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

Table 5

Classification of ratio B.

Impact degree	The ratio of proximity distance to tunnel diameter	Structural state of existing tunnel
I	≤ 0.3	Unsafe
II	0.3 <  ≤ 0.7	Relatively unsafe
III	0.7 <  ≤ 1.0	Relatively safe
IV	1.0 <	Safe