Under the conditions of repeated mining in multiple coal seams, overlying strata experience repeated pressure relief, and the fracture paths are different from that formed in single coal seam mining. The movement of overlying strata, the fracture distribution and evolution, and the shape were investigated by physical tests and theoretic analysis. The results demonstrate that a mobilized elliptic parabolic zone would be formed if the bed-separated fissures and broken fissures are connected in the process of the overlying strata movement. Compacted area gives shrinkage effect with subareas further developing, and fracture of the overlying strata undergoes five stages, i.e. fracture initiation, expansion, compaction, expansion again and compaction again. Equations on voids and permeability in overlying caved rock, residual expansion coefficient, gob depth and the strata movement were established when composite gob becomes basically stable, which can be used to analysis the expanding characteristics of strata and the gap seepage characteristics of composite gob under the condition of repeated mining. At last, a mathematical model of the elliptic parabolic zone in overlying strata is derived, which provides a theoretical basis for determining the location of drilling hole for gas extraction.
Cracks in coal seam and rocks are the main channel of infiltration and fluid migration caused by overlying strata regularly cave, rupture and sink with the mining face advance (Qian & Miao 1996). Bai (1990) and Palchik V (2003) agree that there exist three different mobile belt above longwall mining gobs (caving zone, fracture zone, bending subsidence zone); Liu (1981), Qian (1983) established theoretical system of "Horizontal Three Area" and "Vertical Three Regions" on the basis of overburden failure and conductivity fracture distribution characteristics. Through experimental and theoretical research. Qian (1998 & 1997) put forward the theory of "O" ring. Yuan (2004 & 2008) put forward the concept of "roof annular crack circle". Lin (2007) called it a "recurrent" ringand and Li (1990 & 2000 & 2011) described it as "mining fissure elliptic paraboloid zone" and covered "mining fissure rounded rectangle ladder". Xie (2008) described it as an "arch" cap. The above researches reveal the evolution law of fracture zone in single coal seam mining, and form a more completely theoretical system which is playing an important role in guiding simultaneous extracting of coal and methane.