Rail Engineer has visited the new Crossrail Farringdon station site before (issue 118, August 2014) and has looked at the working of the tunnel boring machines (TBMs). However, the latest visit to Farringdon was a bit different to either previous experience.

This time the TBM was not in a cutting at the start of its drive, rather it was being dismantled deep underground having triumphantly completed its work.

There is a danger of becoming too complacent about tunnelling under London. This is partly the fault of the Crossrail project, the team having done its complex job under the city so well that it hasn’t attracted the publicity that can attend when things go spectacularly wrong.

The Crossrail team has delivered 42km of 6.2 metre bore tunnels using eight TBMs through ground that must be more densely packed with foundations, services and other tunnels than anywhere else in the world. It has done this on time, within the financial envelope and with a great safety record. This success is down to the skill and attention to detail of the client, its suppliers and its staff.

Significant contributions have come from the various joint venture construction teams such as BFK (BAM, Ferrovial, Kier) and DSJV (Dragados Sisk Joint Venture), whose works meet up under the ground at Farringdon. Equally important has been the supply of the efficient TBMs by German specialist Herrenknecht.

Now its ‘just’ a case of turning the tunnels into a railway. Due to open in 2018, the new route will add about 10% to central London’s railway capacity. Estimates predict the creation of as many as 55,000 new jobs and £5bn of new retail real estate.

But first, the tunnel boring machines have to be removed to make way for the tracks. Crossrail project director Simon Wright, eastern tunnelling project manager Roger Mears and Farringdon station project manager Linda Miller were all on hand to explain how this was being done.

Down to the TBM

Access to the works was via the Eastern Ticket Hall site entrance. One immediately noticeable change since the last visit was that no cumbersome re-breather sets were issued. So much more of the underground excavation has now been completed, and the air circulation systems below ground are now so much more comprehensive, that these are no longer required. They are still available below ground, but the risk was now assessed to be too low to necessitate each individual carrying one all the time.

Just getting to the Eastern site from the offices at the Western Ticket Hall site is quite a trek. The Crossrail platforms at Farringdon are to be the longest platforms in the whole of Crossrail at some 350 metres.

The viewing platform which overlooks the works is actually one storey above street level. The view emphasises how confined the site is by the surrounding streets and neighbouring real estate. The project team has worked hard to keep on good terms with the neighbours and avoiding disputes about structural damage, noise and vibration, air pollution and traffic congestion.

From the platform, it is quite a way down to the running tunnels, at present involving many flights of temporary stairs down through the access voids left in the floor slabs of the new structure. These stairs finish 5 levels below the street, on the slab that forms the base of the new ticket hall construction. From there it is necessary to go down a further level into the running tunnel around which the station platform tunnels will be formed. Access during the works is by an adit which will eventually form part of the ventilation system of the new station.

As might be imagined, space is at a premium down in the cavern where the TBM is being taken apart after completing the drive it has made in creating the eastbound running tunnel. Paradoxically, the tunnel was driven from Limmo, 8.3km to the east, towards Farringdon – in the opposite direction to that of the trains that will eventually use the tunnel.

Dismantling TBM Elizabeth

Like her sister, Victoria, which drove the adjacent westbound running tunnel, Elizabeth has done her job and has to go. Victoria actually completed the last tunnel drive of the whole Crossrail project, finishing the drive into Farringdon of the westbound tunnel on 26 May 2015. Elizabeth had finished her drive on the eastbound tunnel a little earlier.

The TBMs could not be driven to one side and buried at this location and, at around 130 metres in overall length, the machines and their support trains were too big to be removed by any means other than total dismantling. In any case, the project wished to recycle as much of each of the TBMs used as was practicable.

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First, the support train behind the cutter head of Elizabeth was taken away. Valuable items such as the electric motors that powered the cutting system were removed intact for reuse. Large and heavy parts of the structure that have little value were cut up into bits and lifted out at Farringdon by the relatively light lifting gear available in the tunnel.

One of the more problematic items was the recovery of the main bearing that hung in the centre of the shield of the cutting head and supported the rotating cutter face of the TBM. This is a huge bearing, weighing about 50 tonnes, and is valuable enough that it had to be recovered intact. A cradle running on rails was inserted beneath it and the bearing was lowered onto this. From there it was dragged back onto a carrier on a temporary rail track, and then hauled along this, back down the new tunnel to a shaft at Stepney Green where there was a crane of sufficient capacity to lift it up out of the shaft. It is due to be taken back to Herrenknecht for refurbishment and reuse.

Not all of the TBM was removed. The main cylinder of steel forming the barrel of the cutter head section could not be extracted safely. It was therefore stripped of all ancillaries and the steel diaphragm that closed most of its face, behind where the cutter disc used to be, was cut away. This left a large steel ring that will remain in situ for ever, behind the secondary tunnel lining.

Victoria is being treated in exactly the same way. Thus the Crossrail trains will, at this location, run through parts of the machines that built the tunnels for them to use.

Unique shuttering

The tunnel lining construction in this section of the works is of interest in itself. Whereas in many other areas the secondary tunnel lining was constructed using sprayed fibre reinforced concrete, similar to that used for the primary lining, BFK elected to use a shuttered concreting method here.

The secondary lining consists of a waterproofing layer encased in a concrete lining cast on a shutter which, once the concrete has cured, could be partially collapsed. It was then moved forward from the completed section to the next portion due for concreting and expanded back to its full size ready for the concrete to be pumped in behind it to form the next section of lining. The contractor elected to use this innovative technique because it was considered to be quicker and to offer a quality result.

This change in tunnel lining from segmental construction to shuttered in-situ concrete marks both the change from bored running tunnel to station tunnel, and also the change in contractor, from the DSJV of the tunnel works to the BFKJV responsible for the Farringdon Station works.

On the western side of Farringdon Station the twin running tunnels resume to Royal Oak, about 6.8km away to the west.

Now that the tunnelling works have been concluded, the focus of the project will switch to fitting out the stations and tunnels ready for the initiation of the train service. The track will be of concrete slab form throughout the tunnels. Construction will commence with a base layer of concrete cast in the bottom of the tunnel ring to create a flat surface, and the slab track will be constructed upon this.

But that’s where the tunnelling stops, and railway construction begins. For Elizabeth, and her seven sisters, their job is done.