Evolution Mechanism of Interconnected Vertical Fractures in the Overburden of Longwall Coal Mining

Table 3

Empirical formula for calculating the height of caving zone.

Lithology of overburden (uniaxial compressive strength and main rock name) (MPa)	Calculation formula (m)
Hard (40-80, quartz sandstone, limestone, conglomerate)
Medium-hard (20-40, sandstone, argillaceous limestone, sandy shale, shale)
Weak (10-20, mudstone, argillaceous sandstone)
Very weak (<10, bauxite, weathered mudstone, clay, sandy clay)