Evolution Of Master Fractures And Water Inrush Forecast For Fully Mechanized Top-coal Caving Mining Of Extremely Thick Coal Seam

The overburden of Jurassic coal bed in Huanglong coal mine area is characterized by short rock-forming age,weak cementation and high shale content.The water-conducting fractured zone is well developed and high-level horizontal separated fractured spaces are easily generated at the interface between soft and hard rock strata.If there would be aquifers around the separated fractures,these spaces will be filled up with water.The water inrush induced by seperation water is characterized by instantaneity,large flow rate and along with the appearance of mine pressure,which has a major impact on the safety of coal mines.Based on the experience of periodical seperation water inrush of Cuimu coal mine,the seperation position and interval of roofing breaking was studied in this paper through theoretical analysis,similar material simulation and numerical simulation.In addition,this study put forward three types of seperation water inrush and predicted dangerous segments of 1307 working face.Through standard calculation,similar conditions mine analogy and numerical simulation,the height of water flowing fractured zone is determined as 280 m and the average affected thickness of Yijun formation is 44.91 m in 1307 working face.The height of the water-guiding fracture zone in the 1307 working face is 280 m,and the average thickness of the Yijun Formation is 44.91 m.It is determined by standard calculations,analogous conditions,mine analogies,and numerical simulations.According to the Key Strata theory,the position of horizontal separated fractures,which mainly occurred at the interface between Anding formation and Yijun formation and the upper Anding formation,was discriminated.Based on the fracture theory of the plate,the ultimate breaking distance and the periodic breaking distance of the inferior key strara layer in the lower part of the seperation were obtained.The general discrete element program 3DEC was used to study the development and dynamics of the overburden seperation during the mining of the working face.The results of this study showed that there was a clear separation between the upper Anding formation and the lower Yijun formation for mined-out lengths of 400 m.The maximum displacement of mining rock stratum,with a “bell-shape” symmetrical distribution,occurred in the upper middle of the goaf.The fracture angles of the rock formations on the left and right sides are basically similar.A “fracture-rich arch” distributed in the overburden nearby coal seam.The fractures developed obviously in both sides of the goaf,while there is a compaction zone in the middle of goaf.The results of physical simulation tests show that the obvious separation occured at the interface between Yijun formation and the Anding formation for mined-out lengths of 390 m.The fracture angles on both sides are not same due to the inclined mining,which caused more obvious seperation in the cut-hole.Different from horizontal mining,the largest subsidence position of the strata is close to the cut-hole.During the entire mining process,transverse fractures always occurred before longitudinal fractures,longitudinal fractures doesn’t appear until the transverse fractures seperated to a sufficient length,then the entire strata breaked in the middle and created more longitudinal fractures.As a result,three types of water inrush controlled by the master fractures(horizontal separated fractures and vertical main water-conducting fractures)were proposed.They are water inrush without high-level transverse seperation,water inrush with vertical main water-conducting fractures,and water inrush with master fractures.Considering the maximum development height of water flowing fractured zone,seperation position and breaking law of inferior key strata,it is noted that vertical main water flowing fractures are potentially to interconnect with upper horizontal separated water body so that the qualification of water inrush with master fractures is relatively satisfied.Considering the breaking distance of the sub key strata and different fracture angles along working face strike,the dangerous segments prediction of seperation water inrush were delimited.