Experimental study on “horizontal U-Y” periodical breakage characteristics of key strata in stope roof

In response to the mining of the water-carrying fracture zone in some hard roof mines and the horizontal “O-X” breakage and movement in the high-level key strata at the height of 8 to 15 times the mining height above the stope in the mining-induced water flowing fracture zone， which caused some dynamic disasters such as dynamic weighting and support crushing in the stope， floor heave happening 100 m to 200 m ahead of working face in the roadway adjacent to gob.The self-made large-scale three-dimensional physical simulation experiment platform was used to study the characteristics of breakage and movement of high-level key strata.The results show that when the key layer is broken horizontally， the long sides of the front side of the key layer and the middle axis direction of the reverse side form a crack at the same time.Then， the curved fracture formed in the two short sides on the front surface simultaneously， and separated fractures from a junction formed in the end point of middle fracture on the reverse surface.An “O”-shaped crack is formed on the front side of the plate and an “X”-shaped crack is formed on the reverse side， while a “U”-shaped crack is formed on the front side and a“Y”-shaped crack is formed on the reverse side.Finally， the positive and reverse cracks penetrate through to form a “transverse O-X” type initial break and a “transverse U-Y” type periodic break， forming a “curved triangular plate” above the working face and a “trapezoid plate”broken block structure above the roadway.The statistical results showed that the initial breakage interval of “horizontal O-X” was nearly the same as the periodical breakage intervals of “horizontal U-Y”.Generally， the value of the vertex angle of the curved triangle plate was between 75°and 78°， which was almost equal to the angle between the waist line and arc line as the value was between 72°and 82°.Therefore， the curved triangle plate could be roughly regarded as a “curved equiangular triangle plate”.The experimental results of the block model show that the curved triangle plate easily slid to instability and caused strong ground pressure behavior in the stope due to the rotation and instability of the “trapezoidal plate” structure， resulting in the detachment phenomenon.The research findings can provide an experimental reference for the prevention and control of strong mining pressure disasters in working faces with large mining height.