Europe’s Biggest TBM to Dig Lower Thames Crossing Tunnel

Tunnel boring machines function by using a rotating cutterhead to grind away the material in front of the machine, while some kind of support is provided to prevent the exposed tunnel face from collapsing before a lining is in place behind it. In soft, water-containing ground there are two main types: earth pressure balance machines, which use the excavated soil itself - this soil being kept under pressure - to support the face, and slurry machines, which pump a fluid mixture, typically consisting of bentonite clay suspended in water, against the face in order to hold it in position while cutting takes place. A variable density machine, which is the type selected for the Lower Thames Crossing tunnel, is an improvement on the slurry method in that it allows engineers to alter the density of the supporting fluid as the ground conditions vary along the route, rather than being restricted to a single slurry mixture for the entire tunnel boring process.

How important that flexibility is when working under a river such as the Thames, since the material under the riverbed is almost never uniform. A bore of this size will have to go through a combination of soft alluvial silts close to the surface, more compact gravels, and chalk or clay strata lower down, each of which reacts in a different way to pressure and therefore needs a different ratio of slurry density to face pressure in order to remain stable. If the face pressure is miscalculated, even for a short time, there is a risk of the cutterhead collapsing or, at the other end of the scale, a blowout occurring which would force the slurry up through the riverbed and might thus disrupt shipping on one of the busiest waterways in the country. It is only because the density can be adjusted on the go, rather than having to stop the machine to reconfigure it, that tunnelling under an active tidal river of this width becomes at all feasible.

The enormous size of the machine brings about its own engineering difficulties well before it gets to the riverbed. Since the finished machine is 16.4 metres in diameter and 120 metres long, it cannot be built in a factory and then transported as one piece, and so Herrenknecht will build it in sections and assemble it within a specially constructed launch shaft close to the Port of Tilbury. The cutterhead alone, with the disc cutters which grind through rock and soil, requires a drive system that can provide a massive torque steadily for months on end without the regular maintenance access that a smaller machine would offer, as it is much more complicated to stop a machine of this size during boring under a flowing river than it is simply to pull it back for servicing.

Deciding to use a single machine for digging both tunnels rather than having two work in parallel is a purposeful compromise: it is cheaper to manufacture one large TBM than two, and it makes the launch shaft and on-site logistics simpler. The drawback is the amount of time taken - the second tunnel cannot start until the first is finished and the machine has been retrieved, refurbished and relaunched, thus extending the total construction programme even though it reduces the number of components that could fail at the same time. Since the project is already aiming for an opening in the early to mid-2030s, the engineers have concluded that the advantages of reliability and lower manufacturing risk justify the extra months caused by carrying out the tunnels one after the other rather than side by side.

For A-level physics or engineering students who are studying the topics of forces, pressure and materials, this kind of project serves as a practical example of pressure balance in operation: the pressure of the slurry at the cutting face must be constantly matched against the total weight of the ground together with the hydrostatic pressure of the water in the river above it, with the use of real-time sensor feedback instead of a fixed setting. The project also acts as a case study showing how civil engineering projects scale in a non-linear manner; since doubling the diameter of a tunnel does not just result in the forces being doubled, they increase by approximately the square of the diameter when it comes to cross-sectional loading, which is the reason why a machine only slightly larger than the existing TBMs in Europe represents a real engineering challenge rather than merely an incremental improvement.