Experimental Test And Analysis Of Coal Deformation And Fracture Evolution Law In Working Face

Due to the increasing scarcity of shallow resources,the mining of most coal resources in China has gradually entered the deep.However,with the gradual increase of mining depth,the in-situ stress of coal seams has also gradually increased,and the geological conditions are complex and changeable.The coal body of the working face is in the state of stress balance before mining.With the progress of different forms of mining operations,the stress balance of the coal body in front of the working face is broken,and a stress concentration area is formed near the mining free face.The coal body is prone to fracture,deformation and slope under the state of high stress,and even dynamic disasters such as rock burst and coal and gas outburst,which seriously affects the normal production of the mine and the safety of underground operators.Taking the coal body of the working face as the research object,this thesis establishes the ZTTS-322 K multi field and multiphase coupling triaxial experimental system by comprehensively using the methods of engineering analysis,experimental research and theoretical analysis.The deformation and failure law of the unloaded coal body exposed to excavation and the deformation and fracture law of the coal body loaded on the coal wall of the working face are tested and analyzed.The main work and conclusions are as follows:(1)By simulating the excavation of working face,the influence experiment of initial load on coal unloading failure is carried out,and the influence law of initial load on coal unloading deformation and fracture is revealed.It is found that the unloading surface of the sample will deform or rupture after unloading,and the initial load has a great influence on the fracture form,fracture distribution and mechanical properties of the unloaded coal surface.The failure degree,crack density and crack type of coal specimen increase with the increase of initial load,and the deformation amount and deformation growth rate of coal specimen increase with the increase of initial load;In the case of no unloading fracture,there is a stress platform in the stress-strain curve perpendicular to the unloading direction,there is no stress drop,and the length of the platform increases with the increase of the initial load;Under the condition of high initial load,the unloading fracture of the specimen will produce stress drop,and the length of the stress platform is significantly smaller than that of the specimen without brittle failure.(2)By simulating the excavation of working face,the influence experiment of unloading speed on coal unloading failure is carried out,and the influence law of unloading speed on coal fracture evolution is revealed.It is found that the unloading speed has a great influence on the surface fracture form,fracture distribution and mechanical properties of unloaded coal.The damage degree of coal specimen surface is positively correlated with the unloading rate.The deformation and deformation growth rate of coal specimen increase with the increase of unloading rate;The length of stress platform increases with the increase of unloading speed without unloading fracture;Under the condition of high unloading speed,the unloading fracture of the specimen will produce stress drop,and the length of the stress platform is significantly smaller than that of the specimen without brittle failure;Coal samples are more prone to brittle failure under high unloading speed,and samples with low unloading speed are more inclined to ductile deformation.(3)Simulated coal pillar,carried out the experiment of the influence of multidirectional unloading path on coal unloading failure.Through the macroscopic analysis of the external crack characteristics of the coal body,the damage degree is as follows:four side stress unloading specimen,three side stress unloading specimen,one side stress unloading specimen and two side symmetrical stress unloading specimen;The strain of coal samples in each stress path is sorted from large to small as follows: four side stress unloading,symmetrical stress unloading on both sides,three side stress unloading and one side stress unloading.In the stress path of stress drop,the stress drop of three sides is the fastest,that of one side is the second,and that of four sides is the slowest.In the stress path of stress drop,the yield platform of one side stress unloading is the longest,the third side stress unloading is the second,and the fourth side stress unloading is the shortest.(4)It is found that the initial load plays a leading role,the unloading speed takes the second place,and the stress unloading path plays the smallest role among the three.(5)Simulating the vertical loading of coal wall in the working face,the experiment on the influence of initial loading horizontal single-sided unloading vertical stress concentration on coal fracture is carried out.The experiment shows that after the coal is continuously loaded by vertical stress,the damage degree of coal specimen increases with the increase of initial load and unloading speed;Compression deformation occurs in the vertical loading direction and expansion deformation occurs in the horizontal unloading direction.The parameters such as deformation in the vertical loading direction,deformation in the unloading direction and peak strength increase with the increase of initial load;The parameters such as the peak stress velocity of the specimen decrease with the increase of the initial load,the parameters such as the peak stress velocity of the specimen and the deformation in the unloading direction increase with the increase of the unloading speed,and the parameters such as the peak strength of the specimen decrease with the increase of the unloading rate;In the variables of initial load and unloading speed,the octahedron strength criterion has good applicability and can be used for strength prediction.The research work and results play an important role and significance in further understanding the evolution process of coal wall deformation,fracture,slope and rock burst,coal and gas outburst disasters and accidents in coal mine on-site working face,effectively preventing dynamic disasters in working face and ensuring safe production.There are 59 pictures,13 tables and 110 references in this thesis.