Managing final tunnelling works at Farringdon 22 July 2015

Peter Kenyon, TunnelTalk
With TBM excavation of the 21km of twin running tunnels under London for Crossrail now complete, attention focuses on station construction, escalator excavation works, installation of the platform tunnel final linings, trackwork and M&E fitout, ahead of scheduled opening of the first services on the central section in 2018. Peter Kenyon of TunnelTalk joined the Crossrail tour of Farringdon Station to find out more.

As work to cut up the cutterheads of the last two remaining Crossrail TBMs into manageable 1-2 tonne chunks draws to a close, and with the main bearings already removed, it will not be long before all evidence of three years of mechanised tunnelling under London is gone.

Dismantling the TBM at Farringdon Station
Dismantling the TBM at Farringdon Station

TBMs Elizabeth and Victoria completed final project breakthrough from the west and into the new Farringdon Station platform last month (June 2015), and to mark the event, Crossrail invited the media to witness the final stages of the dismantling operation some 25m below street level.

In many ways it is fitting that the end of TBM operations should occur at Farringdon, whose central location has presented many special challenges, both practically and logistically.

Farringdon Station is the location where the Bam Nuttall/Ferrovial Agroman/Kier (BFK) Western Tunnel drives from Royal Oak in the west meet the Dragados/Sisk Eastern Tunnel drives from Limmo in the east. The eastern and western ticket halls of the station are separated by the historic Smithfield Market, accommodation of which means that at 280m long, the station platforms are up to 100m longer than any of the other Crossrail stations. The localised geology of alluvial deposits at the Farringdon site is also more highly water-bearing than the more usual London Clay, meaning that the station’s platform tunnels are the only ones along the whole alignment to feature installation of a plastic waterproofing membrane behind a final cast insitu concrete final lining. Finally, as the point of east-meets-west, half of the project’s eight Herrenknecht TBMs ended their journeys here, at a time when station construction was at the stage where onsite shaft removal of the machines in their entirety was rendered a practical impossibility.

12m shutter for cast insitu concrete final lining over a sheet waterproof membrane
12m shutter for cast insitu concrete final lining over a sheet waterproof membrane

Eastern Tunnelling Project Manager Roger Mears told TunnelTalk: “Here at Farringdon the ground conditions are a little bit worse than at the other station sites. There is a greater threat of water ingress at this location, as we are down into granular material and less cohesive ground. A decision was made, early on, by the station contractor [BFK], to use a sheet waterproofing membrane. It is not possible to apply spray concrete on that to form a secondary permanent lining so they are using a sheet membrane and a 12m travelling shutter to cast each pour. At the other station platform sites we are using SCL (spray concrete lining), which is quicker to apply. The decision to use a sheet waterproof membrane at Farringdon was a design decision based on concerns about groundwater ingress control.”

All but 30m of secondary lining in the 280m-long eastbound platform tunnel is installed. In the westbound platform tunnel, presswork and invert work is under way ahead of the start of secondary lining works.

Roger Mears, Eastern Tunnelling Project Manager (left) and Andy Scholes, Eastern Ticket Hall Site Manager
Roger Mears, Eastern Tunnelling Project Manager (left) and Andy Scholes, Eastern Ticket Hall Site Manager

Dismantling of the final pair of TBMs is also creating logistical and scheduling challenges. Those driven in from the west in late 2013 and early 2014 are already buried into offset splay tunnels, and most of their parts have long since been removed, leaving their shields and cutterheads behind. The remaining parts had to be dismantled into very small pieces and removed via the Fisher Street shaft, whose dimensions were prohibitively small for anything else.

Removing the final pair of machines is proving equally complex. Lifting out their screw conveyors and main bearings using either the eastern or western Farringdon access shafts is an impossibility given the fact that construction of the internal works of the eastern and western ticket halls is fairly advanced.

Mears explained: “Most of the large TBM parts have come back to Stepney Green loaded on locos, including the main bearing that can’t be lifted at Farringdon East or West due to the props that are now in place in the shafts.

“Had we waited at Farringdon to complete tunnelling and not done any internal station works, then obviously the station contractor would have been a year or two behind on programme. Using shafts at other locations further east and west along the alignment was the only solution.

Farringdon Station cross section plan
Farringdon Station cross section plan

“The cutterheads of the last two machines are now being cut into smaller 1-2 tonne pieces and these can be lifted out through the bracing props in the shaft at Farringdon West. We will also use the Moorgate shaft at Liverpool Street to move some smaller elements later in the programme. This is because we will need to stop using the running tunnels between Liverpool Street and Stepney Green as a way of extracting the TBMs while the station contractors at Liverpool Street and Whitechapel stations get on with secondary lining of those platform tunnels in order to keep to the project schedule.

“It is quite a complicated strategy that we have had to adopt,” said Mears, “and we are obviously working with all the different contractors to work out the optimal solutions for the dismantling programme going forward.”

As tunnelling reaches its close, and with the machines now either dismantled or lifted in their entirety and returned to Herrenknecht as part of a buyback deal, Farringdon Project Manager Linda Miller paid tribute to German engineering. “Our success so far has been based on four things,” Miller explained. “First, we bought what I believe to be the best machines in the world. They were fabulously crafted and they progressed excavation and tunnel construction with an exacting precision the like of which I have never seen in my 25 years in the business.”

Final Crossrail TBM progress map and station locations
Final Crossrail TBM progress map and station locations

Miller also singled out the pre-excavation grouting regime at Farringdon, which involved the installation of thousands of tube-a-manchette of between 20m–70m in length from various 5m diameter grout shafts around the jobsite. This enabled precise control of localised settlement of the ground. Grout injection through the tube-a-manchette quickly corrects downward ground movements of up to 2mm that are flagged by a complex monitoring system.

This monitoring system, said Miller, represented the third main reason behind the successful completion, without serious incident, of the Crossrail running tunnels through some of the most crowded of underground spaces in the world, and below some of the most historic and expensive real estate on the surface.

Farringdon Station western ticket hall from viewing gantry
Farringdon Station western ticket hall from viewing gantry

Miller said: “Tens of thousands of instruments were laid in advance, all of them giving three dimensional XYZ tilt readings recorded by laser automatic total devices. These send terrabytes of data to a central hub for analysis. One of the features of this project has been the use of a bespoke computing system, the first of its kind in the world, to analyse all this information on a daily basis. Triggers were set up, and project managers would get an immediate text if, for example, one of the thousands of prisms attached to buildings across London suddenly triggered alarms on the system.

“In many cases this might have been a false alarm caused by a bird landing on the prism, but the point is that we had a highly effective early warning system in place. Only five years ago there probably would not have been the computing capacity to process all this information in any meaningful way.”

Finally, said Miller, Crossrail learned valuable lessons from the Heathrow collapse of 1994. “Had they had the right people at the face, who knew what they were doing, the Heathrow collapse would likely have been avoided. It could have been predicted that there were likely to be problems two to three weeks before the collapse actually happened.”

She added: “This was a valuable lesson for Crossrail, and it was a project priority that we had very highly qualified people at the face of every tunnel, at all times, 24 hours a day, seven days a week.”

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