Test And Numerical Analysis Of Mechanical Properties Of Muddy Rock Mass In Fault Fracture Zone

When the tunnel passes through the geological conditions such as fault fracture zone,it is easy to occur engineering problems such as instability of face,water inrush,excessive deformation of supporting structure and even failure of supporting structure.The fault fracture zone is mainly composed of rock blocks with relatively high strength and stiffness and fault gouges with low strength.This kind of mud-entrapped rock mass is essentially a mixture of soil and rock,which has the characteristics of directional arrangement and contains large-size block stones（25-270mm）.At present,for this kind of soil-rock mixture,the most direct and reliable method is to use large-scale triaxial compression test.However,due to the complex topography of the mountain tunnel and the significant different drilling resistance of rock blocks and matrix soil in the fault fracture zone,it is difficult to obtain complete samples through drilling,and the current mainstream large-scale triaxial compression testers are 300mm × 600mm and 300mm× 700mm,which can not eliminate the size effect caused by large-size blocks.In this paper,artificial preparation of sandy muddy rock mass is used to carry out largescale triaxial compression laboratory tests with sample size of 500mm × 1000mm,combined with numerical simulation analysis based on two-dimensional particle flow analysis software PFC2D.The macroscopic mechanical response and meso-mechanism of sandy muddy rock mass under different block stone content,arrangement angle,matrix strength and sample size are studied.Through the quantitative study of the influence of block stone content and arrangement direction on matrix strength,the mechanical strength of fault fracture zone can be inferred under the condition that matrix strength,block stone content and arrangement direction are known.The main research contents and conclusions are as follows:（1）The indoor large-scale triaxial compression test of sandy muddy rock mass was carried out,and the effects of block stone content,arrangement angle and relative compactness of matrix sand on the macroscopic mechanical properties of sandy muddy rock mass were analyzed.The results show that the sandy muddy rock mass with relatively high matrix density has higher peak strength and initial modulus than the sandy muddy rock mass with low compactness.There is a rock content threshold in sandy muddy rock mass,and when the volume content of rock mass is less than 30%,the strength and deformation modulus of muddy rock mass mainly depend on the strength and deformation modulus of matrix soil;with the increase of stone content,the internal friction angle and cohesion of sandy muddy rock mass increase to a certain extent,and the softening characteristics of the sample will become more obvious.When the stone content increases to 70%,the above phenomenon slows down with the increase of stone content.（2）A flexible biaxial discrete element numerical model of sandy muddy rock mass is established based on PFC2D platform.The effects of block stone content,arrangement angle and matrix strength on the macroscopic mechanical properties of sandy muddy rock mass are further analyzed,and the reliability of laboratory test results is proved.The results show that under the condition of fixed stone content,the strength of the sample decreases at first and then increases with the change of the inclination angle from 0° to 90°,and the lowest value is obtained when the arrangement angle is approximately 45°+φs/2.The numerical simulation analysis explains the influence mechanism of block stone content and block rock arrangement inclination angle on the strength of soil-rock mixture from the microscopic point of view.（3）Based on the results of laboratory test and numerical simulation of sandy muddy rock mass,a formula for predicting the strength of sandy muddy rock mass considering stone content is put forward,that is,the law between strength index and stone content of sandy muddy rock mass can be expressed by the formula f_b/s=1+A/(1+100e^-10VBP)·VBP/（VBP+0.01）.At the same time,a flexible biaxial discrete element numerical model of viscous muddy rock mass is established,and the numerical simulation analysis shows that the above strength prediction formula can be extended to the strength evaluation of viscous muddy rock mass.4)When the stone content is less than 70%,the arrangement angle of block stone and the strength index of muddy rock mass can be described by parabolic formula f（θ）＝Aθ²2A（45°+φs/2）θ+f_s+A（45°+φs/2）².This formula can well describe the relationship between the strength indexes of muddy rock mass and the arrangement angle of rock blocks.（5）Based on the above strength prediction formula,the strength prediction model of muddy rock mass in fault fracture zone is established,which provides certain guiding significance for the design of tunnel crossing fault fracture zone.