Appendix A Examples of engineering,technical hazards associatedwith urban tunnelling

A Geological1 Fault zones2 Non-cohesive soils above the tunnel crown3 Variable ground stiness across the tunnel face4 Variation in geology along the tunnel route and at the tunnel

horizon including changes in ground type or behaviour5 Groundwater characteristics (pressure, connections between

aquifers, contamination)6 Pre-existing voids7 Low density and disturbed ground8 Boulders

B Built obstructions to tunnelling1 Foundations2 Piled structures3 Redundant piles and other abandoned structures4 Boreholes that would cause a sudden pressure release or blow-out5 Unexploded ordnance6 Abandoned or existing wells7 Existing underground utilities including water supply and

sewerage tunnels

C Environmental1 Bad air (e.g. due to exhaust of compressed air from a TBM

plenum working through glauconitic sands)2 Natural gases including methane and hydrogen sulphide3 Reradiated noise4 Ground-borne vibration5 Dust6 Airborne and reradiated noise7 Spoil haulage and disposal8 Release of grease from tailseals and TBMmain bearings into the

ground

D Closed-face TBM system1 Inability to see the ground directly2 Poor reconciliation of spoil quantity against progress3 Imperfect conditioning of spoil4 Ground loss at the face5 Ground loss into the annulus around the TBM shield6 Ground loss around the tail skin and imperfect grouting of the

lining annulus7 Complexity of systems (with many systems being critical for

good control)8 Inability to see the TBM cutterhead directly (undetected damage)9 Inaccurate measurement of TBM parameters10 Tendency to over-stress the TBM (e.g. to over-apply the plenum

operating pressure as a contingency against unseen groundconditions)

58 Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005

11 Need for failsafe back-up systems against the risk of power lossor water inundation etc.

12 Inability to maintain a full plenum13 Increasing over-dependency on complex computer and

communication systems

E Pre-existing infrastructure vulnerable to settlement1 Buildings2 Statutory utility pipelines (especially cast iron gas and water

mains)3 Sewers4 Domestic utilities (especially pipe connections at property

margins)5 Buried cables (especially old high-voltage cables)6 Bridges7 Tunnels8 Railways9 Highways

F Monitoring1 Need tomonitor and interpretmultiple simultaneous data streams2 Dependency on limited manual skill base3 Dependency on highly automated systems that are not always

fully understood4 Dependency on high-tech communication systems5 Inability to meet survey sighting requirements6 Vandalism7 Weather8 Safe working environment for TBM operatives9 Inappropriate trigger level settings10 Inability to validate some monitoring systems prior to start of

tunnelling11 Failure to identify signicant trends12 Misinterpretation of monitored data

Closed-face tunnelling machines and ground stability. Thomas Telford, London, 2005 59

A GeologicalB Built obstructions to tunnellingC EnvironmentalD Closed-face TBM systemE Pre-existing infrastructure vulnerable to settlementF Monitoring