Tip-to-face Roof Stability Based Shield-strata Interactions In Top Coal Caving Face

Research on the relationship of shield and surrounding rock is the main content of underground coal mine ground control theory.In this dissertation,the tip-to-face roof stability is taken into consideration with the research of the relationship between shield and surrounding rock in top coal caving face.First,the typical tip-to-face roof instability phenomenon of scene is statistically analysed and the effect forms of tip-to-face roof stability is clarified.By the use of mathematical evaluation method,the defined effect factors are classificated and sorted.Then,the overlying rock mass instability and filure mechanism is analysied by movement mechanics and the research engineering scale is selected.Last,by the use of numerical and physical simulation method,the effect factors of tip-to-face stability are studied.The optimzated roof control measures of top coal caving face has been proposed.The specific research conclusions as below:(1)The tip-to-face roof instability and the relationship of shield and surrounding rock interact with each other.The imbalance of shield and surrounding rock system may induce the collapse of tip-to-face roof,and if the control measures are not in time,the situation will be worse off.(2)At the point of active roof control,18 effect factors of tip-to-face roof stability are proposed,they are:(1)coal seam occurrence condition,include coal strength,primary fissures,feological structure,coal seam inclination,buried depth and thickness;(2)Artificial controllable factors,include canopy inclination,shield setting load,tip-toface distance,shield's front support power,length of workface and shearer cutting height;(3)Uncontrollable factors,include shield stiffness,coal wall slide,unbalance loading of canopy,overlying rook mass instability and abutment pressure.With the use of analytic hierarchy process,the factors weight and sorting are clarified,as Tab.2.13 shows.(3)Based on the scientific mining level,thick coal resvers and existing capacity,the engineering background is identified as the top coal caving face in Lu'an and Jincheng mining area which main coal product is No.3 of Shanxi formation of Permian.With the parameter comparison of top coal caving shield and heigh cutting shield,the study of top coal caving shield's longer canopy loading distrubtion is main part of the relationship of shield and surrounding rock system.(4)The overlying rock mass balance structure model is built for the research and the instability filure mechanism of basic roof block is calculated by movement mechanics.The defined engineering scale is-the shield and coal wall combined support system and top coal.(5)Combined with top coal reload paths and its envolution rules,the feature of tip-to-face roof instability is analysed and its filure structure is a kind of triangle which the two bases are separately located on shield front roof and upper coal wall and its center of gravity is inclined to coal wall.Based ong the top coal's fissure characterisrics,the top coal are divided into four zones,which are top coal caving zone,canopy reloading zone,tip to face roof zone and fracture zone.(6)The shield and surrounding roof interaction rules are analysed by the use of physical simulation method.A high-fidelity shield model is invented and depend on the all time canopy load monitoring system,top coal is crushed by overlying rock mass and shield support.In 2D plane,the stress state of coal block is turned to uniaxial compression from bidirectional state which induced the frcture open.(7)With the load distribution monitoring on the total length of canopy,it is found that the load is bigger in the front of the canopy generally.It is because of the crushed coal block at the back of canopy can not transfer the upper load.The discrepancy of shield support force and load distrubtion reduced the shield's control efficiency and also influenced the canopy support degree.(8)With the simlution of tip-to-face roof fall,it is found that layered roof fall characteristic is more obvious in low cohesion roof mass.The tip-to-face distance is one of the main factors of the stability,and the shield's front canopy only working as shelter of roof fall and inhibition of free surface.(9)It can be concluded from the variable analysis of support facors that the tipto-face roof instability destabilizing the shield and coal wall support system.Once the tipto-face roof falls,the original combined support system is segmented to two mutually independent part.(10)When the factors are analysed,the founded rules include:(1)the canopy pitching angle inhibited the top coal's horizontal displacement;(2)the setting load of shield has threshold value to keep roof stable,even the setting load is much bigger than the value,the efficiency of roof control is not obviously;(3)a short tip-to-face distance is good at roof control,however the price is the increased load and increased roof displacement on the canopy;(4)the front support force is more efficiency than other factors in the control of upper roof even it may induce more serious unbalance loading of canopy;(5)the shearer cutting height influence to the stability of top coal overlying the canopy is much more obviously than tip-to-face roof.