Study On The Mechanism Of Deviatoric Pressure Instability Of Weak Roof And Thick Coal Wall In The Deep Buried Working Face

The coal occurrence conditions in China are generally complex,and the common occurrence of deep buried thick coal seam with weak roof is extremely widespread.Under the action of the non-uniform bearing pressure,the roof caving and the coal wall spalling are easy to happen,which affects the safe production of the working face.Therefore,it is necessary to study the distribution of stress,the instability mechanism and the instability mode of weak roof and coal wall under the deep buried working face with thick coal seam and weak roof.Taking the deep buried working face with thick coal seam and weak roof in Zhaozhuang Coal Mine as the engineering background,a combination of site investigation,theoretical analysis,laboratory test,physical model test and numerical simulation was used to study this research.Based on the statistical analysis of the characteristics of the strata pressure in the deep buried working face with thick coal seam and weak roof,the evolution law of deviatoric pressure zones of bearing pressure in front of the coal wall was studied.The mechanical properties and failure mode tests of coal-rock combination with equal strength had been carried out.The deformation law of the coal wall and the instability mode of the coal wall under the action of the deviatoric pressure were revealed.The interaction mechanism between the soft roof caving and the coal wall spalling was discussed.The instability mechanical model of the coal wall eccentric pressure column and the shear sliding failure mechanical model of the coal wall eccentric pressure column were established,and the coal wall spalling mechanism was revealed.Finally,the evaluation method of coal wall instability in the deep buried working face with thick coal seam and weak roof was put forward.(1)Based on the non-uniform distribution of bearing pressure and the characteristics of weak roof,the deviatoric coefficient and the change rate of deviatoric coefficient were introduced to refine the supporting pressure area into deviatoric pressure zone I,II and III,and the first fracture step and periodic weighting step of the weak roof were determined.Based on the elastic foundation beam theory and the limit equilibrium theory,the stress distribution function and range of the deviatoric pressure zone in the elastic deformation phase and elastoplastic deformation phase of the coal wall were given.The influence law and sensitivity of the factors were analyzed,and the evolution law of the stress and the deviatoric pressure zone range in the mining process was revealed.(2)The similar materials with equal strength of coal and rock were made up with cement,gypsum,quartz sand and other material,as well as the coal-rock combination with different rock-coal height ratio and pre-pressed rock samples.The uniaxial compression tests were carried out under different loading rates and preloading pressures.The mechanical properties,deformation characteristics and acoustic emission evolution of the coal-rock combination were analyzed,and the coupling failure mode of the coal-rock combination was revealed.(3)The unloading test system for simulating coal and rock mining in working face was independently developed.The deformation and failure evolution law of coal-rock mass and coal wall under the action of deviatoric pressure were studied,and the mode of coal wall spalling was obtained.The evolution law of longitudinal wave velocity of the model specimen before and after loading was compared and analyzed,and the integrality coefficient of the specimen was introduced to reflect the internal damage state of the specimen.The dynamic deformation process and influence mechanism of coal-rock mass and coal wall in the working face were revealed.(4)Based on the deformation and failure characteristics of the weak roof,the interaction law between the weak roof caving and the coal wall spalling was studied,and the interaction mechanism between the weak roof caving and the coal wall spalling was revealed.Based on the characteristics of eccentric compression in front of coal wall,a model of eccentric compression column was built.The slenderness ratio of "coal pillar" was introduced to establish the instability mechanical model of the coal wall eccentric pressure column(instability failure type)and the shear sliding failure mechanical model of the coal wall eccentric pressure column(material failure type),and two kinds of instability failure criteria of coal wall failure models were given,meanwhile,the mechanism of coal wall spalling was revealed.The coal wall spalling coefficient was put forward,and the sensitivity of factors was analyzed by numerical simulation method.Based on the weight analysis and fuzzy comprehensive evaluation,the evaluation method of the coal wall instability in the deep buried working face with large mining height and weak roof was established.Then,the risk grade of coal wall instability in No.1307 working face of Zhaozhuang coal mine is evaluated,and its comprehensive score is 0.566,which belongs to E-level high risk and wall instability.The results are in accordance with the engineering practice,and the application effect is good.There are 118 figures,45 tables and 206 references in this dissertation.