Quantitative Evaluation and Case Study of Risk Degree for Underground Goafs with Multiple Indexes considering Uncertain Factors in Mines

Table 2

Classification criteria for evaluation of risk levels with quantitative indexes.

Evaluation indexes	Classification
	Risk level	Risk level	Risk level	Risk level
RQD ()/%	>60	50–60	40–50	<40
Span ()/m	<40	40–80	80–120	>120
Area ()/m^2	<800	800–1200	1200–2400	>2400
Height ()/m	<8	8–20	20–30	>30
Buried depth ()/m	<100	100–200	200–400	>400