Tunnelling risk - Practical Engineering Geology - page 405

(continued)

Hazard Possible mitigation measures prior to

and during construction to reduce
likelihood or consequence of hazard

Impact of
mitigation
measures and
risk

Summary of main
residual risks

Title Example of

circumstances

Notes * indicates technique more likely
than others to be useful

7. geological mapping to anticipate
permeable or fractured structure,
strata distress
8. grouting ahead of face *
9. ground freezing ahead of face
10. advanced dewatering ahead of
tunnel from the surface
11. use permeable liner to reduce
pressure on liner
12. consider also hazard 1 and 2

7 Collapsed ground karst areas

(boulder
chokes),
shafts, areas
of
collapsed
mine
workings

M 1. GI boreholes

2. pre-construction geological desk
study and risk assessment
3. probe drilling ahead of and around
face through TBM head.
Monitoring of rates of drilling and
character of drill cuttings (presence
of clay, in

L M M 1. failure of grouting

programem or strata
control foams
2. stability problems and
damage to cutter head
3. higher than anticipated
loading on tunnel linings
leading to premature
failure
4. higher post construction
maintenance
5. temporary or permanent
loss of drive

ll materials etc )*
4. monitoring of water make from face
5. TSP to detect change in re ection

characteristic of strata
6. geological mapping to identify
disturbed or fractured structure and
strata distress