Study On The Mechanism Of Critical Transition From Solid To Flowing State Of Geo-Discrete Medium

There are a large number of discrete solid particles in weakly cemented granular media.As far as the single particle of loose body is concerned,it follows the stress-strain characteristics of solid mechanics and the motion characteristics of rigid body mechanics.From the point of view of the whole particle system,it also shows the mechanical characteristics of solid-flow transformation under the critical condition.From the point of view of particles,it is the theoretical basis for engineering construction and the key technology of prevention and control of geological granular disasters to study the meso-structure,dynamic process and the relationship between the contact formation of particles and the macroscopic mechanical properties,and establish the relationship between the meso-structure and the macroscopic mechanical properties.To strengthen the mesoscopic study on the localization behavior of weakly consolidated loose media is beneficial to improve the cognitive ability and the level of prediction and regulation of the formation,evolution and disaster-causing mechanism of natural disasters.In this work,by means of laboratory photoelastic experiments and numerical simulation,the force chain network,macro and micro mechanical characteristics and critical conditions in the solid-flow transformation process of weakly cemented loose media are systematically studied,and the formation,evolution and disaster-causing mechanism of geologically dispersed landslides are explained through the analysis of examples.The thesis mainly carries out research from the following aspects:(1)Through the photoelastic experiment of particle flow under biaxial compression,the characteristics of solid-flow conversion interface and load transfer mode were analyzed.It is found that the boundary is arched.Marked by the force chain arch,there are two different states:the particles under the arch is in the state of solid-flow transformation,and the particles above the arch is in the solid stable region.(2)Through horizontal biaxial compression and shear photoelastic experiments,the stress and contact evolution process of granular media were analyzed.With shear,the stress gradually increased,and the average contact number also increased,and the increase rate was related to packing fraction.The lower the packing fraction,the slower the increase of the force and contact number.(3)Combining with the shear photoelastic experiment of two-dimensional particle system,the critical conditions for the conversion of solid state to flow state based on the average contact number and principal stress difference are analyzed,and the range of volume fraction for solid-flow conversion is given.(4)The effects of packing fraction and confining pressure on the macroscopic mechanical behavior of granular bodies were analyzed through the simulation experiments of pure shear and biaxial compression.The critical state equation of solid-flow conversion was proposed,and the volume fraction could be used as a predictive parameter for solid-flow conversion.(5)A numerical simulation of landslide was carried out.The displacement field,damage field and force chain network were analyzed in different stages of a landslide,such as in the initial state,only under the condition of mining disturbance,in combination of mining and heavy rainfall.The space-time evolution of the local volume fraction of the slope and the mean contact number is studied and it may provide a new criterion for landslide warning.