ABSTRACT:

After shallow long wall face mined, the composite sinking motion of covering loose layer and broken bedrock layers were extremely easy to cause serious impact on the mining space, like roof cutting along the coal wall, which resulting supports crushing accident. By comprehensive research methods of similar simulation, numerical simulation and field observation, the evolution characteristics of stress field and overlying strata movement in typical shallow mining working face were analyzed. A "near-far field" composite structure of shallow long wall face was proposed, which based on the special roof movement law. Through the study of its formation and evolution process, the results shows that out-sync instability of "near-far field" composite structure caused obviously different cycles of roof pressure strength. Its essence is the crack zone conducted surface and main roof broken occurs periodically. Small periodic pressure strength indicates the influence caused by main roof periodic breakage, as the near field structure's instability; big periodic pressure strength indicates that after crack zone conducted to surface, the effect caused by composite sinking motion of covering loose layer and broken bedrock layers, as the far field structure's instability.

1 INTRODUCTION

The essential difference of ground pressure behavior between shallow coal seam and conventional coal seam is whether the complete "three zones" (caving zone, crack zone, sagging zone) could be formed in the overlying strata after reaching full subsidence or not (Ren 2011). In general, crack zone would directly conduct to surface in shallow coalface mining, and the complete "three zones" could not be formed in the overlying strata, only "two zones" or "one zone" Huang & Qian (1999). Based on the study of strata breaking process of several typical shallow long wall coalface, it shows that the immediate and main roof would sink and break with coalface advancing, and after the coalface advanced to a certain distance, crack zone would conduct to surface (all bedrock layers broken), composite sinking motion of covering loose layer and broken bedrock layers occurs (Holla & Buizen 1990).

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