Surface Cracks Distribution Characteristics And Development Mechanism In High-intensity Mining Of Nearly Shallow Thick Coal Seam

With the gradual depletion of resources in the eastern mining areas,the strategy of coal westward has become a reality.Surface cracks disaster caused by underground mining in the western mining areas have attracted more attention.In view of the characteristics of shallow depth,thick coal seam and high-intensity mining in western mining area and the surface cracks disaster induced by overburden strata failure breaking through the surface during coal mining,this paper took Daliuta Coal Mine in Shendong mine area as the engineering background,based on the method of field observation,similar simulation,numerical calculation and theoretical analysis,to analyze the law of surface subsidence,spatial distribution and dynamic development characteristics of surface cracks in nearly shallow thick coal seam mining in detail.Also,the formation mechanism and influencing factors of mining-induced surface cracks were systematically studied.The prediction and calculation models of different types of surface cracks were constructed.The main results were as follows:(1)According to the geological and mining conditions and the actual topographic conditions of the surface,the observation station for surface movement was established and carried out by RTK and three-dimensional laser scanning technology respectively.The surface subsidence characteristics in nearly shallow thick coal seam was studied,and the parameters of surface movement were obtained.(2)On the basis of real-time observation record and classification statistics of mining-induced surface cracks in western mining area,the classification method of surface cracks was put forward: according to the development form of surface cracks,they were divided into four types: stretching cracks,compression cracks,stepped cracks and collapsing cracks;according to whether the surface cracks were connected with the water flowing fracture zone of overburden strata,they were divided into two types: penetrating surface cracks and non-penetrating surface cracks.(3)Based on the investigation and analysis of a large number of surface cracks,the spatial distribution characteristics of surface surface cracks and the development process and development cycle time of different types of surface cracks were studied.(4)Similar simulation and numerical analysis method were used to analyze thebreaking movement of single "near-field thick key stratum".Because there was only one key stratum above the roof of the panel which was close to the coal seam,the overhang distance in the initial mining stage of the working face was large,resulting in a larger initial weighting interval of the panel.Later,the "near-field thick key stratum" broke stratifiedly and successively with the advance of the panel,forming the composite structure of the lower "stepped rock beam" and the upper "masonry beam".The roof formed the large and small periodic weighting behavior,in which the large periodic weighting interval was about 30 m and the small periodic weighting interval was about 15 m.(5)The development characteristics of mining fissures in overburden strata during different weighting periods of the panel and its influence on surface cracks were analyzed.During the first period of weighting,the movement of overburden strata was more intense,resulting in large cracks in overburden strata,of which some developed to the surface.Therefore,surface damage was so serious that large surface cracks occurred.After the second period of weighting,in front of the coal seam,it was not easy to develop mining fissures in overburden strata.The cracks in overburden strata developed from the surface down experienced the process of "enlargement-slow closure-non-compaction",and the cracks in overburden strata developed from the bottom up experienced the process of "slow closure-non-compaction".After mining,the cracks in the overburden strata were fully developed,and it was not easy to close and disappear with the advance of the panel.(6)The displacement contour of overburden strata almost expanded forward in a straight line with the failure movement of key stratum until it reached the surface.When the lower part of key stratum was broken while the upper was not broken,the displacement isoline of overburden strata near the coal seam extended very little forward,resulting in non-penetrating surface cracks developed from the surface down.When the whole key stratum was broken at the same time,the displacement isoline of overburden strata rapidly expands forward,and the cracks in overburden strata will develop to the surface,resulting in penetrating surface cracks.(7)The trinity structure model of "Underground-Overburden-Surface" was established to analyze the formation mechanism,influencing factors and distribution ofdifferent types of surface cracks,and the prediction and calculation models of different types of surface cracks were put forward.According to field observation and lab experiment,the formation conditions and development positions of different types of surface cracks were analyzed,the development areas are divided.The relationship between lag distance and development height of the surface cracks and mining speed,surface subsidence and strain,respectively.At last,the prediction and calculation models of different types of surface cracks were put forward.