In Pictures: The Crossrail TBMs

A gloomy Monday in Germany this week presented us with a rare opportunity – the chance to see the first (and, as it happened also the second) of Crossrail’s Tunnel Boring Machines (TBMs) close up.

In total, eight TBMs will be used on the Crossrail project. Six of these (including the example pictured below) are Earth Pressure Balanced Machines (EPBs). These will be used, in pairs, to bore the tunnels from the Limmo Peninsular, Royal Oak and Pudding Mill Lane tunnelling portals. Two Slurry TBMs will also be required for the new “Thames Tunnel” beneath the river. As it stands, all these TBMs will effectively be life-expired once they have completed their respective drives. Herrenknecht were able to confirm that the current contract contains a buy-back clause allowing them to buy the TBMs back upon completion of the tunnelling. Crossrail have confirmed, however, that there have also been very general, hypothetical discussions about the possibility of reconditioning and reusing the TBMs instead, should Crossrail 2 make it definitively off the drawing board.

Although let as a number of separate contracts (based on the portals), all of these were ultimately awarded to Herrenknecht. Herrenknecht have previous form in London, having provided the TBMs for both the Jubilee Line Extension and the DLR Woolwich Arsenal extension. This has actually allowed for some efficiencies – mainly through the ability to establish a shared spare-parts pool. It has also meant that Herrenknecht were able to commit to producing the TBMs in 10 months rather than 12.

The two TBMs currently being prepared are the EPBs for contract C300 – the boring of the tunnels from Royal Oak Portal (we have looked at the portal itself previously here). The first of these now sits at Herrenknecht’s German plant, where it is undergoing final factory testing. It will shortly be dismantled and shipped to the UK, where it will be reassembled on the portal ramp at Westbourne Park.

As the photos above hopefully convey, the TBM’s are grand in scale, with a 7.1m cutterhead and (when fully connected to their conveyor train) 140m in length. They are by no means the largest TBMs Herrenknecht have built (indeed the firm is currently working on a 19m cutterhead TBM for a Russian project), but they are still impressive pieces of engineering. As well as cutting, these TBMs will also lay the precast tunnel segments behind themselves as they pass beneath the capital.

Those familiar with TBMs will notice that these TBMs have an unusually open cutterhead for an EPB – indeed they’re over 50% open. This is because they will need to deal with the significant amount of clay to be found beneath London.

This close up view of the cutterhead hopefully begins to give some idea as to the boring process. The cutterhead and the space behind it are pressurised in order to help prevent the tunnel collapsing before the lining is laid (pressure chambers allow worker access to the area behind the cutterhead if it’s required)

As the TBM slowly advances (they’ll operate 24hrs a day and bore approximately 100m of tunnel a week), spoil accumulates in the space behind the cutterhead. This is then carried away by a large screw and deposited on a conveyor running the length of the top of the TBM.

To advance, the TBM effectively uses exactly the same technique first pioneered by Brunel on the Thames Tunnel over 150 years ago – it jacks itself forward on the tunnel rings it has been laying behind itself.

In order to do this, 22 double hydraulic jacks are located in a circle on the rear of the cutting shield.

Once sufficient space has been excavated, the jacks are retracted and a new tunnel ring is laid. It is to this activity that much of the TBM’s length is effectively dedicated. Inside, the TBM is essentially a long tunnel, complete with its own narrow gauge railway.

The railway brings new pre-cast ring sections forward from the rear. Tunnelling is as much logistics as it is hardcore engineering, and the process of casting and providing these reinforced tunnel segments is as complex as the act of tunnelling itself. These will all be cast on site at the portals, with the casting factory at Old Oak Common already now operational and producing 100 segments a day. All the ring segments are actually slightly curved to either the left or right, the placement of different combinations of these allowing the path of the tunnel to be changed as required.

These segments, once delivered to the front of the TBM, are then taken off of the narrow gauge and lifted onto a short conveyor which carries them forward to where they need to be laid.

Our German hosts were kind enough to fire up the TBM in order to give a better idea as to the final ring laying action. This can be seen in the video below.

Once the rings are in place, a concrete grout mixture is injected between them and the tunnel wall.

Each TBM is operated by a crew of about 20, with about 12 of those on the TBM itself. The TBM itself is controlled from a small cabin located just behind the tunnel head. This can be seen in the picture below – as can a justifiably proud Ralph Lickert, the Project Manager overseeing the construction of the two TBMs for the Royal Oak Portal. As with all the paired TBMs, these will effectively be mirror images of each other inside (the cabin will be on the left on one, for example, and on the right on the other). This is necessary in order to keep the supporting conveyors and logistics separate when crossovers are constructed.

It’s from the cabin that the TBM is piloted and kept on course – a doubly vital activity on Crossrail due to the tight margins the tunnel needs to be constructed within. At Tottenham Court Road, for example, the TBMs will pass within less than 50cm of the active Northern Line tunnels.

Finally, Ralph was kind enough to direct our attention elsewhere on the Herrenknecht site, where this TBM’s twin could be seen nearing completion.

Both TBMs will be in West London early next year, where they will be the first to start tunnelling in March 2012.