Study On The Scale Effect And Anisotropy Of Mechanical Properties Of Jointed Rock Masses

Engineering rock mass usually contains a large number of randomly distributed joints.The existence of joints not only controls the deformation and failure mode of rock mass,but also makes the mechanical properties of rock mass have size effect and anisotropy.At present,it is difficult to consider the influence of each joint on the calculation results when the stability of engineering rock mass was analysed,which is different from the actual engineering.The Representative Element Volume(REV)size and equivalent mechanical parameters considering rock anisotropy can be obtained,when the size effect and anisotropy of jointed rock mass was studied.Then the influence of a large number of joints on the mechanical properties of the rock mass is averaged into the entire rock mass,and the anisotropy of the jointed rock mass is transferred to the anisotropy of the constitutive relationship.Additionally,the equivalent continuous method is used and the main faults or fissures was supplemented to analyze the stability of engineering rock mass,thereby the engineering calculation problem can be simplified.Therefore,it is of great significance to study the size effect and anisotropy of jointed rock mass mechanics characteristics for rock mass engineering deformation calculation and stability analysis.In this paper,the methods of experimental research,theoretical derivation and numerical simulation are applied to study the size effect and anisotropy of mechanical properties of jointed rock masses,and to determine the equivalent mechanical parameters of jointed rock masses.The main work and conclusions are as follows:(1)The physical and mechanical experiments were carried out to investigate size effect and anisotropy of jointed rock masses.Through the in-situ uniaxial compression test of samples with the same joint angle and different model sizes,and the same model size and different joint angles,the relationship between the size effect and anisotropy of jointed rock masses is analyzed.The results show that the size effect and anisotropy of jointed rock mass do not exist independently,but are related to each other.The anisotropy of the jointed rock mass decreases with the increase of the model size and show size effect,while the REV size of the jointed rock mass changes with the joint angle and has anisotropy.(2)A constitutive model of elastic damage considering the size effect and anisotropy of jointed rock mass is established in theory.According to the relationship between the elastic modulus of jointed rock mass with various model size and joint angle,an elastic damage constitutive model considering the size effect and anisotropy of jointed rock mass is proposed.The model can reflect the linear and nonlinear stages of stress-strain curves of rock samples with different model sizes and joint angles,as well as the size effect and anisotropy of mechanical parameters.The rationality of the proposed constitutive model is verified by comparing with the experimental results.(3)The numerical simulation method is used to study the size effect and anisotropic characteristics of engineering jointed rock mass.The results show that the mechanical parameters of REV of jointed rock mass show obvious anisotropy in different research directions.To consider the anisotropy of jointed rock masses,the concept of generalized REV is proposed.The size effect and anisotropy of jointed rock mass decrease with the increase of confining pressure,and have a certain confining pressure effect.Therefore,it is reasonable to use the REV size obtained under uniaxial compression as the REV size of jointed rock mass under different confining pressures.In addition,under the condition of uniaxial compression,the anisotropy of the jointed rock mass(the largest degree of anisotropy)and the isotropic parameter are used to study the stability of the engineering rock mass.The results can be used as two extreme values to evaluate the stability of engineering rock mass,and provide a reference for the stability analysis of engineering rock mass.(4)A three-dimensional joint network model of equivalent rock mass was constructed,and the size effect and anisotropy of three-dimensional joint rock mass was inverstigated.Based on the Baecher model and the Monte-Carlo method,the reconstruction of a random threedimensional random DFN model can be achieved used RFPA3D software,and a calculation module for generating a three-dimensional random DFN model can be provided.Shpaemetrix3d is used to obtain the geometric characteristic parameters of rock mass joints of the slope at 100 m downstream of the dam site of the left bank of Lianghekou Hydropower Station.Then as an example,the rationality of using RFPA3D software to generate 3D random DFN model is verified.Compared with the mechanical parameters of rock mass obtained based on the generalized Hoek Brown strength criterion,the validity of the equivalent threedimensional joint network model for calculating the mechanical parameters of joints is verified.Finally,based on the 3D joint network model of equivalent rock mass,the size effect and anisotropy of 3D jointed rock mass are studied,and the equivalent mechanical parameters of jointed rock mass in the study area are determined.(5)The sensitivity of rondomly joint geometrical parameters to the mechanical parameters of jointed rock mass was developed.To study the sensitivity of rondomly joint rock mass mechanical parameters to joint geometric characteristic parameters,the orthogonal test method was used to analyze the influence degree of rondomly joint control factors such as joint length,inclination,dip angle and body density(spacing)on jointed rock mass mechanical parameters.The results show that the elastic modulus of the rock mass is insensitive to the geometrical parameters of the jointed rock mass,while the uniaxial compressive strength is highly sensitive to the joint length,volume density and dip angle,but not to the joint tendency.This provides a reference for the measurement and statistics of geometric parameters of joints in engineering rock masses,and provides an effective method for improving the accuracy analysis of mechanical parameters of jointed rock masses.(6)Application study on joint rock mass engineering of the right bank slope of the inlet of Xiaowan Hydropower Station.Taking the right bank slope of the inlet of Xiao wan Hydropower Station as an example,based on the equivalent mechanical parameters considering the size effect and anisotropy of jointed rock mass,the transverse isotropic model and orthotropic shear yield criterion are used to study.The deformation and shear zone of the slope rock mass in the area is analyzed and compared with the traditional isotropic calculation results.The results show that the maximum and minimum values of the initial vertical stress in the study area have little difference based on the two constitutive models.The existence of faults makes the horizontal displacement distribution nephogram in the isotropic constitutive model more complex than that in the transverse isotropic constitutive model.The horizontal displacements along the surface and inside of the slope obtained by the transversely isotropic constitutive model are much larger than those of the isotropic constitutive model,and the maximum differences are 21.28 mm and 7.54 mm,respectively.