The basic principle of roof strata control in fully mechanized caving mining of extra thick coal seam

Based on the inability to change the initial occurrence state of coal and rock mass in mining activities,and a lack of technical means to accurately understand its occurrence characteristics and engineering response characteristics,the theoretical analysis method is used to study the basic principle of roof strata control in the fully mechanized caving mining of extra thick coal seam. The study concluded that "temporary stability" state,'timely caving" state and a periodic transformation of the two states are three typical characteristics in extra thick coal seam roof strata using fully mechanized caving mining. The basic principle of roof strata control in the fully mechanized caving mining of extra thick coal seam is to control the "temporary stability" of the roof rock structure during the mining process. To maintain the "temporary stability",it is needed to avoid the sliding destabilization of the upper immediate roof "combined short cantilever beam" structure and the destabilization of the basic top "articulated rock beam" structure.Based on the analysis of the two instability conditions of the "articulated rock beam" structure and three instability modes of the "combined short cantilever beam" structure,the conditions of the deformation pressure on the hydraulic support to maintain the "temporary stability" of the roof rock layer structure in a fully mechanized top coal caving min-ing is proposed. It is concluded that external intervention can be used to promote the "articulated rock beam" structure to further shift upwards,reduce the "articulated rock beam" fracture step and "combined cantilever beam"length. These measures can slow down the pressure intensity and maintain the roof structure stability of fully-mechanized top coal caving. The field test results show that taking some measures,including external intervention to promote the "articulated rock beam" structure to further shift upwards and reduce the "combined cantilever beam" length,can ensure safely and efficiently coal mining at the follow up working face where has shown severe pressure behaviors and multiple support damage,on the permission of keeping original fully mechanized caving equipment.