The countdown is underway for tunnel boring to commence later this year on London’s Silvertown road tunnel following TBM lowering. Everything needed has been organised for supply chains to focus upon the southeast London site, close to the existing Blackwall tunnel, also passing below the River Thames.

Vital to success during construction of the 1.4km long tunnel and long-term operational success is the precast structural concrete rings to be built behind the 1200 tonnes, 11.92m diameter Herrenknecht TBM, ‘Jill’, presently being constructed within the launch chamber on at the riverside in Silvertown.

The concrete rings for the project’s twin tubes are mostly of a standard design with fibres for much of length of the road tunnel.

Extra reinforcement is in some rings that need to be erected at a few specific locations, Mark Thompson, tunnelling manager for Silvertown Tunnel at Transport for London (TfL), tells T&T.

In addition to fibres helping reduce the carbon footprint of the precast segments, the environmental load is further reduced with the concrete mix design using ground granulated blast-furnace slag (GGBS) to replace 40% Portland Cement.

PREPARING FOR SILVERTOWN PPP

TfL procured Silvertown on a public private partnership (PPP) concession contract with a private consortium – Riverlinx.

The Riverlinx consortium holding the Design, Build, Finance, Operate and Maintain (DBFOM) contract has as equity partners Macquarie Capital (part of the Macquarie Group), Cintra (part of Ferrovial Group), Aberdeen Standard Investments, BAM PPP-PPGM JV and SK Engineering & Construction (SK E&C).

The turnkey contract to create the infrastructure was awarded by Riverlinx on an Engineer, Procure and Construct (EPC) basis to a joint venture – Riverlinx Construction – which comprises: BAM Nuttall; Spain’s Ferrovial Agroman (part of Ferrovial Group); and, South Korea’s SK E&C (and an equity holder). As part of the Royal BAM Group, BAM Nuttall has a sister company involved for the tunnelling – Wayss & Freytag – and another involved in the equity stake, BAM PPP.

Riverlinx has raised £1.2 billion of funding against the Silvertown budget that remains at approximately £1 billion, says TfL. The extra is to cover the budget plus risk through the construction period until opening, on schedule for 2025, and then 25 years of operation afterward, per the concession agreement.

Silvertown is the biggest investment in road infrastructure in that area of London for about 30 years. Procurement for the PPP concession project was highly competitive and ran for an extended time.

SILVERTOWN SEGMENTS

The 10.66m i.d. concrete tunnel lining has been designed by COWI and Gall Zeidler Consultants, and a total of 1122 rings are required to be built – involving erection of more than 10 000 precast segments, says Thompson.

He adds that the rings will be built within the 11.92m diameter excavated tunnel, each ring consisting of nine segments, including the key.

The segments are being manufactured by Banagher Precast Concrete, in Ireland, with a strictly monitored process of quality control, says Thompson.

Of the total number of rings, he says 70 are of “an innovative design with significant heavy reinforcement to facilitate a prop-free opening of the tunnel’s cross-passages.”

Most of the segments, and therefore rings, are of a standard design with fibre reinforcement on a 35kg/m3 dosage, “which is typical,” says Thompson.

“There are also a further 315 rings that have additional reinforcement for the start and end of the tunnels.

“They will also be used at particular sections of the tunnel where the heavier duty sections have been identified as necessary within the design.

“In addition, 50 rings will have an epoxy resin coating which will help them resist chemical attack from potential contaminated ground.

The heavy-duty rings have 150kg/m3 of steel reinforcement and the prop-free specials have up to 330 kg/m3 of steel rebar within them, he adds.

Each standard segment will weigh just over 8 tonnes, with the special segments weighing up to 8.5 tonnes.

Sealing gaskets are cast into the precast segments at manufacture.

“The rings are now arriving on site at a rate of 6-7 rings a day, with the site having storage for up to 250 rings at any one time,” Thompson tells T&T.

During tunnel boring, as the TBM advances, the fibre reinforced concrete segments will be loaded into the shaft using an overhead gantry crane – three segments at a time – onto one of two multi-service vehicles (MSV) capable of accommodating a full ring.

“Each MSV, currently being built by Metalliance, will then haul the rings up the back of the TBM where they will be off-loaded onto the segment feeder.

“Banagher have currently manufactured around 340 of the rings, which is around 30% of the total and they continue production at around 20 rings per week,” says Thompson.

As well as the Herrenknecht TBM being assembled – including lowering by crawler crane of the tunnel eye in May, in a four-hour lifting operation, and then sections of shield and the 250 tonnes main drive, also being put in place are supporting systems, such as conveyors, grout plants, water treatment and gantry cranes.

Thompson says, “They are also all being installed on site at the moment, ahead of tunnelling set to start later this summer.”

In a statement in June, Riverlinx’s project director, Juan Angel Martinez, said: “There has been a lot of hard work involved to get us to this point, and the achievements so far are a great example of what collaboration and excellent teamwork can deliver.”

The TBM passed its Factory Acceptance Test (FAT) at the end of 2021. It will bore from Silvertown under the river to Greenwich Peninsula to excavated and build the first segmentally lined tube and then be turned to drive back for the second tunnel. The shield is bigger by far than any used in London TBM bores before – such as Tideway (8.85m dia.), Crossrail (7.1m.), Northern Line Extension (6.03m).

While the project advances, and then operates for years ahead, a further ongoing benefit of the project in reducing the environmental footprint of the asset comes from Banagher’s commitment to plant a hardwood tree for every precast concrete ring manufactured. Riverlinx has said such trees absorb about 20 tonnes of carbon annually and, with about 1200 trees to be planted that would mean an ongoing saving of 24 tonnes per year.