A second TBM excavating the main 25km long Tideway CSO interceptor under the River Thames in London has broken through. Breakthrough was at the end of the 7km drive of the West Contract from the launch shaft at Carnwath Road to Acton Storm Tanks for the BAM Nuttall-Morgan Sindall-Balfour Beatty (BMB) JV. The 8.8m diameter machine began tunnelling in May 2019 and excavated about 725,000 tonne of material to reach the reception shaft at Acton, the westernmost point of the Tideway route (Fig 1).

Breakthrough at Acton Storm Tanks

The breakthrough completes TBM excavations on the West Contract, after a refurbished Lovat EPBM completed the 1.1km long Frogmore connector between King George’s Park and Carnwath Road in July 2020.

The first Tideway TBM breakthrough on the 7.3m i.d. main line was the 5km reach between Kirtling Street and Carnwath Road on the Central Contract, completing its drive in November 2019 for the Ferrovial-Laing O'Rourke (FLO) JV. Casting of the 250mm thick secondary concrete lining on this section is expected to begin "within the next month or so," Tideway told TunnelTalk. The lining is to be cast using two 12m shutters supplied by TecoZam of Spain. Each 12m long shutter is supported on a 34m traveller (Fig 1). The traveller was split into four parts for lowering down the shaft and reassembled underground. The secondary lining is reinforced with 30kg of ArcelorMittal steel fibre/m³ of concrete.

Two of the four main TBM drives for the project now remain to be completed. An NFM EPBM is currently 6.3km into the longest 7.5km Tideway drive on the Central Contract between Kirtling Street and Chambers Wharf and a Herrenknecht slurry machine is underground but is yet to launch on its 5.5km drive for the East Contract between Chambers Wharf east to Abbey Mills pumping station, for contractor Costain-Vinci-Bachy Soletanche (CVB) JV. A final TBM, another Herrenknecht slurry machine, is also underground and preparing to launch on the 4.6km long x 5.5m o.d. Greenwich connector on the East Contract (Fig 2). Both East Contract TBMs are expected to launch by the end of the year.

“Having broken through, it is easy to forget the time and effort that goes into making these excavations possible,” said Neil Binns, Senior Project Manager for Tideway. “It is an example of the teamwork required to clean up the River Thames.”

The last TBM to work on the main tunnel for the Tideway CSO project under the River Thames in London arrived on site in Bermondsey from manufacturer Herrenknecht in late July 2020. The slurry machine will drive the 5.5km section of the main interceptor from its Chambers Wharf launch site to the Abbey Mills pumping station (Fig 1). As part of the East Contract, being constructed by the Costain-Vinci-Bachy Soletranche JV, it is the last TBM to launch on the main 25km x 7.2m i.d. Tideway alignment, which extends from Acton in the west to Abbey Mills, where it connects with the existing Lee Tunnel for conveyance of combined storm water onwards to the Beckton sewage treatment works (Fig 1).

Final Tideway TBM arrives in London

Launch of the TBM is expected in late 2020 with completion in 2022. Manufacture of the precast segmental lining is already underway at a plant in Tallington, Lincolnshire, where they are also being stored, before transport to Thamesport in Kent, and then up the river to Chambers Wharf.

The final main line TBM was “always expected to begin later in the programme than at other launch sites,” said Tideway in an email statement. “The preparatory work required at Chambers Wharf was more extensive. A cofferdam had to be built before construction of the shaft could start and the shaft itself is the deepest on the project,” at about 64m deep.

Of the four TBMs working on the main Tideway CSO alignment, there are currently two in operation, after the 5km Kirtling Street-to-Carnwath Road drive of the Central Contract was completed by its NFM EPBM for contractor Ferrovial-Laing O'Rourke (FLO) JV in November 2019.

Fig 1. The 25km long Tideway route and drives by contract

A second NFM EPBM on the Central Contract is currently about 5.5km into its 7.5km drive between Kirtling Street and Chambers Wharf after a stoppage between February and June. The stoppage was originally planned to prepare the cutterhead for the different ground conditions under east London but was extended beyond its scheduled restart after Tideway closed all construction as a result of the coronavirus pandemic.

The work to the cutterhead included replacing more than 300 tools, including drag bits, arm/nose rippers, bucket scrapers and disc cutters, in preparation for the drive into the Upnor Formation and Thanet Sands, both of which are known for their abrasiveness to TBM cutting tools.

Restarting this element of the project, the longest of the main Tideway drives, was “one of the most challenging under new safety guidelines”, said Paul Hallows, Site Manager at Kirtling Street for the Ferrovial-Laing O'Rourke (FLO) JV in a June 2020 press release. “There is still much work required to get us to Chambers Wharf, but we are confident we can finish this section of tunnel, meeting the same standards as we have already set.” Tideway expects breakthrough of this eastward drive of the Central Contract later this year.

Casting of the inner in-situ concrete lining in the main drive of the Central Contract from the Kirtling Street to the Carnwath Road is also getting started. The first of two 34m long shutters, weighing about 240 tonne each, has been lowered to the bottom of the Kirtling Street access shaft following completion of the westbound TBM at Carnwath Road in November 2019. Due to the size and weight of the full round shutter, it was split into four pieces for lowering into to the bottom of the shaft for reassembly in the starter tunnel.

The shutters will cast a 250mm thick concrete lining inside the primary segmental lining, reducing the i.d. to 7.3m diameter. The secondary lining provides further resistance to abrasion to ensure the 120-year design life of the infrastructure. Sewage in Tideway will flow by gravity eastward with no need for intermediate pumps until lifted by pumps into the Beckton sewage treatment works at the end of the Lee Tunnel.

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Central Contract Kirtling Street to Chambers Wharf drive restarted in June 2020 after COVID-19 shutdown (left) ; First part of two shutters lowered into Kirtling Street shaft (centre) for reassembly in the starter tunnel (right)

The final TBM on the main Tideway alignment, a Herrenknecht EPBM on the West Contract, is almost 5.5km into its 6.5km drive between Carnwath Road and Acton Storm Tanks. Its progress was also halted by the pandemic in March with contractor, the BAM Nuttall-Morgan Sindall-Balfour Beatty, restarting tunnelling on 13 May. Tideway is still assessing the impact of the shutdown on the breakthrough date.

Frogmore connector breakthrough at Carnwath Road

Excavation on the TBM connectors

In addition to the main interceptor, Tideway includes two long TBM excavated connectors, the Frogmore long connector as part of the West Contract and the Greenwich connector as part of the East Contract.

The 1.1km x 2.5m i.d. Frogmore connector broke through to the main CSO alignment at the Carnwath Road shaft in July 2020. The connector was excavated in two drives from a central launch shaft at Dormay Street by the BAM Nuttall-Morgan Sindall-Balfour Beatty JV using a refurbished Lovat EPBM. The first 500m southward drive to King George’s Park was completed in October 2019 and the second 600m drive from Dormay Street northward to Carnwath Road in January 2020.

TBM to drive Greenwich connector is finally underground

Meanwhile, the Costain-Vinci-Bachy Soletranche JV on the East Contract is preparing to launch a Herrenkencht slurry TBM on the 4.6km x 5.5m i.d. Greenwich connector between Greenwich pumping station and Chambers Wharf where it will join the main Tideway alignment. In July, its tailshield and cutterhead were finally lowered into the launch shaft, after a delay in transporting the machine the 3 miles from its Victoria Deep assembly site in North Greenwich, where it had arrived a year ago in late July 2019.

The time taken to transport the machine from assembly to launch site was explained by Tideway as “a number of logistical challenges in getting the TBM to the launch site in one piece.” With lockdown reducing the size of crew available to transport the machine, “the decision was made to split the machine down into a number of parts, transport to the launch site, and re-assemble on site.” Tunnelling is now expected to begin on the Greenwich connector in November 2020 for completion in Summer 2022.

Tideway and the pandemic

Although unavoidable, Tideway is counting the cost of the coronavirus pandemic shutdown. The predicted budget for the project had already increased from an initial £3.4 billion (£3.1 billion in 2014/15 prices) to £3.9 billion. Following the pandemic shutdown Tideway has released a ‘plausible downside scenario’ and ‘severe downside scenario’, as part of its latest annual report.

Connector reaches main Tideway segmental lining

Under the plausible downside scenario, the costs to complete the project are 9% higher than forecast, taking the total cost to £4.1 billion. Under the worst-case scenario, costs to completion increase by 24%, taking the total to £4.3 billion.

On the advice of the UK Government, non-essential construction on the project was paused during the week of 23 March, with only the West Contract TBM drive to Acton Storm Tanks and pipe-jacking work at Putney continuing. “We could not stop the TBM in its position beneath the river wall, so it was progressed at the start of lockdown and then paused. Stopping the pipe-jacking may have risked settlement,” Tideway told TunnelTalk.

Resuming activities at all working sites has involved a “site-by-site approach to the implementation of new safety measures,” said Tideway, including the introduction of staggered shift patterns, one-way walkways, contact-free entry to site, social-distancing in its welfare facilities, and encouraging workers to avoid public transport.

“We did what we had to,” said Andy Mitchell, Tideway CEO, in the annual report. “We paused our work on site and brought tunnelling to a safe stop. Every member of staff who was able to, began working from home. We gave assurances to our supply chain that we would continue to pay them, so they in turn could protect their own workforce. This was the right thing to do.”

As a result, Tideway reports that it was able to “safely re-mobilise our operations, with the same, highly skilled team in place. We remain on track to deliver all works by 2024.” A detailed assessment of the impact of the pandemic is expected to be published later in the year with Tideway “working with all stakeholders including our contractors and our regulator” to prepare this.