Abstract: The geological condition of shallow thick-hard roof is very common in Shendong mining area, which would be particularly conductive to form a large area hanging roof especially when the Wongawilli production practice is adopted. If one of the working face sections stopped to limit area, large-area caving zone will come into being on the thick-hard roof, which will result in casualties and equipment damage. In this paper, fracture mechanics theory is adopted to study the cracking of thick-hard roof. Based on energy balance in the process of crack extension, the minimum duration from initiating cracking to large area caving of thick-hard roof on Wongawilli face under ideal conditions without blasting disturbance is derived, and it provides a theoretical basis for reasonable control of thick-hard roof.

Key words: shallow Wongawilli face, thick-hard roof, energy balance, time difference