Risk Assessment of an Existing Metro Tunnel in Close Proximity to New Shield Tunnels following Construction

Table 6

Classification of seepage water.

Impact degree	Seepage velocity V (drops/minute)	Safety status of existing tunnel structures
I	V > 300	Unsafe
II	60 < V ≤ 300	Relatively unsafe
III	5 < V ≤ 60	Relatively safe
IV	V ≤ 5	Safe