Prediction of the height of destressed zone above the mined panel roof in longwall coal mining

Longwall mining is one of the most widely used underground mining methods most suitable in relatively flat-lying, thick, and uniform coalbeds. Due to extraction of the coal seam, the panel roof strata above the mined zone will be destressed and then the roof loads will be redistributed and transferred to the front abutment and neighboring solid sections with higher load bearing capacity where the adjacent access tunnels and the corresponding barrier pillars are located. The height of destressed zone (HDZ), in this paper is taken as equivalent to the combined height of the caving and fracturing zones above the mined panel roof induced due to longwall mining. The height of destressed zone plays a vital role in accurate determination of the amount of loads being transferred towards front abutment and panel sides. The paper describes the mechanism of development of the height of this zone. Finally, five new simple, yet conclusive, mathematical approaches to estimate the height of destressed zone are presented. The results of the methods proposed are compared with each other and with the comparable methods. The methods proposed are further compared and verified with in-situ measurements reported in the literatures. The comparative results confirm the agreement that exist among the methods and those with the in-situ measurements as well. Finally the methods have shown that, in short term, the height of destressed zone ranges from 6.5 to 24 times the extraction coal seam height; while, in long term, the height of destressed zone ranges from 11.5 to 46.5 times the extraction coal seam height. Therefore, beyond this height the overburden pressure will be transferred towards the front abutment, the adjacent access tunnels, the intervening barrier pillars as well as the panel rib-sides.