Study On Top-coal Fracture And Drawing Law Of Longwall Top-coal Caving Mining Method In Thick Coal Seam

The estimated total reserves of the coal resources in China can be up to 5.06 trillion tons,and the thick coal seam reserves account for about 50%of the total reserves.Longwall top-coal caving?LTCC?mining,which is a core technology for mining the thick coal seam,had been introduced into our country in the 1980s.The fracture characteristics of the top-coal in fracture zone and the drawing law of the top-coal in the crushed zone are important for the top-coal recovery ratio,which is an important indicator in LTCC mining.Therefore,it is significant in theory and reality to reveal the top-coal fracture and drawing law in LTCC mining.To analyze the fracture characteristics of the top-coal in fracture zone,the stress intensity factor expressions for the tips of the offset parallel cracks in coal mass were derived based on the classic Kachanov Method,and the impacts of crack length,crack angle,vertical interval and level interval on the crack propagation were analyzed.In order to obtain the propagation modes of the offset parallel cracks under different loads,the RFPA^2D numerical simulation and uniaxial compression test were conducted,respectively.The initial stress represents that the crack begins to propagate,and the impacts of the level interval and vertical interval on the initial stress were studied in detail to verify the correctness of the theoretical analysis.The RFPA^2D numerical simulation was carried out to analyze the propagation law of the single crack in the coal mass under unloading condition,and the impacts of the crack angle,crack length and unloading rate on the initial stress were analyzed,and then the load carrying mechanism of the single crack under unloading condition was analyzed using the 2D sliding crack model.To research the drawing law of the top-coal in the crushed zone,the shape characteristics of coal particles moving boundary,the top-coal loss,the top-coal boundary and the drawing body were studied based on the stochastic medium theory.The theoretical model for calculating top-coal recovery ratio was established,by which the interrelationship between top-coal recovery ratio and rock mixed ratio was analyzed,and the criterion for terminating the drawing process was proposed.Taking the coal working face 8102 of the Tashan coal mine in Datong as the engineering background,the model experiment was carried out,by which the shapes of the coal particles moving boundary and the top-coal loss were obtained.Then the dynamic evolution of top-coal boundary and the interrelationship between the top-coal recovery ratio and rock mixed ratio were analyzed using the model experiment,and the experimental results were compared with theoretical analysis,which indicated that the theoretical analysis is correct.To further validate the theoretical analysis,the PFC^2D numerical simulation was conducted,by which the shapes of the coal particles moving boundary and top-coal loss were obtained,and the evolution processes of the top-coal boundary and drawing body were visually reproduced,and then the interrelationship between the top-coal recovery ratio and rock mixed ratio was analyzed.The major results obtained from the doctoral thesis are as follows:?1?The offset parallel cracks in coal mass exist reinforcing effect,shielding effect and null effect.When the level interval is large enough,the interaction between offset parallel cracks can be neglected.As the level interval decreases,the interaction between offset parallel cracks first presents reinforcing effect and then presents shielding effect.When the offset parallel cracks overlap with each other,the shielding effect and reinforcing effect both decrease with increasing vertical interval.When the offset parallel cracks do not overlap with each other,the inner and outer tips both present shielding effect,which first increases slightly and then decreases with increasing vertical interval.The shielding area and the reinforcing area both increase with increasing crack length.As the crack length changes,a certain fixed location may present reinforcing effect,shielding effect or null effect.?2?When the offset parallel cracks are subjected to uniaxial tension loading,there are 3 following characteristics:when the two offset parallel cracks overlap with each other,as the crack angle increases,the mode-I shielding effect exerted on the inner crack tip increases obviously,while the mode-II shielding effect decreases obviously.When the two offset parallel cracks do not overlap with each other,the mode-I shielding effect exerted on the inner crack tip increases slightly,while the mode-II shielding effect decreases slightly.The interaction between the two offset parallel cracks is the most violent when the crack angles are 60°and 30°.?3?The initiation and propagation of the single crack in coal mass under unloading condition are induced by the unload differential rebound deformation.The unload differential rebound deformation increases the tensile stress and the shear stress on the crack surface,decreasing the shearing resistance.When the crack angle is small,the location of initiation is on the crack,and as the crack angle increases,the location of initiation moves towards the crack tip.When the crack angle is large enough,the location of initiation stays at the crack tip.When the crack length is small enough,even the crack angle is small,the initiation may begin at the crack tip.When the crack length is large enough,even the crack angle is large,the initiation may begin at the middle of crack.Unlike during the uniaxial compression process,coal mass can not break completely along the crack during the unloading compression process.In this case,the crack propagation speed will reduce and even stop after extending to a certain length,and many wing micro-cracks are formed at the tip and other parts of the crack,decreasing rapidly the compressive strength.The final result is that the coal mass subjected to high axle pressure is crushed,which is accompanied by the phenomenon that the crack may be closed due to the high pressure.The crack with a small crack angle,small length,and fast unloading rate is more prone to initiate.?4?Aimed at the top-coal drawing process of the LTCC mining in thick coal seam,the equations of the coal particle movement,the coal particle discharged quantity,the coal particles moving boundary,the top-coal boundary and the drawing body were derived,and the theoretical model for calculating top-coal recovery ratio was established,which is of great significance for promoting the theoretical research on the LTCC mining.?5?The top-coal boundary in initial drawing process has the following characteristics:When the drawing volume is small,the lowest point of the top-coal boundary is close to the y-axis.As the drawing volume increases,the lowest point of the top-coal boundary moves towards the goaf,and the offset value increases,suggesting that the effect of the caving hydraulic support on the lowest coal particle movement become more and more obvious.When the drawing volume increases to a certain value,the offset value of the lowest point begins to decrease,suggesting that the effect of the caving hydraulic support on the coal particle movement decreases but the effect of the drawing window on the coal particle movement increases.During the whole initial drawing process,the offset value of the lowest point of top-coal boundary is very small,and the top-coal boundary is approximately symmetrical.?6?The top-coal boundary in normal drawing process cycles has the following characteristics:The slope of the tangent line of any point on the starting top-coal boundary is negative.As the drawing process continues,the top-coal boundary gradually concaves to the drawing window,and the concave point of the top-coal boundary is the vertex of the drawing body in theory.The straight line passing through the concave point of top-coal boundary and the midpoint of drawing window is defined as the center axis of the top-coal boundary,of which the slope is always positive during the whole drawing process.The center axis divides the top-coal boundary into two parts:the upper-left part and the bottom-right part.The slope of the tangent line of any point on the upper-left part is negative when the drawing volume is small.As the drawing volume increases,the slope of the tangent line of the lower point on the upper-left part transforms to positive value from negative value,and the upper-left part presents the tendency that the upper part is flat but the lower part is steep,i.e.the upper-left part is convex towards the goaf.The slope of the tangent line of any point on the bottom-right part mutates to be a positive value from a negative value and increases with increasing drawing volume,i.e.the bottom-right part gets steeper and steeper.When the drawing process is terminated,the coal surrounded by the starting top-coal boundary,the floor?x-axis?and the bottom-right part forms the top-coal loss,which is zonal distribution and tilts toward the goaf.?7?As the boundary effects of the shield beam and the drawing window,the drawing body is mainly composed of the top-coal above and behind the shield beam.For a certain cutting height-caving height ratio,when the drawing volume is small,the drawing body is an approximate sphere cut by the shield beam and its deflection angle is relatively large.The deflection angle decreases and the drawing body gradually changes into an approximate cut ellipsoid with increasing drawing volume.For a certain height of the drawing body,the deflection angle and the drawing volume decrease with increasing cutting height-caving height ratio,and the drawing body with a small cutting height-caving height ratio is more like an ellipse than that with a large cutting height-caving height ratio.?8?The top-coal recovery ratio increases non-linearly with increasing rock mixed ratio during the drawing process.The height of the drawing body,at the exact moment when the waste rock begins to be drawn out,decreases with increasing cutting height-caving height ratio,while the top-coal recovery ratio,at the exact moment when the waste rock begins to be drawn out,increases with increasing cutting height-caving height ratio.The concepts of the critical rock mixed ratio and critical top-coal recovery ratio were proposed.No matter what the cutting height-caving height ratio is in production practice,the rock mixed ratio of 10%-15%can be used as the criterion for terminating the drawing cycle.