Study on termination principle of coal caving in thinner thick coal seams considering correlation between step distance

In order to study the termination principle for fully mechanized top coal caving in thinner thick coal seams with a coal seam thickness of about 4～6 m,taking the No.12309 working face of Wangjialing Coal Mine as the research background,the distribution law of top coal block was obtained through on-site measurements. There is a relationship between the step distance and the next step distance of coal caving. The PFC numerical simulation method was used to invert the excessive coal caving discharge body,and the discharge body was divided into 4 sub-areas,and the research was carried out around the proportion of each sub-area and the mark of the loss position of the remaining coal that can be drawn,and the reasonable principle of coal caving termination was explored. The results show that when the block size of the top coal increases,the number of released blocks decreases;the number of blocks with a top coal particle size of 4.0-9.2 cm is the largest,while the average mass percentage of top coal blocks with a particle size of 9.2-14.4 cm is the largest. The overall shape of the discharge body is an ellipsoid cut at the lower part by the support beam;Most of the top coal released from excessive coal discharge is the top coal that can be discharged in the next step,and on average,only 1/3 of the particles are releasable residual coal that can be released from excessive drawing coal;The difference between gangue content rate and residual coal drawing quality temporarily decreases at the initial stage of drawing coal,but as the coal caving continues,the difference increases rapidly,and the overall curve of the two shows a "scissors-like" change;The residual coal is divided into floor coal and high-level coal,and it is found that the floor coal will be released quickly after the gangue is found,and the low gangue content can be maintained at this time;However,the discharge of high-level residual coal is accompanied by a large amount of gangue,so the excessive coal discharge time should be strictly controlled,and it can be terminated only for a short time;Combined with the characteristics of fully mechanized top coal caving in thinner thick coal seam and the process of coal caving at the production site,it is considered that strictly implementing the coal caving termination principle of “seeing the gangue and closing the window” in actual operation can effectively control the gangue content and achieve higher top coal mining rate.