Safety Threshold Determination for Blasting Vibration of the Lining in Existing Tunnels under Adjacent Tunnel Blasting

Table 9

Criteria for determining crack grade.

Crack grade	Judgment factors
	Damage degree	Damage development trend	Safety threats to pedestrians, vehicles, structures, and facilities
B	Slight	None or tend to be stable	None or slight
1A	Commonly	Slower	None, it may pose a threat in the future
2A	Heavier	Fast	A certain threat and danger
3A	Serious	Rapid	More dangerous