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Tunnelling: How engineers at Silvertown rotated the UK's largest TBM

Jul 13, 2023Jul 13, 2023

The Silvertown Tunnel project in east London involves innovative solutions for a range of engineering and logistical challenges. They include rotating a 1,800t tunnel boring machine on nitrogen skates.

Transport for London's (TfL's) Silvertown Tunnel will be the first road crossing to be constructed under the River Thames east of Tower Bridge in more than 30 years.

The 1.4km twin bore road tunnel will connect Silvertown in Newham with the Greenwich Peninsula. When it opens in spring 2025, the tunnel is expected to improve cross-river public transport and ease pressure on the chronically congested Blackwall Tunnel further west.

TfL awarded the Riverlinx consortium, comprising Ferrovial subsidiary Cintra, Abrdn, Invesis, Macquarie Capital and SK Ecoplant, the Silvertown Tunnel design, build, finance, operate and maintain contract in 2019.

Riverlinx Construction Joint Venture, a partnership between Bam Nuttall, Ferrovial Construction and SK Ecoplant, is designing and building the tunnel.

The Riverlinx team is using the largest diameter tunnel boring machine (TBM) ever employed in the UK. TBM Jill weighs 1,800t, is 82m long and 11.9m in diameter. A small section around the tunnel entrances will be built in a cut and cover tunnel.

Jill has already completed the first bore, working from Newham to Greenwich.

Since the team is using only one TBM to build both bores, a major challenge has been how to turn this enormous machine around for its return journey back to Newham.

To add further complexity, various land right constraints in Greenwich meant that this rotation had to be achieved in a very tight space inside an access shaft.

Riverlinx has overcome this problem with ingenuity, designing and building a chamber in Greenwich where the TBM was rotated in a subsurface chamber using "nitrogen skates" and a complex system of hydraulic jacking.

The TBM in the rotation chamber in Greenwich

Jill launched from the north side of the river at the start of September 2022. As the TBM made its way under the river from Newham, construction of the Greenwich chamber got underway. This was where the TBM would break through and then be relaunched for its 1.1km journey back towards the Silvertown site.

Most twin bore tunnels either use two TBMs or dismantle a single TBM and return it to the initial launch point for its second drive.

But Riverlinx operations director Borja Trashorras explains that using two tunnel boring machines would have been too expensive, while stripping and rebuilding a single TBM could take up to five or six months. On top of this, existing assets in the vicinity, such as roads and utilities, made it impossible to construct a bigger chamber at the site on the Greenwich Peninsula.

"Rotating the machine was therefore the best solution from a cost perspective and programme perspective," explains Trashorras. "And in doing so, we’ve done it in the most innovative way that we could, in a way which has never been done before in the UK."

As a result Spanish firm Ayesa designed an oval shaped chamber that would avoid existing utilities in the area while providing enough space for the TBM to be rotated 180˚ in pieces. It is 18m deep from base slab to surface, 40m long from headwall to rear wall and 39m wide. The chamber will eventually have two openings – one for the TBM tunnels to the north and the other to provide access for the cut and cover tunnel section to the south.

Construction of the chamber's diaphragm walls involved installing 23 panels containing a total of 6,580m3 of concrete and 800t of steel reinforcement and glass fibre reinforced plastic. It was completed by Bachy Soletanche in July last year.

Trashorras continues: "As the shaft was in permeable ground, we had to install surface dewatering measures to be able to dewater the upper and the intermediate aquifers, firstly to get a dry excavation, and second, to lower the water level to be able to construct the base slab at the bottom of the rotation chamber.

"Once we finished the diaphragm wall, we installed the dewatering wells from the surface. Then we started the active dewatering and after that we started excavation works."

The excavation work, carried out with clamshell excavators, took around four weeks and removed around 30,000m3 of spoil.

The team completed the base slab and the concrete headwall – the reinforced concrete eye that the TBM breaks through – between September and January.

The tunnel boring machine being held by the cradle which rests on the nitrogen skates

Once TBM Jill broke through into the Greenwich rotation chamber in February, the team faced the daunting task of turning the machine around.

Riverlinx project manager for the tunnels Ivor Thomas explains that this had to be done by breaking the machine into four sections, starting with the 1,400t shield and then the three backup gantries.

To control the front of the machine as it emerged into the shaft, the team used a platform-like restraint system that fastened onto the underside of the TBM's bulkhead, behind the cutterhead. As the machine was pushing itself forward the restraint system prevented it from moving ahead too quickly.

"We have a 12m [diameter] machine building rings behind it, so once we’ve broken through, we have no restraint load at the front," Thomas explains. "So, there's a tendency even for this 1,400t machine to move forward of its own accord when it's pushing on the rings behind it."

When the shield came clean from the tunnel, it was removed from its adjoining gantry.

But because the TBM climbs up from the flat base of the tunnel under the river on a 4% gradient, it was still at an angle once it emerged from the tunnel, so the team had to get it as horizontal as possible.

This first involved using a large metal band that caught the shield as it emerged from the tunnel. Hydraulic rams were placed directly beneath this band and used to jack up the rear of the shield, so it was horizontal.

When the shield was entirely horizontal, a steel cradle was placed beneath it on "nitrogen skates" – an innovative system of hydraulic feet with a rubber skirt. Nitrogen was pumped into the base of each skate, sealed by the rubber skirt, and maintained at 250bar.

When the cradle was ready to be moved, the hydraulic system was turned off and the skates were floated on the cushion of compressed nitrogen. Steel plates that had been installed on the floor of the rotation chamber during its construction were oiled to ease the movement of the skates as the shield was rotated.

"We use nitrogen because it's commonly available. It's inert, it's not poisonous, it's lighter than air and it's highly compressible," explains Thomas.

He adds that the nitrogen skate system "is about breaking down the friction between the steel plates and the machine; it's not about liftingthe machine".

While rotating a machine on nitrogen skates has been done twice before, once in Paris and once in Stuttgart, this is the first time that the technique has been used on a machine of this size.

"Machines have been rotated on air before," notes Thomas, "but to rotate a machine of this size on compressed air, we’d have probably needed to hire all of the compressors in the UK."

The team then used 25t remote controlled air chain pulls anchored to the wall of the rotation chamber to drag the shield out and turn it around in stages until it was facing the opposite direction. That process took a day. It was then repeated with the relatively light backup gantries which were placed behind the TBM shield.

The tunnel boring machine shield facing north after being turned around inside the rotation chamber

To prepare the TBM for its northbound drive, the team is using another unusual innovation – dubbed a "flying launch" system.

"With a normal launch, you stick up a frame, build up rings and you push the machine in," explains Thomas. "But the problem here is that system would fill up our pit bottom with rings.

"So, we’re using a flying launch system where we build the propulsion frame very close to the tunnel eye."

The TBM shield is then pulled forward towards the tunnel eye using a steel pressure ring, tension bars and hydraulic jacks.

As a result of using this system, a temporary tunnel is not needed to start the bore. Because this system requires less space, it is also ideal for constrained sites.

"The idea is that we launch and get ready to bore with one gantry, drive, and then we turn the other gantries around," explains Thomas.

This means that after the first gantry was reattached to the TBM shield, the team started what is called an "umbilical launch", where the front half of the machine bored an initial 70m of the drive. Then the second and third gantries were added to the machine. After all parts of the machine are fitted back together, the TBM will be put into "full mode" and officially launch north, which is expected to be in June.

Thomas puts the success of the rotation operation down to a "marriage of key supply chain partners", with Herrenknecht supplying the TBM and the temporary works. Max Wild was involved in the TBM rotation, Mammoet supplied heavy lifting services and PHL Hydraulics supplied the equipment that was used for the flying launch.

As a result of this collaborative approach, the project has broken three construction records.

"It's the first time a machine of this size has been used in the UK, it's the first time a machine of this size has been rotated using nitrogen skates, and it's the first time a machine of this size has been launched using the ‘flying launch’ system," notes Thomas.

To add to this list, a complex system for removing tunnel spoil had to be devised as spoil cannot be removed from the rotation chamber. Instead, it will be fed back through the first bore on a conveyor and transported to the north side of the river where it will be loaded onto barges and taken to a reclamation site in east London.

When the team starts tunnelling again this summer, it will face more challenges – this time from ground conditions. Indeed, the ground under the Thames on the tunnel route is "as tricky as it gets east of Canary Wharf", says Thomas.

It is the mixed face and high water content of the ground that make tunnelling and excavation particularly hard at this point on the river – and why British civil engineer Ernest William Moir struggled so much when building the original Blackwall Tunnel beneath the Thames in the 1890s, Thomas comments.

The team must also contend with other obstacles, such as avoiding the piles for the cable car that passes above the river and directly over the tunnel's alignment.

Unlike Moir's Blackwall tunnel team the Silvertown engineers have modern innovations at their disposal to make tunnelling operations easier and safer.

To overcome dewatering issues, for example, they have installed sump units in the tunnel at its lowest point.

Thomas says: "The low point in the tunnel means that water flows downwards. Because of this we’re installing 160 precast concrete units in the base of the tunnel's mid section to form a sump. They will be bolted into the tunnel lining and then grouted."

The sump units will collect run-off water that will then be pumped out of the tunnel.

The team started installing the precast concrete units in the first bore in March while the rotation was happening to "steal time on the programme", says Thomas. Then the tunnel will be backfilled to get it to pavement level, with all the main civil works within the first bore expected to be completed before the TBM starts full mode tunnelling the second bore in June.

The team will also be using ground freezing – what Thomas calls the "Rolls-Royce" of ground treatment options – on four cross passages in the tunnel. The technique will be used to freeze the water-saturated sediments of the Lambeth Group to enable the team to excavate the cross passages.

The ground freezing will be carried out when both bores are complete so that the team can run the freeze pipes in between them.

Another UK first, adds Thomas, is that the team will not be propping the cross passages but will instead be building all the temporary propping into the cross passage linings.

At each end, cut and cover tunnelling work for 600m of approach roads has also started.

The walls of the cut and cover sections are being built with secant piles. Excavation works for the cut and cover section in Greenwich started in March and is due for completion this summer.

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Rotation chamber Nitrogen skates Flying Launch Ground freezing Thames Menteth