Height of the Fractured Zone in Coal Mining under Extra-Thick Coal Seam Geological Conditions

Table 3

Initial failure of each key strata with advancement of the working face.

Advancement of face (m)	Failure height of overlying strata (m)	Initial failure of key strata
55	32	Y2 (inferior key strata I)
109	143	Y9 (inferior key strata II)
193	174	Y22 (inferior key strata III)