Once the TBM gets started, it does not simply keep going on until the completion. It usually stops from time to time for a procedure called ‘face intervention’.
The main reasons for face intervention could be:
Inspection of face conditions or probing ahead
Maintenance or replacement of cutter head tools. The cutting tools are consumables during the tunnel drives and need replacement from time to time
Treatment of the face such as grouting to reduce permeability of the ground
Removal of obstacles at face, such as foundation, karst features
Clean out of the excavation chamber
Repair of structural damage of the cutter head
Resolving mechanical faults of the TBM
Face intervention can be categorised into the following from a project management perspective:
Planned intervention, where the stoppage and intervention happens by design, and well prepared for in advance
Unplanned intervention, where the stoppage and intervention happens in response to unexpected change in conditions or breakdown/emergency. Needless to say, unplanned intervention is less favourable in comparison.
Safe havens
Face interventions are only permitted where it is considered sufficiently stable with controllable water inflow to back the TBM shield away from the ground. The so-called ‘safe havens’ are such places, usually used for planned interventions. These can be naturally provided in the ground, or man-made. In certain situations of unfavourable conditions, additional structural support to the ground or modifications to it (such as ground treatment) may be required. These additional supports can be done well ahead of tunnelling, in anticipation of the planned interventions.
How frequent should the planned intervention be?
This is a question extremely dependent on the specific situation, including a number of factors in consideration listed below. The answer usually lies somewhere in the order of hundreds of meters to several kilometers.
The specs of the TBM. E.g. whether cutting tools can be replaced from within the TBM
Degree of abrasively of the ground
Whether/how often ground conditions permit planned interventions
How effective are the contingencies expected to be
Risk appetite of the project team
What to do in the case of unplanned intervention?
Unplanned interventions can take place in natural safe havens, where the ground is naturally stable with minimal water inflow. Where needed, compressed air can be used to stabilise the ground, and this is normally a built-in function of the TBM. It must be noted that this method has favourable health and safety implications where man entry to the face is required. The HSE’s guideline of upper limit of safe compressed air pressure for face intervention is 3.45bars. Above this pressure, additional safety measures are required. Compressed air can be used in combination with other measures such as ground treatment, done either from within the TBM or from the ground surface down.
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