Principle And Application Of Rockburst Control By Weakening Static And Dynamic Loading Using Top-coal Caving In The Mining Of Thick Coal Seams

Rockburst is one of the characteristic coal-rock dynamic disasters. It occurs in coal seams with various thickness, among which rockbursts occurring in thick coal seams reach a majority and the induced damage is severer. Hence, rockburst control in the mining of thick coal seams is an urgent problem in need of solution. Currently, in situ application has been put into practice to reduce rockbursts occurring in thick coal seams by using fully mechanized top-coal caving mining in our country. Although certain positive effect has been achieved, rockbursts occurring in thick coal seams have not been eliminated. Furthermore, as a basic scientific problem, principle of rockburst control with top-coal caving mining lacks of systematical study. To solve this problem, a study aiming at rockburst control in thick coal seam mining was carried out in this work, by means of in situ and literature investigation, theoretical analysis, laboratory experiment, numerical simulation, and engineering practice etc.An investigation was firstly conducted on rockburst characteristics and factors in the fully mechanized top-coal caving mining and fully mechanized mining of thick coal seams, which was aimed at providing a basis for studying the mechanism and control of rockburst. Results show that when comparing with fully mechanized mining, in top-coal caving mining rockburts occurring in coalfaces take a much smaller proportion, damage is less disastrous, and damage ranges are shorter. Additionally, damage locations in roadways generally start at the face line in fully mechanized mining, whereas they mostly start at a certain distance ahead of the face line in top-coal caving mining. Both in the two mining methods, gob-side rockbursts take a large majority and damage is mainly characteriszed by floor heave. Differences of rockburst factors bewteen the two mining methods lie mainly in the mining height and roadway layout.Experimental study was conducted on coal specimen burst failure with over-strength loading. It was revealed the over-strength loading effect and mode of surrounding rock imposed on coal seam, and corresponding model and criterion of rockburst was established. Experimental results show that the testing machine can impose over-strength loading on the specimen under conditions including the machine stiffness is small, force is chosen as the programmed control variable, and displacement is chosen as the programmed control variable with high loading rate. Surrounding rock can impose over-strength loading on the coal seam, leading to the dynamic failure of coal mass. The over-strength loading contains three modes: the strain softening behavior of coal mass develops to the state that surrounding rock stiffness is smaller than coal mass stiffness, overburden fracture or local failure generates seismic propagation leading to instantaneous dynamic disturbance imposed on coal mass, and mining or local failure causes instantaneous drop of confining stress of coal mass leading to instantaneous drop of coal mass strength. Coal mass is easier to fail dynamically under conditions including higher abutment stress, higher stress gradient, steeper coal strength drop, stronger dynamic disturbance, and smaller ranges of cracked zone etc.It was studied and revealed characteristics of overburden structure and its influence on the over-strength loading imposed on coal seam in fully mechanized top-coal caving mining. Lower key strata fracture and formed structure are in the form of voussoir beam and cantilever beam in top-coal caving mining, whereas they are in the form of voussoir beam in fully mechanized mining. Ratios of fracture span(and released energy) of strata between voussoir beam and cantilever beam are(0.632~0.577)/1(and(0.860~0.813)/1, respectively). The fracture span and released energy increase with the increase of mining height. Hence, the key strata fracture-induced dynamic load source is more intense in top-coal caving mining. However, the dynamic load attenuates rapidly along with the propagation and the weakening effect of overburden structure on static stress concertration is obvious in top-coal caving mining, leading to a small stress concertration degree, a large stress concertration range, a far distance of stress peak from the coal wall. Numerical results show that the stress concentration factor drops from 1.97~2.04 to 1.44~1.60 when the mining height rises from 3 m to 20 m. Therefore, it is not easy for surrounding rock to impose over-strength loading on coal seam in the top-coal caving mining of thick coal seams, and thus coal mass is not easy to achieve dynamic failure when compared with fully mechanized slice mining.It was proposed the principle of rockburst control in thick coal seam mining, which states that top-coal caving should be adopted for rockburst control in the mining of thick coal seams. Stress concentraton is reduced by increasing the mining height within a reasonable up limit which can be 11~13 m by numerical results. Additionally, dynamic disturbance is weakened by taking roof pre-splitting measures which are aimed at eliminating tremors with energy higher than 104 J. Formulae were given for the determination of pre-splitting interval. Research findings in this work were put into application in Xing’an Coalmine and achieved remarkable effect.