Tunnel boring machines: an underground damage survey

A machine capable of excavating tunnels up to 50 metres below sea level can leave a loss adjuster perplexed when a claim occurs. While tunnel boring machines (TBMs) quite often experience conventional mechanical failures, the difficulty lies more so in the sensitive handling of cases that commonly involve downtime with significant financial implications. In the midst of the Grand Paris construction project, let’s take a look at how TBMs function with Philippe Azoulay, loss adjuster and deputy managing director.


How does a TBM work?

Up to 21 TBMs will be working simultaneously on the Grand Paris Express project. The machines will work practically around the clock, digging 10 to 12 metres per day at a depth of 20 to 30 metres. The TBM accomplishes several tasks, from boring through rock to putting up retaining walls while also removing material.

This cylindrical metal machine weighs several tons and has a cutterhead, a shield, an extraction system and a propulsion system connected to a back-up train made up of rail cars carrying all the equipment the TBM needs (power supply system, driver station, hydraulic supply system, auxiliary fluid system). The installation system for the tunnel ring (concrete segments) is also part of the back-up train.


The TBM works in three separate phases.

1) Boring

This operation is completed with the cutterhead, which is the same diameter as the TBM and located at the front of the machine. The head is equipped with protruding high-strength steel discs (cutting tools). It completes between 0 and 2 rotations per minute. (This setting depends on the hardness and consistency of the material to be bored.) The shield contains openings through which excavated material passes. It is then loaded into electric rail cars and conveyed for removal via the back-up train.


2)Installing the ring segments

Once an opening is made, boring stops so a ring segment can be installed. A tunnel ring is composed of cement segments that form a circle matching the tunnel’s diameter. The final component that closes the ring, called the keystone, makes the ring freestanding once installed.


3)Injecting cement between the ring segment

Grouting is injected to fill the small remaining space between the outside of the ring and tunnel sides. While the ring segment is being placed, the excavated material is removed using the rail cars. Once placement is complete, the boring work then restarts as the cutterhead begins turning again.



Managing complex TBM claims

These impressive machines have the same types of failures as other construction equipment: a mechanical part can break and require repair or replacement. The complexity lies in managing cases that require high levels of technical expertise and the ability to quickly intervene.

Repairing a TBM is very complicated and can take several months because it is heavy mechanical work in a very confined environment. The loss adjuster works in extreme conditions up to 50 metres below sea level, with the tunnel entrance as the only access point.

The TBM cannot move in reverse and the damaged section is usually at the front of the machine. When we worked on a gas terminal construction site following a TBM shutdown, our loss adjusters conducted “core sampling” operations consisting of cutting out a cylindrical sample section of the machine in order to analyse the cause of the incident.

All factors were considered in order to suggest several technical repair solutions. No hypothesis could be ignored given the significant financial implications of a delay on the project. We were able to isolate the cause of the incident, enabling excavation work to restart.

These days, new types of vertical shaft sinking machines (VSM) are starting to appear on worksites, requiring our loss adjusters to adapt their logistical planning and interventions.


Philippe AZOULAY – Deputy Managing Director – Customer Relationship Department

Find out more about his area of expertise