Seismic Quality Factor Analysis Of Rock Mass Based On Discontinuous Numerical Method

When the stress wave caused by earthquake or blasting propagates in the ground,the amplitude will attenuate obviously with the increase of the propagation distance,even if the diffusion effect of the wave front is not considered.The main reasons for this attenuation are:1.Scattering attenuation in inhomogeneous media,usually expressed as a scattering quality factor.2.intrinsic attenuation due to absorption,which is usually expressed in terms of intrinsic quality factor.In addition,since it is difficult to separate the scattering attenuation from the inherent attenuation,the quality factor obtained by the general measurement method reflects both attenuation and is usually indicated by the apparent quality factor.For the study of seismic quality factor(quality factor),the main methods are theoretical research,in-situ test,seismic data inversion,laboratory test and numerical method.The widely distributed joints in rock mass have a significant effect on the propagation of stress waves,but the study on the effect of joint properties on seismic quality factors is still rare.The discrete element method regards jointed rock mass as composed of discrete blocks and joint surfaces,which can truly express the geometric characteristics of jointed rock mass and become a favorable tool for studying wave propagation in rock mass,therefore,the discrete element method(DEM)is used to study the quality factors of rock mass.In this paper,a method for calculating the seismic quality factor of rock mass based on the UDEC is presented,and the method for distinguishing the scattering attenuation from the apparent attenuation is given,the feasibility of obtaining rock seismic quality factor based on UDEC simulation is verified.Then,for parallel through joints,the dimension problem of element body which should be used in numerical simulation is discussed,and the influence of joint characteristics(mechanical properties,geometric distribution)and incident wave characteristics on seismic quality factor is studied.The results show that the intrinsic quality factor is highly correlated with the joint characteristics and is influenced by the incident wave frequency and amplitude,these factors also have significant influence on the parameters such as scattering attenuation,wave velocity and transmissivity,which reflect the dynamic properties of rock mass.Then,a discrete fracture network is established in UDEC for the complex rock mass with intermittent joint,and the effects of joint characteristics and incident wave characteristics on seismic quality factors are analyzed,at the same time,it is concluded that the scattering attenuation is significantly reduced when the joint is not fully connected.Finally,based on a subway tunnel construction project,the P-wave apparent quality factor in the test area is calculated by using the ascending time method.The discrete fracture network is generated in UDEC,and the p-wave apparent quality factor is obtained by numerical simulation.Comparing the simulation results with the field results,it is found that the simulation results are in good agreement with the measured results.