Study On Mechanical Failure Characteristics And Constitutive Model Of Layered Rocks

Layered rock masses are widespread in nature and are commonly found in large projects such as tunnels,underground structures,petroleum exploration and development,unconventional oil and gas development,water conservancy and hydropower.Unlike conventional rock masses,the deformation and strength of layered rock masses are obviously anisotropic,and their failure mechanisms are also relatively complex.The mechanical properties of layered rock masses are greatly affected by bedding inclinations and rock materials.Many engineering problems are related to layered rock masses in recent years.It is of great significance to correctly understand the basically mechanical properties of layered rock masses for stability calculation and safety evaluation of engineering.Thus,a combination method of laboratory test and numerical simulation are used to investigate the anisotropic characteristics of deformation,strength and failure modes of layered rocks.An anisotropic elastoplastic constitutive model of layered rocks is proposed based on the Drucker-Prager criterion.And the anisotropic characteristics of layered rock mass deformation,compressive strength and tensile strength are comprehensively considered in this model.The following main issues are completed in this paper:(1)Based on the laboratory test of uniaxial compression of shale samples with different bedding angles,the anisotropy characteristics of wave velocity,deformation,strength and failure modes are discussed.RFPA3D is used to study and explore the influence of confining pressure on deformation,strength and failure modes.The progressive failure process of the shale specimena is analyzed based on the uniaxial compression stress-strain curves.Several strength failure criterions for layered rock mass are summarized.By introducing anisotropic parameters,the Jaeger strength criterion is revised and an improved Jaeger strength criterion is proposed.(2)The anisotropic characteristics of the Brazilian split failure modes and indirect tensile strength of layered shale are discussed based on the Brazilian dics tests of shale samples with different bedding angles.Direct tensile and Brazilian dics test are carried by using RFPA2D to analyze the differences of failure modes and failure mechanisms between the two methods.The failure modes of Brazilian dics test are mainly divided into longitudinal split tensile failure along the center of the disc,tensile-shear composite failure and shear failure;the direct tensile failure modes are mainly divided into the tensile fracture failure along the bedding plane,the serrated progressive failure mode and the tensile failure of the rock material.A modified single plane of weakness failure criterion(MSPW)is proposed based on the single plane of weakness failure criterion(SPW).On the basis of the MSPW criterion,a method for determining the tensile strength of layered rock mass is proposed in this paper.(3)Numerical simulations are conducted on shale samples with different bedding angles to discuss the anisotropy of the failure mode and shear strength of layered shale under direct shear test.According to the Mohr Coulomb criterion,the cohesion and internal friction angle of the layered rock samples under direct shear conditions are obtained by theoretical solution.At the same time,the variation of the cohesions and internal friction angles of the layered rock samples under the triaxial condition are obtained on the basis of theoretical analyze.It is founded that under direct shearing conditions,the cohesive c of the layered rock samples gradually increases with the increase of the bedding angle,and the internal friction angle ? of the sample firstly decreases and then increases with the increase of the bedding angle;the cohesive c of the layered rock sample exhibits a changing rule that firstly decreases and then increases with the increase of bedding angles under the condition of triaxial compression,the curve behaves like a typical "U",and the friction angle ? of the samples increases gradually with the increase of the bevel angle.According to the variation of cohesive force and internal friction angle under triaxial conditions,an empirical formula of shear strength parameters variation with the bedding inclinations is proposed.(4)The law of energy evolution of the layered rock specimens under uniaxial compression was analyzed and a failure criterion based on energy dissipation is proposed according to the results of uniaxial compression test and energy calculation principle of the layered rock specimen.This criterion is verified on the basis of the uniaxial compression test results of the shale.(5)Based on the results of uniaxial compression,Brazilian dics test and direct shear test of layered rock specimens,anisotropic elastoplastic constitutive model of layered rock specimens is proposed,and numerical realization and verification are carried out through ABAQUS software by writing the UMAT constitutive program.The elastic anisotropy constitutive matrix of the layered rock sample,the anisotropic characteristics of the tensile strength and shear strength parameters are included in this consecutive model.It can better simulate the anisotropic behavior of the deformation and strength of layered rocks.