A Method for Determining Feasibility of Mining Residual Coal above Out-Fashioned Goaf under Variable Load: A Case Study

Table 1

Stability determination methods of the upward mining of longwall goaf.

Methods	Calculation formula and application condition
Rational analytical method [6, 7]	K > 6(single seam); K > 6.3(multiple seam)
Mathematical statistics method [8]
The “three-zone” method [6, 9–13]	Caving zone 1:
	Caving zone 2:
	Fracture zone 1:
	(here (a) strong and hard: c1 = 2.1, c2 = 16, c3 = 1.2, c4 = 2; (b) medium strong: c1 = 4.7, c2 = 19, c3 = 1.6, c4 = 3.6; (c) soft and weak: c1 = 6.2, c2 = 32, c3 = 3.1, c4 = 5)
	Fracture zone 2:
	(here m is the mining thickness and hi is the stratum thickness i)
The method of equilibrium surrounding rock [14–16]
	(here K1, M, and hp are, respectively, the bulking coefficient, mining thickness, and equilibrium surrounding rock thickness)