A Ground disturbance induced by the following tunnel when the preceding
tunnel passed beneath
B Ground disturbance induced by the preceding tunnel when the preceding
tunnel passed beneath
C Ground settlement trough over twin tunnels when the preceding tunnel
passed beneath (i.e. A+B)
D Ground disturbance induced by the preceding tunnel when both tunnels
passed through
E Ground disturbance induced by the following tunnel when both tunnels
passed through
F Ground settlement trough over twin tunnels when both tunnels passed
through (i.e. D+E)