Development Laws And Control Of Overlying Strata Water Flowing Fractures In Repeated Disturbance Zone Of Shallowly-buried Short-distance Coal Seams

In the mining area of northwest China, the first main mineable coal seam has been mined out, and the senond main mineable coal seam will be excavated in large scale. Aiming at this current situation, the comprehensive study methods such as field research, analog simulation, numerical simulation, theoretical analysis and field measurement are adopted to analyze and research systematically the control and development law of overlying strata water flowing fractures in repeated disturbance zone of shallowly-buried short-distance coal seams. These research results can be used as a reference for water resource conservation mining in shallowly-buried short-distance coal seams.Main deliverables of this dissertation have been displayed as follows:(1) The mining of the lower coal seam of short-distance coal seams affects secondary development of water flowing fratures of the upper coal seam under the condition of shallowly-buried deep. Considering this, the shallowly-buried short-distance coal seams and repeatedly disturbed zone are definited. In addition, surveying the eological condition of shallowly-buried short-distance coal seams in the partial coal field of Shendong mining area, based on the classification method of the single shallow buried coal seam, and considerating the water swelling of aquiclude, the shallowly-buried short-distance coal seams is classified into three types, and in which the type Ⅰ and type Ⅱ are the main research objects.(2) The similar materials of non-hydrophilic aquifuge are invented. Through the solid and solid-liquid coupling physical simulation experiments, in combination with solid-liquid coupling numerical simulation experiments, and introducing the coal-rock infrared detection technique into physical simulation of water flowing fractures experiment, the development laws of water flowing fractures of overlying rock in repeated disturbance region have been maken clear. In the process of the lower coal seam mining,the water flowing fractures, which have been compacted steadily after the upper coal seam been mined in overburden rocks, will regenerate with different levels in different mining regions. The fractures’ development and its water swelling in mining-induced water-resisting layer is synchronised to each other. Fractures in the mining boundary of water-resisting layer can heal under the influence of water swelling while mining the upper coal seam. By contrast, fractures in lower coal seam cannot heal due to the lack of water swelling capacity. Fractures in mid goaf water-resisting layer can heal under the mutual function of rock rotary extrusion and the swelling of water-resisting layer.(3) The overlying rock mechanics model is built, and the relation of extremity breaking distance of rock breakage or tensile failure and its beneath sinking space have been analyzed. On the basis of defining the no-load lateral restraint expansion elongation and the equivalent fracture width of aquiclude, and establishing the mathematical simulation between the fracture width of aquifuge and aquifuge’s maximum subsidence, the open-close development mechanism of overlying aquiclude fractures has been analyzed, and the development mechanisim of water flowing fractures of overlying rock in repeated disturbance region has been revealed.(4) Based on the overlying strata movement characteristics of the close shallow burried coal seam, and considering the strata movement angle and the full-mining angle, the reasonable range of the lower coal seam’s internal and external critical displacement distance that can prevent the development of water flowing fractures has been given, the discriminated and computering methods of the critical layer production ratio that guarantee the overlying rock aquiclude will not produce the water flowing fractures under the condition of the internal displacement layout has been determined. Moreover, the methods of remove, transfer and local processing for vulnerable water resources loss area, including ‘One divides into two, Narrow section coal pillar, Interval concurrent mining’ and mining boundary step stype filling method, which can help to accomplish water-preserved mining, are proposed. Working faces layout in mining district or panel has been designed and the applicable condition classification for water-preserved mining in shallow buried short-distance coal seams has been finished.