The Analysis And Application Of Mesoscopic Structure Characteristic And Deformation And Failure Mechanism Of Weak Cemented Sandstone

Weakly cemented strata are a type of special sedimentary sandstone strata which are widely distributed in the Jurassic and Cretaceous Era strata in western China.The strata are mainly composed of weak sandstone cementation with low maturity in terms of both the composition and structure.Rich with rigid particles,it features weak cementation,low strength,argillization with water,and sensitivity to disturbance,etc,which has a huge impact on support models for deep underground engineering and the stability control of the wall rock.This paper focuses on the weakly cemented sandstone as the object of study.By using techniques such as mesoscopic experiment,macroscopic rock mechanics experiment,theoretical analysis and numerical simulation,it studies the mineral components,microstrucutre features,and their influences on statics and dynamics properties.The mutual influence mechanism between sandstone and water is revealed,as well as the effect on permeability.This paper also establishes the renormalization model of weakly cemented sandstone.The critical state of phase transition during the sandstone deformation and failure process under press and its telling characteristics are elaborated.Based on the phenomenon of such critical state and its physical implications,this paper analyzes mutual disturbance in tunnel excavation in the weakly cemented strata,and proposes effective active support measures for weakly cemented wall rock.The mineral composition,chemical components,crystal structure,and mesoscopic characteristics of weakly cemented sandstone are quantitatively and qualitatively studied by means of QEMSCAN electron microscopy,SEM scanning electron microscopy,and Zoom—stereo microscope.The results show that the mesoscopic of the weakly cemented sandstone is mainly composed of the cementation and bond structure formed by compaction and cementation of particulate matter and cementing material,and that the differences among mineral components of the weakly cemented sandstone and the contact state between the particles are important for the mechanical characteristics.It is also identified through comparison and analysis that its maturity is much lower than the sandstone in eastern and central China.Through the rock mechanics static load testing,dynamic impact test and discrete element numerical analysis method,the deformation and failure characteristics are analyzed.The results show that the macroscopic mechanical behavior of weakly cemented sandstone is mainly determined by inter-granular contact:When the boundary load changes,the interaction force between the weakly cemented sandstone particles also changes,resulting in changes of the mesoscopic of the granular system.Particle contact parameters of the mesoscopic of weakly cemented sandstone exert great influences on the mechanical properties as well as the deformation and failure characteristics of weakly cemented sandstone.The effects of water-rock interaction and its impact on the mechanical properties of weakly cemented sandstone are studied through water immersion test,microscopic structural analysis test,shear test with water saturation,acoustic emission test and infiltration test under rotating wet and dry conditions,as well as other mechanics test.The results show that the argillization and expansion of cementation material caused by water,as well as the high porosity of the weakly cemented sandstone makes it easier for water to enter the internal structure of the rock.The structure of the mineral particles corrosion then damages the structure of the minerals,which leads to the changes of the mesoscopic in the sandstone.As a result,the strength of weakly cemented sandstone is reduced.It's also found that the interaction between water and rock has important influences on the shear failure characteristics of the morphology features,the characteristics of acoustic emission,and the permeability.Based on the analysis of the mineral composition and mesoscopic of the weakly bound sandstone,the mechanics experiment,and the microstructural stress-bearing theory analysis,the renormalization group model of the weakly cemented sandstone is established.Combing the acoustic emission features of weakly cemented sandstone during loading,it is proved that there is a critical state of phase transition state in the process of deformation and failure of weakly cemented sandstone,that is,the process of the transition of cemented particles from a continuous state to a discrete state.The difference of cementation of weakly cemented sandstone causes the difference of the corresponding critical stress and critical strain,as well as the failure status when reaching the phase transition critical state.When the weakly cemented sandstone reaches a critical state,the specimen is very sensitive to the tiny strain ??,and the specimen is prone to the transition from the continuous to the discrete state,namely,the phase transition critical state.Based on the analysis of regional geostress characteristics,the testing of on-site loosened rock circles,and the method of numerical simulation,this paper studies the mutual disturbance factors of tunnel excavation adjacent to the weekly cemented strata.Based on the existence of critical state of phase transition during the stress loading process,it is concluded after analysis that during the roadway engineering in such strata,traditional yielding support or passive support are inapplicable as a support measure in weakly cemented strata.Instead,an active support should be used,and the strength of support should ensure that the stress status of the wall rock should not cause the critical state of the phase transaction from the continuous to the discrete state.An appropriate active support measure is proposed.