics of typical granularity,heterogeneity,structure,mutil-scale and non-uniformity.These characteristics make the soil show unique engineering characteristics.The root of these characteristics of soil is the particle surface effect and interface effect caused by the difference in soil geometry and mineral composition,that is,the scale effect.This paper starta from the mineral composition and the geometric scale of soil particles,a method of comprehensive ratio of micr-force to gravity energy scale division method(CMGESD)for the soil is established,which can reveal the root cause of the multi-scale characteristics of the soil properties.And based on the energy scale method,a matrix membrane-reinforced granular soil scale model(membrane model)which can simultaneously reflect the properties of the microscopic discrete particles and the macroscopic continuum of the soil was established,then the constitutive equation of the membrane model is deduced.Through a large number of direct shear tests and triaxial tests,the force and deformation characteristics and influencing factors of the membrane model soil under shear load and triaxial compression are analyzed.At the same time,CT scanning is used to analyze the internal structure changes of the"membrane"model during the whole process of the triaxial compression test.Finally,on the basic"membrane"unit,the meaning and influence of the parameters in the"membrane"model constitutive equation are discussed,and the rationality of the"membrane"model constitutive equation is verified by triaxial tests.Through the above research methods and works,this article has mainly obtained the following results:(1)The geometric scale and mineral composition of particles can produce particle interface effects and surface effects,which is the internal cause of the scale effect of soils.The CMGESD method takes the above factors into account and forms a new standard for the classification of soils at different scales.The experiment shows that when the soil is classified by the CMGESD method,I_pindex is higher consistency of the same comprehensive micr-force to gravity ratio soil.The CMGESD method provides a new theoretical basis for the division of matrix materials and reinforced granular materials.(2)The direct shear test,triaxial test and theoretical calculation of the"membrane"model all show that the volume ratio has the most significant influence on the soil strength:in the filled state,as the volume ratio of the matrix increases,the shear strength value and triaxial yield stress increase;in the film state,as the ratio of reinforced particles increases,the shear strength value and triaxial yield stress increase continuously.The volume ratio also affects the structural changes under load,which makes the soil stress-strain curve show the phenomenon of hardening or softening.Theoretical calculations show that:the volume ratio and the load adjustment coefficient are almost linear,and the change of the volume ratio will affect the volume of the"membrane"element and the load adjustment coefficient at the same time.The combined effect of the two is that as the volume ratio of the reinforced particles increases,the value of the stress calculated of the"membrane"unit is also increasing.(3)The direct shear test of"membrane model"soil show that:reinforcement particles and matrix particles have different contributions to the shear strength of the soil:the reinforcement particles are mainly manifested as the internal friction effect of the soil;while the matrix is mainly manifested as cohesion effect of soil.When the proportion of reinforced particles increases,the internal friction angle?of the"membrane model"gradually increases;when the proportion of the matrix increases,the cohesion c value of the"membrane model"gradually increases.(4)The direct shear test,the triaxial test and the theoretical calculation of"membrane"element all show that the reinforced particle size d has no obvious influence on the strength of the soil.Under the same strain condition,the larger the d of a single particle,the larger the particle's rotational torque,Hertzian contact force and spring contact force,but when the volume ratio is constant,the larger the d,the smaller the number of particles.After taking the statistical value of the particle group,the impact of d on the strength of the soil is not obvious.The"membrane"element theory shows that the spring force and the Hertzian contact force are second power of d,and the cross-sectional area of the"membrane"element is second power of d.Therefore,change of d has no obvious influence on the calculated stress value of the"membrane"element.(5)When processing CT scan images for contacted particles,the"box"search method and boundary domain prediction method can reduce the amount of calculation by about 20%of the original.(6)The CT scan results show that the internal structure of the soil is constantly changing during the triaxial test.The change trend of the internal structural parameters(CN,S_R-R,S_R-M)of membrane model shows that when the axial strain is small,the volume of the soil sample is compressed;when the axial strain increases,The shear rotation of the particles causes the soil to swell,and the contact between the particles in the swelling area becomes soft.Therefore,during the triaxial test,CN,S_R-R,S_R-M appear to increase first and then decrease.(7)The constitutive equation of membrane model can accurately predict the whole process of the stress-strain curve when the soil undergoes strain hardening or softening in the test.When the load is applied,the internal contact structure of the membrane model is constantly changing,the Hertzian contact force of the reinforced particles and the spring force of the matrix is also constantly changing.Therefore,when the load adjustment coefficient is expressed as a form that can reflect the change of soil structure,the membrane model constitutive equation can obtain an accurate prediction of the whole process of stress-strain curve.