Characteristics Of "Two-zone" And Formation Mechanism Of Overburden Due To High-intensity Mining Of Thick Coal Seam

The "three-zone" mode caused by longwall mining in northwest mining area with high-intensity mining of thick coal seam is different from that in traditional geological mining conditions.Sometimes the fractured zone caused by mining is directly developed to the ground surface,or connected with the surface crack directly,forming only the "two-zone" failure characteristics.Based on the mining geological characteristics of high-intensity mining in typical thick coal seam,the methods of physical simulation,numerical simulation,theoretical analysis and field measurement were adopted in this paper.The failure characteristics and formation mechanism of "two-zone" of overburden in high-intensity mining of thick coal seam were studied systematically,and the following research results were obtained:1.Through physical simulation,it was found that the failure of "cantilever beam" of the low key strata of in working face would cause the instability of overlying "voussoir beam" structure and increase the failure height of overlying strata.The evolution process of "two-zone" of overburden through four stages: the gestation stage,formation stage,transformation stage and the stable stage.In the transformation stage,the fractured zone was transformed into caving zone,and the height of the caving zone increased significantly.2.A structural model of overburden pressure arch was proposed and its structural characteristics were analyzed.The key strata of overlying rock in could inhibit the development of pressure arch.When the keystrata was not break,the top ofpressure archwas located near the key strata;on the contrary,the pressure arch would re-arched at the fault position of the key strata.3.Based on the characteristics of high-intensity mining in thick seam,the influence of mining height,advancing speed,mining depth and overburden structure on the "two zones" of overburden is analyzed.The sensitivity of the main control factors from large to small was analyzed by orthogonal test,which was as follows: mining height,overburden structure,mining depth and advancing speed.4.According to a large number of engineering practice,there were three failure types of overburden "two zones" : overburden "two zones" directly through the surface,"two zones" connected with ground fissures and overlying strata "three zones" failure type.Based on the key stratum theory,voussoir beam theory,breaking law of hard and soft rock strata,a method for calculating the height of water conducting fracture zone was proposed..The prediction model of "two zones" failure type of overlying stratawas put forward.5.The formation conditions of the "two zones" failure mode of overburden were analyzed,and the mechanics and cusp catastrophe model of overburden shear collapse were established.The sudden instability of “cantilever-voussoir” beam system led to the shear of cantilever beam and the instability of voussoir beam structure.The occurrence of overburden shear collapse disaster formed the failure type of overlying rock "two zones" straight through the surface.6.The key strata of overburden formed bearing structure without sliding instability,bedrock fissures and surface fractures connected,forming the failure type of overburden "two-zone" fissures connected surface fissures.After mining,the surface tension cracksconnected surface fissures,and the failure types of overburden "two-zone" fissures connected surface fissures were formed.7.Based on the 11915 working face of Gequan east Mine,this paper described in detail the scheme,the composition of detection equipment,the detection method,the problems easily occurring in the detection process when the borehole method was used to detect the height of the "two-zone" and the solutions,which provided a detailed reference for the field detection of the "two-zone".