Study On Fracture Mechanism Of Surface Borehole And Inversion Of Overburden Movement In Response To Underground Mining

In view of the application of the current borehole camera technology in the exploration of overburden movement,there is a lack of information utilization and incomplete interpretation about the deformation and failure of the borehole,especially the dynamic evolution process of the borehole fracture in response to underground mining disturbance and the indistinct mechanism of the connection with the overburden movement.The relationship between the characteristics of the surface borehole fracture and overburden movement is studied by means of theoretical analysis,numerical simulation,physical simulation and field measurement,and the systematic research on the mechanism of borehole failure and the inversion of overburden movement is carried out.The main achievements are as follows:(1)Typical fracture characteristics and temporal and spatial evolution process of surface borehole are revealed.Under the conditions of different geological and mining technology,the typical fracture characteristics of borehole in long wall mining area mainly include four forms: fracture,dislocation,separation and collapse.The spatio-temporal evolution process of borehole deformation and failure in response to the underground mining is fracture development,fracture expansion,dislocation,separation,collapse and stoppage.(2)Fracture mechanism of surface borehole in response to the underground mining is revealed.Based on the Mohr Kulun criterion,laminated plates and elastic thin plate structure,mechanical models are established for the fracture,dislocation and collapse of the borehole wall under the mining stress.Shear stress exceeds the shear strength of surrounding rock,the hole wall fracture and dislocation.There are two forms of the hole wall fissure with circumferential fissure and the symmetrical distribution of the vertical fissure.The distribution of dislocation displacement after sufficient mining is "external dislocation-internal dislocation-none dislocation-internal dislocation-external dislocation ",and the collapse is caused by the rotary instability(tensile stress exceeding tensile strength)and shear instability(shear stress exceeding shear strength).(3)Response characteristics of borehole fracture and overburden move are revealed.With the advancing of working face,the vertical displacement of borehole increases monotonously,and the direction of horizontal displacement fluctuates,and the amplitude of increase and decrease is different.There is no obvious regularity.The failure forms of borehole wall in the caving zone is mainly fracture,dislocation and collapse,in the fracture zone is mainly fracture,dislocation and separation,in the bend subsidence belt mainly produces crack and fault.(4)Inversion relationship between surface borehole fracture and overburden movement is established.A key index and criterion based on the horizontal movement(u),horizontal deformation(?)and separation(s)in the inversion of the overburden movement is proposed.When the borehole is first dislocated and then collapses,and ?u?/h > 1.2%,which is the criterion for the lower limit of the fracture zone;?u?/h < 0.5%,s/h < 10%,???/h < 2.1,which are the criterion for the upper limit of the fracture zone.The value of discriminant index decreases linearly or logarithmically with the increase of the buried depth of the coal seam,decreases logarithmically with the increase of the thickness of the key layer,and does not change with the increase of mining height basically.(5)Field test results show that the failure characteristics of surface borehole can effectively inverse the law of overburden movement,and the inversion results are roughly same with the measured results,which confirms the inverse relationship between the borehole failure and overburden movement,and further enriches and expands the function and application scope of the peep hole information.It has a important reference value and reference value for water conservation mining,gas extraction drilling design,separation and filling drilling layout,mine water prediction and prevention and control.