Research On Mechanism And Application Of Roof Cutting And Support Crushing In Fully-Mechanized Caving Face

Fully mechanized caving mining is the main coal mining method for thick and extra-thick coal seams in China.The thickness of the fully mechanized caving mining is more larger,and the active space of the overlying strata is more larger,which is more prone to roof accidents.At present,the researchers have carried out more research on the large-scale sudden collapse of the hard roof and support crushing,and the hard roof disaster has been effectively controlled.However,the research on the mechanism of non-hard roof cutting and support crushing is still very rare.The accident of large-area roof cutting and support crushing frequently occurs.Therefore,the study on the breaking law of overlying strata movement,the mechanism of roof cutting and support crushing and the prevention technology of non-hard roof has important engineering and scientific significance.Under the support of the national key science and technology research and development project(2017YFC0804301)and the National Natural Science Foundation(51504136),this paper conducts similar material model test on the movement and breaking rules of the roof and overburden during the advancement of the fully mechanized caving mining face.The mechanical mechanism of the roof cutting and support crushing is resaearched,and the dynamic model of the roof rock layer is established.Based on the measured ore pressure data,the evolution mode and law of the working resistance of the bracket are analyzed.The roof moving bracket resistance increasing model and the roof cutting pressing method are proposed to invert the roof activity law.Combined with engineering examples and the above research,the comprehensive prevention and control technology for large-area press-fitting accidents in fully mechanized caving face is proposed.Through the research,this paper has mainly achieved the following conclusions:(1)It is found that the roof of No.3 coal seam in Cuimu Coal Mine is non-hard.The large-area roof cutting and support crushing of the 301 and 302 fully mechanized caving face is a relatively slow process.During the process of support crushing,there is no “air wave”,and the bracket is not subjected to shock load.This is essentially different from the hard roof large-area roof disaster.The F-T curve of the bracket is characterized by the logarithmic function and exponential function during the normal production period,and the F-T curve during the maintenanceperiod and the long-term shutdown period all conform to the logarithmic function.At the same time,the F-T curve that meets the logarithmic function during normal production and the F-T curve during the maintenance period and long-term shutdown period satisfy the generalized Kelvin model,which indicates that the F-T curve of the fully mechanized caving face in Cuimu Coal Mine is affected by time and has a strong time effect.According to the analysis,the F-T curve of the scaffold can be used to evaluate the active state of the roof in working face and the working resistance of the scaffold,and the quantitative analysis and evaluation of the cause of the working face can be realized.(2)Through similar simulations,it is found that the roof fracture of the fully mechanized caving face may occur behind the bracket,at the coal wall and in front of the coal wall.For the non-hard roof in fully mechanized caving face,the roof is not easy to break in front of the coal wall,and the fracture is more likely to occur above the coal wall and behind the bracket.If the roof breakage occurs behind the bracket,the roof cutting and support crushing will not occur normally;the fracture of the roof plate at the coal wall may cause the roof cutting and support crushing,and the working face with lower bracket rigidity,the roof is more likely to break at the coal wall.(3)Theoretical analysis shows that the stiffness of the bracket has a great influence on the bending moment and sinking amount of the roof.The greater the stiffness of the bracket,the smaller the bending moment and the sinking amount of the roof,and the more difficult the large-area cutting press is.The experimental analysis shows that the average linear stiffness of the four supports is137.8~538kN/mm,and the average stiffness is 20.3~68.5MPa/m.It is inferred that the current support line stiffness range is about 100~600 kN/mm,and the stiffness range is about 10~80MPa/m.(4)Based on the elastic foundation model and the cantilever beam model,the mechanical criterion of the fracture(in front of the coal wall or at the coal wall)and the instability of the roof(slipping instability and rotation instability)are analyzed,and the corresponding critical support strength calculation formula is proposed.Theoretical analysis shows that for non-hard roof,due to the low tensile strength and shear strength,the thickness of themain roof follower layer is large,the roof is more likely to break;the compressive strength is small,the friction coefficient is small,thickness of themain roof follower layer is large,the fractured roof is more prone toinstability.These factors are the main factors that cause large-area roof cutting and support crushing with non-hard roof.(5)It is considered that the 302 fully mechanized caving face is prone to the main roof front fracture(the critical support stiffness is 45 MPa/m,whih is greater than the ZYF10500/21/38 bracket stiffness 27 MPa/m);the main roof is easy to tensile failure above the coal wall(The critical support strength is 0.69 MPa,which is greater than the actual initial support strength);the sliding instability is easy to happn(the critical support strength of the slip instability is 2.58 Mpa,whic is greater than the limit support strength of the support 1.13MPa),which indicates that if the roof is broken,the fracture roof will be sliding instable,and in turn will cause the safety valve to open and roof cutting and support crushing.(6)Aiming at the actual conditions of the 303 working face of Cuimu Coal Mine,it is proposed to select reasonable brackets,apply enough initial supporting force to ensure the balance of the front and rear columns and the pressure of the column,reasonable working face advance speed,and accurate warning index of roof cutting and support crushing,and emergency plan,forme complete set of technical measures for the prevention and control of roof cutting and support crushing disater in fully mechanized caving face,which is applied in the 303 working face to ensure the safety mining.