Research On Geometric Characteristics Of Rock Mass Joints Based On Close-range Photogrammetry

Due to various geological effects,there are often a large number of joint surfaces in the rock mass.The existence of joint planes greatly reduces the strength and stability of rock masses,and has become the main cause of various rock mass engineering accidents.The geometric form of the joint surface is an important factor that affects the safety and stability of the rock mass engineering.As a physical quantity describing the smooth and undulating state of the joint surface,the joint roughness is an important parameter reflecting the geometric form of the joint surface.Therefore,the evaluation of the roughness of the joint surface of the rock mass is of great significance for the stability evaluation of the engineering rock mass and the safety risk assessment.This paper takes the granite joint surface of Changheba in Sichuan as the research object,collects the data of the joint surface morphology based on close-range photogrammetry technology,studies the influencing factors of the roughness of the joint surface,and conducts a study on the influencing factors of the roughness based on the discrete element software PFC verification.The main research contents are as follows:（1）Large-scale exposed joint surfaces of natural rock masses are selected through on-site field surveys,and the selected joint surfaces are photogrammed based on close-range photogrammetry technology.The three-dimensional image synthesis and three-dimensional coordinate extraction of the images taken on-site are carried out using the Sirovision software.Perform data preprocessing such as accuracy check,coordinate conversion,and image cropping on the extracted point cloud data.Based on the Matlab platform,a 3D reconstruction program of the joint surface was compiled,which improved the work efficiency,and compared the modeling results with the Surfer software modeling results.The two have good consistency.（2）Taking the typical joint planes（JT₁,JT₂,JT₃）selected on site as the research object,the joint roughness under different sampling directions was calculated and studied on the two-dimensional joint section line and the three-dimensional joint surface angle.And comprehensively considering the various shear directions,a parameter reflecting the anisotropy of joint roughness is proposed.Based on this parameter,the degree of anisotropy of different joint surface roughness is analyzed.The study found that the size of the joint roughness in different directions is different,and it has obvious anisotropy characteristics;The joint roughness anisotropy radar chart calculated from the angle of the joint section line is abrupt,while the angle of the three-dimensional joint surface is gradual;Finally,a method to identify the shear direction of the joint plane is given.（3）Taking the typical joint planes（JT₁,JT₂,JT₃）selected on site as the research object,the influence of sampling size and sampling position on the evaluation of joint roughness was studied on the two-dimensional joint section line and the three-dimensional joint surface angle,and based on the proposed The anisotropy parameter of joint roughness,the influence of sampling size on the anisotropy of joint surface roughness was studied.The study found that the unevenness of the joint surface morphology is the cause of the different roughness results at different measurement positions.When the sampling size and the sampling position are different,the roughness measurement results vary greatly;Sampling size and sampling position will also affect the roughness anisotropy;Finally,based on the progressive coverage method and random sampling method,the error caused by the size effect of the joint surface roughness is reasonably and effectively reduced.（4）The particle flow software PFC was used to establish a numerical model of the jointed rock mass direct shear test,and the model was calibrated according to the laboratory test data,the influence of the joint roughness,the joint inclination angle and the normal force on the shear strength characteristics of the jointed rock mass was studied.The numerical simulation of the direct shear test of the joint surface in different directions and different sizes was realized.Taking the JT₁joint surface as the research object,the shear strength under different directions and different sizes was studied.Research shows that the peak shear strength and residual strength of joints are positively correlated with joint roughness,joint inclination and normal force.When the joint surface is sheared in different directions,it has different joint peak shear strength and different failure modes,that is,it has obvious anisotropy;When the joint surface is sheared in different sizes,the peak shear strength will increase with the size of the joint surface decreases,which has an obvious size effect on the shear strength.Numerical verification of the anisotropy of joint roughness and size effect in the previous article was carried out.