Influence Of Granite Fracture On Elastic Wave Propagation Characteristics

Fracture is a relatively weak part of mechanical strength,which not only destroys the continuity of rock mass,but also determines the medium characteristics and mechanical properties of rock mass.Because the propagation of elastic wave in discontinuous rock mass is closely related to the geological engineering problems such as blasting construction,mine exploiting,seismic landslide,rock burst prediction,geological exploration and advance geological forecast.Elastic wave test results are widely used in engineering to evaluate mechanical properties of rock mass.The study of longitudinal wave propagation of fracture rock mass has important guiding significance for inferred the physical and mechanical state of fracture rock mass,analyzed the structure and anisotropy of fracture rock mass,determined the damage state of fracture rock mass,definited the dynamic elastic parameters of fracture rock mass,and carried out the quality classification of engineering rock mass.Taking the granite fracture rock mass as the research object,the theoretical formula of longitudinal wave propagation velocity of elastic wave in tight fracture rock mass is firstly derived.Then,the discrete element method was used to simulate the propagation process of elastic waves in rock mass with single and multi-fractures,and the influences of the dip angle,amount and normal stiffness of fractures were analyzed,which were verified with the theoretical results by comparison.Finally,the results are applied to the statistical analysis of the wave velocity of granite fracture rock mass based on field tests to verify the numerical simulation results.The main research and achievements are as follows:(1)In the theoretical and numerical study of elastic wave propagation in tight fracture rock mass,it is found that:(1)Based on the elastic wave propagation theory,the formulas of longitudinal wave velocity of tight fracture rock mass concerning normal stiffness,height of rock mass and amount of fractures were deduced.(2)When k_n=18GPa/m is taken as the boundary,the velocity and amplitude of primary longitudinal wave in single-fracture rock mass increase rapidly at first and then slowly with the increase of k_n.(3)The theoretical formula of wave velocity for single-fracture rock mass and multi-fractures rock mass is in good agreement with the numerical results,which confirms the reliability of the simulation program.(2)In the numerical study of elastic wave propagation characteristics in filling fracture rock mass,it is found that:(1)The velocity and amplitude of primary longitudinal wave are positively correlated with the Poisson’s ratio and the elastic modulus of the filling medium.The wave velocity decreases with the increase of the density of the filling medium,while the amplitude of primary wave increases with the increase of the density.(2)The change of normal stiffness of filling fracture has little influence on wave velocity and the amplitude of primary wave.(3)It is found that in the study of wave velocity characteristics of granite fracture rock mass based on field tests:(1)The variation curve of wave velocity in tight fracture rock mass with respect to the amount of fractures is well fitted with the quadratic function,which is closed to the variation trend of numerical simulation results.(2)The normal stress is higher,the fracture rock mass with larger dip angle and smaller opening width has higher wave velocity.(3)When dip angleθ≤45°,filling type has little effect on wave velocity.When the dip angleθ>45°,the filling type has obvious effect on the wave velocity.The research results are helpful to consummate the theory of stress wave propagation,and have guiding significance in improving the blasting effect,enhancing the level of flaw detection and maintaining the safety of rock engineering.