Mechanism And Control On Mining Induced Influences Around Longwall Top Coal Caving Face In Extra Seam With Complex Structures

Violent mining induced responses and difficult stability control is always seen in the surrounding rock in the practice in longwall top coal caving face(LTCCF) with extra thick coal seam in Datong Coal Mine Group. This doctoral dissertation, comprehensive methods including field observations, theory analyses, numerical simulations, field practice, etc., are employed to study the mechanism and control on mining induced responses systematically in the following aspects, intensive spalling of high coal wall in LTCCF, poor broken of top coal with strong rock partings, maintenance headaches even instability of roadways and chain pillar under the hard roof conditions. The following innovations can obtain:(1) The goaf compaction theory was proposed to describe the compaction characteristic of caving zone in LTCCF with extra thick coal seam. A numerical algorithm and program was developed to express compaction process of the caving zone. Response characteristics, such as plastic zones and strength degradation of coal around longwall face, stress recover in goaf, stress distribution, roof rupture up goaf side, top coal broken in LTCCF, were analyzed by the influence of coal thickness, mining height in the face, width of longall face and thick hard roof strata. The influence mechanism for the surrounding rock by great range, long time and intensive disturbance, was revealed in LTCCF with extra thick seam.(2) A mechanical model was established to analyze the coal wall stress in LTCCF in extra thick coal seam, so stress and deformation distribution in coal wall can obtain. Then coal wall deformation was analyzed under the effect of shield support resistance, face gard stress, main roof breaking location, mining height in face, etc. The failure mechanism of coal wall was expounded, tensile and compression-shear forms. Three spalling shapes were concluded on the basis of field observations. The failure evolution was analyzed in intact and crack coal wall. For the contact coal wall, large block sliding from coal wall was induced by failure localization, dominantly. For crack coal wall, cracks induced two spalling patterns, entire coal wall spalling with large blocks and coal wall spalling in crash pattern. The coal wall stability influenced by support resistance, parameters of face gard, stopping time of mining was expounded. Some targeted control techniques to increase the coal wall stability were put forward.(3) Theoretical analysis, physical simulation and numerical simulation were comprehensively employed to analyzed top coal crash affected by hard rock partings parameters like thickness, number of layers and locations. Three structures: two ends simply supported, one end fixed and the other simply supported, and one end clamped and the other free, were proposed formed by rock beams which composes of upper thick rock parting and top coal. The equilibrium conditions and caving evolution characteristics of these three structures were analyzed theoretically. Hanging arch of rock parting and top coal forms easily, and because of the hanging structures, top coal above structures were restricted in deformation, broken and migration. The material and its match of physical simulation were determined to exactly present the whole process of top coal: deformation→rupture→crash→caving. A physical support for top coal caving simulation was developed with the same function of real shield support. Rock partings crash was modeled numerically by the influenced by long hole presplitting blasting parameters. Long hole presplitting blasting scheme was designed to fracture the hard rock parting, and this scheme was employed successfully in the field.(4) Aiming at chain pillar instability under the thick hard roof strata condition, a comprehensive model for evaluating pillar stability was proposed, and evaluation indexes in this model contained the whole rigidity of pillar, elastic core width in chain pillar and width of prone rid zone. Also, judge criterion of these three indexes was determined. Then, chain pillar stability was classified. The instability mechanism of slender pillar large rib then inducing large deformation of roadway was revealed. Bearing capability affected by pillar width sizes was analyzed and suitable size of coal pillar was determined. Stress concentration regularities were studied under the influence of roof strata structures above goaf side. On the basis of these work, a surrounding rock control approach, optimizing coal pillar stress, was proposed, executing hard roof strata rupture using hydraulic fracturing around roadways.The dissertation contains 157 figures, 16 tables, and 218 references.