Research On Mechanical Characteristics And Stress Shadow Effect In Hydraulic Fracturing Of Layered Rock Masses

Layered rock masses have a wide distribution in nature,acting as a construction objects in large-scale engineering projects inevitably.As the actual scale of project is getting larger and larger,and resource exploitation depth is getting deeper and deeper,the study of the mechanical properties and failure mechanism of layered rock mass is getting more and more important.As a complex transversely isotropic material,the study of its mechanical properties and failure pattern is of great academic significance and practical value for ensuring the safety of engineering with layered rock masses and improving the efficiency of deep resource exploitation.In this doctoral dissertation,the layered rock mass is taken as the research object.The combination of laboratory test and numerical simulation is used to study the mechanical characteristic of layered rock masses under compression and shear loading.The crack propagation and stress shadow effect with hydraulic fracturing in layered rock masses are analyzed.The main research contents and results are given as follows:(1)Combining the advantages of the existing specimen preparation methods for layered rock mass simulation test,a new specimen preparation method is proposed.The cement mortar is used to simulate the intact rock,and the epoxy resin cementing material is used as the bonding material.With the processes of pouring,assembling,pasting,cutting,grinding,modeling,etc.,the artificial specimens of layered rock mass are finished,of which both the mechanical properties of the intact rock and bedding plane can be controlled.The artificial specimens of layered rock mass are then used for uniaxial compression and direct shear laboratory tests.The results of uniaxial compression tests show that the trend of elastic modulus and peak strength of the specimens with different inclination angles has a U-shape variation.It conforms to the deformation and strength characteristics of natural layered rock masses.The results of direct shear tests show that,with the increasing of inclination angles,the shear strength of layered rock masses increases first,then decreases and then increases again,taking a maximum value at around 30 degree and a minimum value at around60 degree.The spacing of the bedding planes has small effect on its shear properties.The increase of the bonding strength of bedding planes will generally enhance the shear strength of the specimen under all inclination angles.(2)The existing modeling methods for bedding planes in particle flow discrete elements are summarized.Based on these,an improved modeling method of bedding planes is proposed,which can overcome both the defect of the rigid particle occlusion which is innate or occurred during late loading process in the traditional and popular simulation method.At the same time,the improved simulation method can ensure the sufficient contact of the particles in the upper and lower intact-rock along the bedding planes.The sensitivity analysis between the macroscopic and microscopic parameters in the uniaxial compression test of layered rock mass is carried out.Based on the results,a set of calibration methods between the macroscopic and microscopic parameters for the numerical model of layered rock mass is designed.The microscopic parameters of the intact rock are calibrated with the specimen when inclination angle ? equals to 90 degree.And the microscopic thickness of bedding plane d_sj is introduced.Through these ways,the simulation of the anisotropic strength characteristics of the specimens when inclination angle ? equals to 0 degree and 90degree can be simultaneously simulated.Using the macroscopic mechanical parameters of Boryeong shale as the calibration object and comparing with the calibration results in related literatures,it can be proved that the microscopic parameters from the calibration method designed in this dissertation can simulate the mechanical properties of layered rock masses better.(3)Taking advantage of particle flow discrete element numerical simulation method,a series of uniaxial compression and direct shear numerical tests of layered rock mass are carried out.The microscopic particle velocity distribution,microscopic contact force distribution,microcrack development and macrocrack propagation evolution of numerical specimens under different bedding plane conditions are analyzed during the loading process,to explore the mechanical properties and failure mechanism of layered rock masses with different inclination angles.With the increasing of the inclination angles,the failure modes of the specimen under uniaxial compression can be verified into four patterns:the tensile failure of the intact rock,the mixture of shearing failure along the bedding planes and tensile failure in intact rock,the shearing failure in bedding planes,the shearing failure along the direction of bedding planes.The direct shear failure of the layered rock can be summarized into four failure patterns according with the different of inclination angles:Sliding failure along the bedding planes when ? equals to 0 degree.When ? equals to 15 to 30degree,the bedding planes and intact rock are simultaneously fractured,and then only the shear fracture in intact rock occurs,with the rupture surface through the specimen.When ? equals to 45 to 75 degree,the intact rock fractures at the end of the ruptured bedding planes,with the direction perpendicular to bedding planes,the shear fracture zone is formed in the middle of the specimen.Shearing failure occurs in the intact rock only when?equals to 90 degree.The macroscopic failure patterns are basically consistent with laboratory results.The increase of spacing and strength of bedding planes inhibits the formation of shear fracture zone in the layered rock specimens during direct shear tests.(4)Modifying and perfecting the existing numerical method of fluid-mechanical coupling in PFC,the numerical model of hydraulic fracturing of layered rock mass is established.The numerical simulation of hydraulic fracturing in layered rock masses under different initial stress field and different inclination angles are conducted,and the results show that both the inclination angles of bedding planes and initial stress field affect the direction of the hydraulic fracture propagation.When the initial stress ratio is relatively large,the influence on the hydraulic fracture propagation is dominated by initial stress ratio.Besides,the existence of the bedding planes can change the propagation of hydraulic fractures.The stress distribution in the specimens under different inclination angles is monitored by measuring circles which are arranged rationally.The stress shadow effect of the single hydraulic fracture in the layered rock masses with different inclination angles is studied in detail,compared with the results in heterogeneous rock masses.The results show that the existence of the bedding planes enhances the stress shadow effect,and the influence of bedding plane is gradually weakened with the increasing of the inclination angles.