The Experimental And Numerical Study On Scale Effect Of Rock Joint Mechanical Properties

In rock engineering,the joint feature is a very important petrophysical property.The large number of complex joints with complex distribution,multi-size and cross-scale make the rock mass scale effect and high degree of inhomogeneity and discontinuity,which greatly reduces the resistance to deformation and mechanical strength of the rock mass.The characteristics of rock joints affects the stability and safety of underground engineering projects.When analyzing the stability of surrounding rock in underground engineering,the strength of surrounding rock and the correctness of its mechanical parameters are the basis for ensuring the calculation results.The lack of consideration of rock and joint size effects will have an impact on the accuracy and reliability of the mechanical parameters.Great influence.Therefore,it is of great practical significance to study the scale effect and application methods of rock joints.Based on the above background,this paper takes the scale effect of rock joints as the main line,and uses a combination of laboratory tests and numerical simulations to study the shear strength of rock joints with different sizes.The main work is as follow:1)The scale effect of surface morphology of rock joint is studied.The surface morphology of rock joints through 3D scanning is digitally sorted and calculated.The size effect of red sandstone joints and granite joints is studied,and the size effect of roughness under the influence of the anisotropy of the joint surface is further considered.The test results show that the joint roughness of red sandstone decreases as the size increases;when the granite joint size reaches the critical size,the granite joint roughness remains stable.2)3D printing technology is used to reconstruct high-strength rock-like materials.The influence of printing direction and post-processing methods on 3D printed samples is studied.The mechanical parameters of the 3D printed samples are tested.The dried samples have similar strength to the grey sandstone,and the vacuum infiltration treated samples have a higher degree of similarity to the red sandstone in physical and mechanical properties.A method for 3D printing to make rock-like joint samples is proposed and 3D printing joint samples are highly similar to natural sandstone joints.3)The samples of rock-like joints with similar surface morphology but different sizes are produced,and the direct shear tests of 3D printed rock-like joints of different sizes are carried out.When the surface morphology is similar,as the joint size increases,the joint shear strength gradually decreases.4)According to the results of uniaxial compression tests and Brazilian split tests of samples,the input parameters of the models of different sizes are calibrated,and the FDEM parameter section method considering the size effect of the complete rock mechanical strength is established.5)The prediction model of the shear strength of red sandstone joints is proposed.The mechanical parameters of red sandstone in different sizes is collected to build a rock parameter database.According to the red sandstone strength parameters in the database,the direct shear tests of red sandstone joints under different sizes,roughness and normal stress are simulated,and the peak shear strength of the corresponding joints is obtained.The joint size,roughness,normal stress and shear strength are added to the rock parameter database.Based on this database,a BP neural network is used to establish the prediction model of the shear strength of red sandstone joints,which is easy to use and has small errors in the prediction results.6)The joint shear strength prediction model considering the joint size effect,roughness and normal stress is applied to the stability analysis of the surrounding rock of a tunnel.The immediate failure process of the surrounding rock during the excavation at the engineering scale is simulated.The considered parameters are compared.The surrounding rock crack propagation law and the displacement change around the cave without considering the parameter size effect clarify the significance of the size effect and verify the engineering applicability of the tunnel surrounding rock stability analysis described in this article.