Study On Coal Caving Law Of Fully Mechanized Caving Mining In Ultra-thick Coal Seam Under Different Overburden Conditions

This paper takes the Buliangou ultra-thick coal seam fully mechanized caving face as the engineering background to carry out relevant researches on coal caving laws under different overlying rock conditions.Firstly,through theoretical analysis,the formation structure and movement characteristics of the overburden rock are determined,the bulk density changes during the top coal caving process,combined with the Bergmark-Roose movement model of the top coal particles,the top coal caving law is theoretically analyzed,and the uniformly varying density function is established.Body model.Secondly,with the help of numerical simulation of different mechanical contact models,the influence of broken rock layer,layered overburden and layered overburden with broken layer on coal caving law is further studied.It reveals the internal connection of coal caving laws under different overburden conditions,and proposes a quantitative model for the average top coal recovery rate of the full face under the condition of broken rock layers.The cantilever structure in the layered overburden has the strongest effect on restraining the top coal caving,followed by the hinged structure.The broken layer in the layered overburden can cushion this inhibition.Finally,based on the research results,technical measures for hydraulic fracturing to weaken the roof and increase the thickness of the broken rock layer are proposed,which not only helps to weaken the strength of the working face,but also can effectively improve the top coal recovery rate.The main research results of the thesis are as follows:(1)Establish a discharge volume model of the uniform density function that reacts to the change of the bulk density during the discharge of crushed top coal particles.Constrained by the particle moving boundary and support in the goaf,the top coal accumulation density is positively correlated with the distance from the coal caving opening.At the same time,the "cantilever-articulated" structure formed by the fracture of multiple hard rock layers in the layered overburden further changes the packing density of the top coal during the caving process.The distance from the coal caving port is positively correlated with the caving section of the stacked top coal.A density change function with the caving section radius as the independent variable is established,combined with the Bergmark-Roose bulk movement model,and a uniform density function discharge volume model is established.The uniform density model reveals that the lateral expansion area of the top coal discharge body is larger than the radial expansion area during the development process.Compared with the traditional constant density discharge body model,the uniform density model has an increased lateral expansion rate of 37.14%;the radial direction is compressed State,the compression increase rate is 27.27%.The essence of this change is the function of the change of the distance between the piled broken top coal and the caving mouth,thus changing the shape of the caving body.The introduction of the uniform density discharge body model can more truly reflect the top coal discharge rule during the discharge process.(2)Under the condition of broken rock layer,the thickness of the layer,the particle size of the rock block and the friction coefficient are in a quadratic function relationship with the amount of top coal released.However,the opening direction of the quadratic function is different,that is,there are maximum and minimum values that affect the amount of top coal discharge.When the thickness of the broken rock layer and the thickness of the top coal are 1:1,the initial coal caving amount,periodic coal caving amount and recovery rate of the top coal are the largest.The particle size ratio of broken coal is in the range of 1:1.6?1.7,and the initial amount of top coal caving,the amount of cyclical caving and the recovery rate are the smallest;when the ratio of friction coefficient of broken coal is 1:4.2,the initial caving of top coal is The coal volume,cycle coal caving volume and recovery rate are the largest.(3)The height of the top coal caving body under broken rock formations shows different changes with the advancement of the working face,but the average caving body height in the periodic coal caving cycle is greater than 1 times the height of the support and less than 2 times the height of the support.The release body is similar to the combined morphological characteristics of "lower triangle-upper partial circle".The height of the discharge body of the initial coal caving cycle is approximately equal to the thickness of the coal seam,and the shape of the discharge body presents the characteristics of "lower triangle-upper part ellipse".Therefore,a quantitative model of the top coal recovery rate of the full face under the condition of broken rock layers is established.(4)The law of coal caving under the condition of layered overburden and layered overburden with broken rock layers was compared and studied.Under the condition of layered overburden,the embedment of broken rock blocks in the irregularly collapsed rock strata has been the development of the top coal crow body shape,so that the crow body shape in each coal caving cycle is "triangular" and "triangular-partial circle".And "triangle-partial ellipse" are randomly formed.The instability of the cantilever structure increases the strength chain in the top coal area to be caving at the upper part of the support tail beam and shifts to the support tail beam,which changes the trend of weakening of the top coal force chain in the area to be caving with the top coal caving in the traditional research.The increase of the strong chain restrains the moving trend of the broken top coal to the caving opening,and the embedding of broken rock blocks changes the coal-rock boundary line,so that the top coal caving amount is drastically reduced.The instability of the articulated structure increases the strength chain in the top coal area to be caving at the upper part of the tail beam of the support,but the increase is less than the instability of the cantilever structure,and the rock blocks in the regular caving zone at the lower part form a squeeze arch,which makes the top coal discharge amount although The reduction is smaller than the instability of the cantilever structure.The instability of the cantilever structure has the most significant impact on the top coal recovery rate.The existence of the broken layer can effectively weaken the influence of the overlying rock movement on the top coal caving process,thereby increasing the top coal recovery rate.(5)The influence of different overburden conditions on the top coal caving law is ultimately reflected in the top coal caving body shape of the periodic caving cycle,which has little control effect on the top coal caving body shape in the initial coal caving stage.(6)Through on-site monitoring and analysis,the recovery rate of top coal decreases when the "large-small" cycle of fully mechanized caving face in extra-thick coal seams comes down.The use of hydraulic fracturing technology to weaken the roof technology can not only reduce the strength of the working face,but also weaken the influence of the overburden movement on the top coal caving law.Comparative analysis shows that the weakened top coal recovery rate has increased by 15.08%.