May 23 2015 will go down in history as a landmark date not just for Crossrail but also for Herrenknecht. It was on this day that TBM S-720 – Victoria – broke through at Farringdon at the end of an 8.3km drive from the Limmo Peninsula.
A successful break through is always a cause for celebration but this one was different. Not only did Crossrail’s 18th break through join the eastern and western tunnels but when Victoria’s job was done three days later, after progressing into Farringdon station, it marked the end of three years and 42km of tunnel excavation underneath some of the most valued – and valuable – real estate in London.
The three joint ventures working for project owner Crossrail Ltd on Europe’s largest infrastructure project – Bam/Ferrovial/ Kier (western tunnels), Dragados/Sisk (three sections of the eastern tunnels) and Hochtief/Murphy (Thames tunnel) – specified eight TBMs between them. And Herrenknecht supplied all eight.
The first order was received at Herrenknecht’s Schwanau factory in early 2011 and the first TBM – Phyllis – was completed in October the same year. The factory had ample capacity to manufacture all eight at the same time but this wasn’t necessary.
"They were rolled out at intervals, although the timing of the delivery of the first TBM was fairly critical as the portal at Royal Oak had already been completed so the contractors were ready to start," said Roy Slocombe, director at Herrenknecht International Ltd (Great Britain). Boring work commenced at Royal Oak in May 2012.
The TBM shopping list comprised six earth pressure balance (EPBM) shields for the construction of the eastern and western tunnels through London clay, sand and gravel; and two Mixshields for the Thames tunnels, which were excavated at depths of up to 15m below the river bed.
"The slurry system was selected mainly because of the flints in the chalk under the Thames," said Slocombe. "The flints are very abrasive and have caused problems with some tunnels in the past but both we and the Hochtief/Murphy joint venture were able to draw on our experience of working on the Channel Tunnel Rail Link tunnels beneath the Thames and, as a result, the Mixshields coped very well."
The eight machines had shield diameters of 7.08m, were up to 147m long, weighed up to 1,100 tonnes and had a power drive of up to 1,920kW.
Guiding these beasts through London’s crowded subterranean infrastructure called for millimeter precise steering and all eight machines were equipped with laser-guidance navigation systems from Herrenknecht subsidiary VMT.
New features on the TBMs included the provision of refuge chambers in accordance with recommendations in the 2011 revision of BS 6164 (Code of practice for health and safety in tunnelling in the construction industry).
"These were the first machines in the UK to be equipped with a refuge so that if there was a _ re in the tunnel behind the TBM, the crew could take cover in a safe haven that was supplied with scrubbers and an air supply," said Slocombe.
Another key amendment was to the belt weighing equipment. "Crossrail wanted to monitor ground settlement as carefully as possible so the belt weighers, weighing the amount of material extracted in every ring, were to a higher standard than had previously been used."
Finding a route through existing infrastructure meant that the ability to follow tight curves as well as straight lines was paramount and the machines had double articulation to enable them to follow curves as they were needed.
Setting up
In terms of assembly on site, each contract was handled in a different way, depending on site conditions and programme requirements.
"For example," said Slocombe, "the portal for Contract C300 [the western tunnels between Royal Oak and Farringdon west] was sandwiched between main railway lines and the Westway overpass, so the two machines, Ada and Phyllis, for Bam/Ferrovial/Kier, were assembled some metres away and then rolled into place on heavy transport ready for the launch."
"The TBMs launched from Limmo by Dragados/Sisk (Elizabeth and Victoria) for the eastern tunnels were assembled in a small launch adit at the bottom of the shaft and the shields themselves were fully assembled on the surface and then lowered down in one big lift."
Jessica and Ellie, which were deployed by Dragados/Sisk on the drive from Pudding Mill Lane to Stepney Green and Sophia and Mary the mixshield TBMs used by Hochtief/Murphy for the Plumstead to North Woolwich leg under the Thames, were assembled in the portal area and launched complete.
The TBMs were required to work around the clock, seven days a week and they were guaranteed to be operational 90 per cent of the time. In fact, Herrenknecht surpassed that, achieving 95 per cent TBM availability.
This was down in no small part to Herrenknecht’s team, which provided the service and maintenance requirements of the machines and kept a ready supply of spare parts at all the Crossrail worksites in London throughout the course of the project.
The machines were maintained on a regular basis and the cutter tools were replaced as necessary. When the machines passed through the previously constructed stations, the opportunity was taken to check the wear protection on the cutterheads.
The TBMs’ performance was exemplary, said Slocombe. "They were up to the specification and all the machines did everything that was expected of them. The fact that S-722 [Ellie] was able to advance 72m – 45 segmental rings – in just 24 hours is quite an achievement and not just for the machine but for the team that was operating her."
The machines were operated by 20 men per shift, with 12 on the TBM and eight on the gantry. And Herrenknecht also played its part from an operational point of view by training TBM operators, accompanying them on the machines and giving advice during the drive.
This typically included changing small parameters in the control system, such as air pressure, water or foam level where the ground was wet, sandy or chalky, to make the drive more efficient and safer. First stop Elizabeth’s first port of call, 2km after launching from Limmo en route to Farringdon east, was the newly constructed box for Canary Wharf station. This marked Crossrail’s first breakthrough into an existing structure and had to be executed with millimetre precision. Once through, the TBM had to cross the 250m-long station before it could continue on its way to Whitechapel, Liverpool Street and its final destination, Farringdon.
Tracks were laid, consoles were welded to the machine’s shield and moving skates were placed underneath. For the back-up, engineers in Schwanau designed special shunting units and in this way, over the course of seven days, it was possible to shunt the entire 150m-long TBM through the station. This solution avoided costly and time-consuming disassembly and re-assembly. Victoria was transported through the Canary Wharf station box in the same way, just after Elizabeth.
The redeployment of Jessica and Ellie saved the purchase of another two TBMs and also recouped some time from the original plans.
"At the time of the contract it hadn’t been decided what to do about the short G drives [Limmo to Victoria Dock portal] but then Dragados/Sisk elected to re-use Jessica and Ellie, which had come in from Pudding Mill Lane to Stepney Green on the two Z drives," said Slocombe.
The two machines were dismantled and transported on heavy trucks to the Limmo portal and relaunched in order to tunnel the 930m to Victoria Dock.
The original plan was that Ellie would be assembled in the auxiliary shaft while the back-up would be lowered through the main shaft at Limmo. However, because Jessica finished her work at Limmo faster than expected, there was enough space to lower and assemble all of Ellie through the auxiliary shaft. This freed up the main shaft for other work.
"The whole process took three months and the change in plan saved the contractors a week and could only have been achieved with input from Herrenknecht," said Roger Escoda, Dragados/Sisk tunnel manager.
"They knew the details of the machine and were able to tell us whether and how our plan was technically feasible," he said. "It’s not a unique situation but we had extra people involved so that if anything needed refurbishment it could be done as the TBMs were reassembled," said Slocombe. "And, as they had only run 2.7km they didn’t require any major repairs." Once they had finished their drives, the TBMs had differing fates. In chronological order, the cans and cutterheads of the first machines that came in from the Paddington direction – Phyllis and Ada – were buried at Farringdon.
"By electing to do that it meant that Farringdon station was freed up for other construction to proceed there," said Slocombe. "The back-up was taken back in the tunnel, although not all the way out as by that time there was other construction going on, so they came out in smaller pieces at Fisher Street."
The two Mixshields, Sophia and Mary, were dismantled in the open at North Woolwich. The next two to be dismantled (prior to being redeployed), were Jessica and Ellie, which were removed at Stepney Green. Here the shaft was so narrow that the back-up had to be lifted out at an angle and, in order to facilitate this, Herrenknecht engineers in Schwanau designed stiffeners, which were then welded on to the equipment on site. Finally, Elizabeth and Victoria were dismantled underground. The cutterheads were cut into small pieces and removed via the shaft at Farringdon, while their trailers were removed from the tunnel via the shaft at Stepney Green.
Many of the major components will be remanufactured and will be used again in new projects. "Martin Herrenknecht is very proud to have supplied all eight machines for this prestigious project," concludes Slocombe.
