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

Table 1

Classification criteria for evaluation of risk levels with qualitative indexes.

Evaluation indexes	Classification
	Risk level	Risk level	Risk level	Risk level
Rock mass structure ()	Complete structure	Stratified structure	Cracked structure	Loose structure
Geological structure ()	Without faults or folds	Impact of folds is small	Impact of folds is huge	The faults penetrate surrounding rock
Groundwater ()	Without water spraying	Little water spraying when rainfall exists	Water spraying when rainfall is huge	Water spraying in rainy season
Impact of groundwater ()	Without impact for surrounding rock	Little impact for surrounding rock	General impact for surrounding rock	Huge impact for surrounding rock
Mining impact ()	Little blasting impact	General blasting impact	Huge blasting impact	Great blasting impact
Condition of adjacent underground goaf ()	No more underground goafs in the influencing area	The area of underground goafs is medium and the number is small	The area of underground goafs is large and the number is great, while the distribution is scattered	The area of underground goafs is large, the number is great, and the distribution is concentrated
Pillar size and layout ()	The pillar exists and the layout is standard	The pillar exists but the layout is nonstandard	Without pillar or the layout is nonstandard; the pillar begins to be damaged	Without pillar or the layout is nonstandard; the pillar is damaged seriously
of underground goaf ()
Engineering layout ()	Reasonable	Generally reasonable	Partly reasonable	Extremely unreasonable

is the ratio of span to height.