Thick-hard strata movement law and multi-dynamic load mechanism in longwall face with ultra-large cutting height

Large mining height technology is characterized by high yield, high efficiency, and high recovery rate. It has become the preferred technology for mining thick coal seams in Yushen mining area. The mining space size and equipment power have increased exponentially. Especially in the working face influenced by ultra-large mining height and thick-hard roof, strong mining occurrences happen frequently, threatening mining safety. To improve the control effectiveness of thick-hard roof in ultra-high working face, 122104 longwall panel in Caojiatan is taken as the engineering background. In situ measurement, lab test and theoretical analysis are used to study the motion law of thick-hard roof and the characteristics of support load in 10 m ultra-high working face, which aims to explore the control method of strong mining occurrences. The results show that large mining space, high advance speed and multi-layer thick-hard roof led to strong dynamic load, high resistance increase rate, large static load and high-frequency cyclic load of the ultra-high working face. Roof load is transferred to the ultra-high coal wall rapidly due to support failure, which triggers splitting failure of the hard coal. The ultra-large mining height causes the three thick-hard rock groups 80 m above the seam to enter the influence zone of the hydraulic support. The thick-hard strata groups show small deformation rupture mode, mainly dominated by elastic deformation before failure. There is no separation in rock strata within the same group, but the linkage effect is obvious. After failure, the fracture development speed is fast, but strength deterioration of the broken rock is low. Thus, the load transfer capacity is strong, which can form a stagger-shaped intermittent balance structure. The co-evolution characteristics of the support load and the roof structure are obtained. The support load is small before the breakage of the thick-hard roof, which mainly distributes in supporting area of the column. After the breakage, the rock load is transferred downward rapidly, and support load increases to the maximum resistance. Multiple sets of hard rock breaking cause the hydraulic support to bear multiple dynamic load impact effects. Roof load deviates from the column support area, causing abnormal support conditions. A short beam structure model for thick-hard roof is proposed by considering interlayer shear force. Internal tensile stress decreases while the shear stress increases, which reveals the principle of small deformation of thick-hard roof and the mechanism of tensile-shear mixed failure. The distribution characteristics of elastic strain energy in the thick-hard roof is obtained, and the conditions of dynamic breakage and strain energy release are revealed. The elastic small deformation breaking characteristics of thick-hard roof determines that the strain energy is mainly released instantaneously, and the conversion rate of dynamic energy reaches 21%. The calculating method of dynamic load impact force is put forward, and the high energy release rate of thick hard roof and the large free space of the broken rock block are the direct reasons of multiple dynamic loads in ultra-high working face. After the advanced regional fracturing, the fracture of thick hard rock group changes from small elastic deformation to large plastic deformation mode. The high plastic dissipation work and high deterioration degree of the thick hard roof reduces the dynamic load impacts strength, and the surrounding rock control effect of the super high working face is significantly improved.