Formation of fractured zones in overburden due to longwall mining

The fractured zones caused by mining were studied in the overburden of the Torezko-Snezhnyanskaya area, Ukraine, through the change in natural gas emission from these zones during longwall coal excavation. Zones of interconnected fractures and separate horizontal fractures were studied with vertical wells drilled from the ground surface down to active underground workings. The maximum heights of the zone of interconnected fractures and separate horizontal fractures may reach 19-41 and 53-92 times the thickness of the coal seam respectively. It was found that the ratio between the maximum height of the zone of interconnected fractures and the thickness of the extracted coal seam increases with the increasing number of rock layer interfaces and decreases with the increasing stiffness of immediate roof. It is shown that the growth of the zone of interconnected fractures occurred during 17-39 days at an average rate of 0.94-1.97 m day(-1) and it was accompanied by increasing methane emission from overburden. Observation shows that the formation of separate horizontal fractures began only 11-49.5 days after the height of the zone of interconnected fractures reached its maximum value. Formation of separate horizontal fractures in overburden over the longwall excavation occurred as a stepped process from lower to upper sandstone-sandy shale layer interfaces in the direction of the ground surface.