Research On Mechanical Properties Of Slab Phyllite And Genetic Mechanism Of Collapse Disaster

Wenchuan earthquake destroyed the rock mass structure,resulting in the reduction of slope stability.Under the influence of rainfall and engineering activities,collapse disasters occur frequently,which seriously affects the highway construction and safe operation.In order to reduce the adverse effects of collapse disasters,it is necessary to study the law of disaster,the mechanical property of rock,and the genetic mechanism of collapse disaster.This paper takes the Wenchuan to Maerkang expressway under construction as the research object.Firstly,the geological disasters along the highway are investigated,the law and influencing factors of the disasters are studied,and the risk assessment model is established;Secondly,the mechanical properties and fracture mechanism of phyllite are deeply studied through a series of indoor tests;Finally,genetic mechanism of collapse disaster is analyzed,and the relationship between mechanical properties of phyllite and collapse disaster is revealed.The main conclusions are as follows:(1)Disasters are mainly concentrated in Wenchuan to Li County,followed by Li county to Miyaluo,and Miyaluo to Maerkang.Geological disasters are mostly concentrated in the basic geological environment with slope gradient of 40°-60°,slope height greater than 150m,seismic intensity of class VIII,elevation of 1500-2000 m and lithology of slab cracked phyllite.(2)The analytic hierarchy process(AHP)and entropy weight(EW)are coupled to determine the comprehensive weight of influencing factor,realizing the combination of subjective and objective weight methods,which not only makes up for the defects of data,but also reduces the influence of human factors.The evaluation result is closer to the real situation.The evaluation system can provide new ideas for the prediction and prevention of collapse disaster in Northwest Sichuan.(3)Phyllite has significant anisotropy and water sensitivity.The mechanical properties of phyllite are affected by both water content and schistosity plane.The greater the schistosity effect of phyllite,the higher the deterioration degree of mechanical properties by water.The softening coefficient of compressive strength and tensile strength reaches the minimum when the included angle of schistosity plane is 30°and 0°respectively.(4)The mechanical properties of phyllite are closely related to the cyclic loading and unloading path.1)With an increase in the number of cycles,the elastic modulus first increases,then basically tends to be constant,and finally decreases slightly,while the Poisson’s ratio gradually increases.The damage stress of phyllite increases first and then decreases with the number of cycles.2)With the increase of plastic shear strain,the dilatancy angle increases gradually,and has obvious nonlinear characteristics.Confining pressure and water weaken the dilatancy effect of phyllite.3)Based on the dissipated energy,a new damage variable(D_P)of phyllite is established.the damage variable(D_P)increases in S-shape.The critical damage variable is about 0.80,and the damage variable(D_P)can be used as an index to predict phyllite failure.(5)Water has a significant effect on the energy release characteristics and fracture propagation mode of phyllite.1)AE energy of water-saturated phyllite has obvious"step"disturbance.The fracture distribution is relatively concentrated,with obvious nucleation area.The crack gradually diffuses from the center to ends.Shear fracture events can be detected repeatedly.The dry phyllite forms dispersed microcracks,and only when failure occurs the high-energy fracture events can be detected.2)Under the influence of expansion stress,pore water pressure and friction coefficient,the stress intensity factor at the tip of water bearing fracture increases and the fracture toughness decreases.Water promotes the expansion and connection of fractures,accelerates the localization of damage,and changes the release characteristics of AE energy and fracture propagation mode.(6)The formation mechanism of collapse disaster includes cracking-topple mode,sliding mode,scattering model and falling model.The formation process of collapse disaster can be summarized into four stages:the formation stage of original structural plane,the initiation and expansion stage of fracture,the formation stage of dangerous rock mass and instability failure.Through the laboratory test of phyllite and analysis of collapse disaster,the relationship between the mechanical properties of phyllite and collapse disaster is revealed,which provides ideas for the further study of slope failure mechanism.