Study On The Multiscale Mechanical Behavior And The Meso-mechanical Model Of Soil-rock Mixture

Soil-rock mixture which was usually classified between fine-grained soil and severely broken rock mass is a special composite geomaterial widely distributed in geotechnical,geological,mining and other engineering fields.Heterogeneity and nonuniform structure are fundamental mesoscopic characteristics of soil-rock mixtures,resulting in significant randomness and uncertainty of their macroscopic mechanical behavior.The mechanical theories and analytical methods developed around homogeneous rock or soil mass are of poor capacity to effectively reveal the macro-to-micro multiscale characteristics of deformation and failure of soil-rock mixtures.As a distinguished meso-mechanical feature between soil-rock mixtures and homogeneous geomaterials,stress transfer between soil matrix and rock inclusions under loading condition is the fundamental source for illustrating the meso structural dependence of the macro-mechanical properties of soil-rock mixtures,which induces complex deformation and failure modes at the meso and micro scale.Theoretical analysis,numerical simulation,and laboratory tests are adopted in this thesis to clarify the influence of meso fabric on the deformation and failure mechanism of soil-rock mixtures through the multi-scale mechanical perspective.The construction of meso-mechanical models and analytical method is highlighted in this thesis,which intends to provide scientific analytical methods and theoretical support for the prevention and control of geological disasters in soil-rock mixtures.The main research results of this thesis are summarized as follows:(1)The analytical solution of the stress transfer model of soil-rock mixture is established by the basic use of theory of elasticity and composite material mechanics.By means of the volumetric homogenization of stress and strain,the homogenized elastic constitutive equation of soil-rock mixture is constructed at the mesoscale.The model well clarifies the general impact of composition conditions on the meso stress distribution and the macro deformation characteristics.The stress transfer characteristics depend on the meso mechanical parameters such as stiffness contrast between soil and rock,volumetric proportion of rock blocks,and distribution structure of rock blocks.Changes in stiffness ratio of rock block to soil matrix will adjust the stress transfer mode,and the volumetric proportion and distribution characteristics of rock block will affect the intensity and anisotropy of stress transfer.(2)Based on the average stress field,a method for determining the failure mode of soil-rock mixture under uniaxial compression is proposed.The equivalent inclusion theory is used to calculate the stress in the phase of soil matrix and rock inclusion,and prediction model for the uniaxial compressive strength of soil-rock mixture is established.Studies indicate that the macroscopic compressive strength of soil-rock mixtures can be scaled down to the strength of the matrix or rock blocks.Using Weibull functions to describe the randomness of the matrix strength can better predict the dispersion of the overall compressive strength of soil-rock mixtures.The mechanical effect of the soil-rock interface strength is manifested by the evolution in bearing capacity of the internal components of the soil-rock mixture,as well as the change in the stress transfer characteristics.(3)In the multiscale mechanical framework,the bearing characteristics and the mechanical effects of key mesoscopic factors of soil-rock mixture under confined compression are clarified.When rock blocks are dispersed within the matrix,the bearing mechanism of the soil-rock mixture under confined compression is mainly related to the matrix bridge formed between the rock blocks.The size of the rock blocks has little impact on the stress transfer properties and the macroscopic deformation modulus.The sliding of the soil-rock interface will change the stress transfer behavior.With a rougher the soil-rock interface,the greater stress is shared by the rock blocks,but even for smooth soil-rock interface,the deformation behavior of soil-rock mixture is still governed by the rock blocks.Under confined compression,the rock block is in a three-dimensional compressive state,and the matrix filling effect makes the blocks less vulnerable to be damaged.(4)Based on the laboratory direct shear test,the influence of the structure of rock blocks on the shear characteristics of soil-rock mixtures has been explored,and the nonlinear strength characteristics and the corresponding physico-mechanical basis of soil-rock mixtures are discussed.The influence of structural effect of rock blocks on the shear properties of soil-rock mixture is related to block size,volumetric block proportion,normal stress,and initial density of matrix.Significant difference is observed in the shear strength between soil-rock mixtures and sand-gravel mixtures,since block size has an important impact on the local shear dilatancy effect caused by the occlusion of rock blocks.The shear strength envelope of soil-rock mixture has significant nonlinearity.Stress dependence exists in the strength parameters of soil-rock mixture generated by the Mohr-Coulomb criterion,and the strength parameters estimated under low pressure will overestimate the strength characteristics under high pressure.(5)The micro and meso mechanical characteristics of shear deformation and failure of soil-rock mixtures are revealed by the mesoscopic numerical experiments.The results show that the distribution of particle contact forces within the soil-rock mixture is significantly uneven.With the increase of rock content,the shear band evolves from a smooth regular shape to an irregular one accompanied by secondary shear bands,and the phenomenon of local shear dilatancy formed by the occlusion of rock blocks is obvious.Under high pressures,the rotation of particles inside soil-rock mixture is restricted resulting from the shrinkage of the shear zone,and the uneven shear stress transmission is suppressed.A conceptual model of equivalent roughness is proposed to analyze the shear stress transfer characteristics and to construct a new shear strength criterion whose rationality was verified by laboratory and field test results.The dissertation contains 121 figures,10 tables,and 279 references.