Tunnelling has now been completed on Tideway, London’s 25km “super sewer” that will reduce the reliance on the capital’s Victorian sewage system, reducing overflows and slashing the amount of untreated waste entering the Thames.
The £4.2bn Thames Tideway Scheme was given planning permission in 2014 and began construction in 2016. Considering the havoc created by the pandemic, it’s impressive that the 25km of new tunnels have been completed in six years – and they’re deep underground, some as low as 70m.
The hard work has often flown under the radar given the noise around London’s other major tunnel project Crossrail and the furore caused by boring through the Chilterns for HS2.
While the number of people who will actually enter the Tideway tunnels will be significantly smaller than those that pass through the Crossrail and HS2 tunnels, the number of people that will benefit from the sewer will be in the tens of millions, year after year.
We’ve taken a look back at how the massive project has come into being and the hurdles its overcome in the last six years.
From the start, one of Tideway’s expressed aims was to “reconnect London with the River Thames”. One of the main ways that it did this was to transport over 90% of the materials to site by barge, reducing the number of HGV road journeys by over 300,000.
The contractors on the east section of the Tideway project is a joint venture between Costain, Vinci Construction Grands Projets and Bachy Soletanche.
In the central section, it is FLO, a joint venture between Ferrovial and Laing O’Rourke, that is carrying out the work.
The western section is overseen by BMB, a joint venture between Bam Nuttall, Morgan Sindall and Balfour Beatty.
Before work began underground, the first works for Tideway began in the middle of the Thames, as contractors constructed a new pier for the Blackfriars stop of the Thames Clipper. The 84m pier was towed from the Netherlands and is supported by six metal struts each weighing more than 40t that arrived from Italy. The new pier was created to ensure that users of the transport could continue uninterrupted as the work continued beneath he ground and river in the Blackfriars area.
Tideway provoked innovations in tunnelling and construction throughout its progress. One of the earliest saw the use of the world’s first mains electricity powered hydrofraise diaphragm walling machine. The electric hydrofraise diaphragm was developed by the Costain Vinci Bachy Soletanche JV, and starting in August 2017 it was used to dig the shaft for the sewer’s main tunnel at Chambers Wharf in Bermondsey.
The innovation not only significantly reduced carbon as compared to standard drilling machinery, but was also much quieter – a massive plus for the central London location.
The world’s first mains electricity powered hydrofraise diaphragm walling machine
There were six tunnel boring machines (TBMs) used in the process of creating Tideway.
The first arrived on site at Carnwath Road in west London in November 2017, having been delivered by barge along the Thames. The 1,350t tbm was manufactured and tested in Kehl, Germany, before making the 850km voyage.
The TBM was named Rachel after Rachel Parsons, an engineer and advocate for women’s employment rights. Once re-assembled on site, the TBM was 147m long and 8.13m in diameter.
The first TBM, Rachel, arrives by barge
Two TBMs named Millicent and Ursula - after suffragist Dame Millicent Fawcett and cryobiologist Audrey ‘Ursula’ Smith - followed, arriving at the Kirtling Street site near Battersea. In June 2018, they were the first to be carefully lowered down to site level, a distance of 53m managed at increments of 400mm to 500mm at a time.
Millicent became the first TBM to get tunnelling on the project in November 2018.
Tideway celebrated its first fully-bored tunnel in February 2019 with the completion of a passageway between Hammersmith Pumping Station and the Thames. At 300m long and 5.1m in diameter (before thinning to 4m), it’s by no means the longest tunnel in the project, but it was the first of many tunnelling milestones. It also boasted the first use of sprayed concrete lining in the UK.
A new phase of tunnelling began in March 2019 as TBM Rachel was lowered 35m into its starting point in Fulham. Over the following months it dug 7km from Carnwath Road to Acton Storm Tanks.
TBM Rachel being lowered into place in Fulham
One of the other TBMs was a refurbished 3m wide machine that had been previously used by Thames Water. It was named Charlotte after suffragist Charlotte Despart and started boring in March 2019, cutting the Frogmore Connection Tunnel, a 1.1km shaft that connects overflow sewers in the Wandsworth area to the main Tideway tunnel. It completed its drive in October, making it the first of Tideway’s TBMs to break ground.
Charlotte becomes the first Tideway TBM to break ground
In April 2019, just as Tideway announced that it had completed the “first and most unpredictable phase of the project”, which included 40% of the construction, it also revealed a cost hike of 8% – £280M. This put it up to £3.8bn from the initial estimate of £3.52bn.
At the time, Tideway chief executive Andy Mitchell said: “We are working in an unforgiving tidal river in the centre of one of the busiest cities in the world. We have got train lines and bridges above our heads and the tube and utility points below our feet. As we approach the half way point of construction, the time is right to update our cost estimate.”
Technical difficulties were also at fault for the cost hike. One of the issues occurred at the Blackfriars site, where the original plan was to build the shaft by constructing a cofferdam in the river and backfilling it to ground level to create a working platform, from which a diaphragm walled shaft could be constructed.
However, contractors Ferrovial and Laing O’Rourke found that the action of backfilling and the diaphragm wall’s movement would cause the rupture of two Victorian gas mains, 5m deep into the wall. Diverting the gas main was not an option, so the contractors were forced to create a new shaft design which involves a secant pile wall solution down to 25m with a sprayed concrete lining solution beneath to 52m. This created a need for dewatering and jet grouting in two rings to stabilise the ground before it was sprayed. At 55m below river bed level, this is believed to be the deepest jet grouting ever to take place.
A similar issue occurred at the King Edward Memorial site, where the cofferdam was to be backfilled but it was discovered that the sheet pile walls deflected more than was expected. This was due to there being thicker and softer alluvium layers than had been designed for, meaning the ground had to be strengthened.
For the temporary works, 150 piles of 25m depth were installed along the bottom perimeter of the cofferdam with a beam and slab on top to support the pile sheets. For the permanent works, the 100m x 50m area of the cofferdam had to be strengthened using deep soil mixing, which involved installing 1,400 columns, each 1.2m diameter and between 8 and 12m long.
It is estimated that both work arounds cost over £100M. In both cases, the delays meant that the TBMs arrived at the shafts before the works were finished.
With the TBMs set on their way from the shaft at Chambers Wharf in Bermondsey, the first major concrete pour took place. Over the course of a 36-hour period in July 2019, 1,500m3 of concrete was poured. It was done inside a specially-designed noise enclosure to limit noise and dust pollution. This concrete formed a platform for a giant digging machine to sit on at a later stage of construction.
March 2019 saw TBM Ursula begin its 7km journey from Battersea to Bermondsey.
In May 2019, Tideway became the longest tunnel under the Thames as Millicent passed the 2km mark on its drive. However, it still had 3km to go to reach its destination at Carnwath Road in Fulham.
On the Eastern end of the project, TBM Annie – in honour of astronomer Annie Scott Dill Russell – arrived in Greenwich in July 2019, having been shipped from Germany by barge. It was assembled on a site next to the O2 Arena and then transported to 4km to the Deptford Creek site, where it began tunnelling the connection tunnel between Deptford and Surrey Quays.
TBM Annie ready to be shipped from Europe
A couple of excavating machines, known as pipe-jacking machines, arrived at the Barn Elms site, west London in November 2019. Unlike TBMs, these are controlled from surface level and make shorter tunnels connecting sewer overflows to the main sewer.
The first pipe-jacking machine arrives in West London
As 2019 drew to a close, Millicent completed its 5km drive from Battersea to Fulham and Tideway passed a total of 10km of tunnels excavated, with four TBMs underground at the time.
Charlotte, meanwhile, was lifted out from its completed tunnel at Fulham and moved Dormay Street to begin another 600m drive northwards to Fulham once more. This drive was completed in July 2020.
Tideway capped off the year by completing the excavation of shafts at three of its sites: Albert Embankment (50m), Heathwall Pumping Station (28m), and Victoria Embankment (48m). In each case, concrete was poured continuously to ensure consistent quality slabs up to 3m thick.
The project’s milestones kept coming as 2020 began, with Ursula making it to its rest point, having drilled 4km of the tunnel from Battersea to Bermondsey.
Tideway officially announced it had reached the halfway mark in February 2020.
Just like everything else, Tideway ground to a halt in March 2020 as the pandemic dug in, forcing a nationwide lockdown and the ceasing of all works.
During a five-week stretch between March and April, the workforce dwindled from 2,800 people on site to 150.
However, it didn’t remain severely hampered for too long, with workers getting back underway at 21 sites in May 2021 – with new testing and social distancing protocols in place.
Due to the impacts of the Covid-19 pandemic, the cost of the Tideway project rocketed up once more. At the end of June 2020, a Tideway report said it could rise to between £4.1bn and £4.3bn – although stressed this was a worst case scenario.
The report also revealed that the project had been delayed by three months and seen a £100M cost increase due to complications in the secondary lining operations at Carnwath Road and Chambers Wharf drive sites.
A few months later, in August, the full impact of the pandemic was revealed as Tideway gave a shareholder announcement in which it said the cost would rise by £233M to £4.13bn. It also said the project would be delayed by nine months, pushing its delivery into early 2025.
The sewer’s giant stainless steel vortex system arrived at Tideway’s Hammersmith site in June 2020. It also includes a 21m long, 65t pipe section that was installed at the Hammersmith Pumping Station in a vertical manner.
At the pumping station the sewage drops down a 15m shaft, which would wear away the bottom over time. The vortex generator is there to mitigate this by spinning the flows down the pipe, removing its energy and preserving the base.
It was lowered into place down the Hammersmith shaft in late July. It had over 1,000 stainless steel studs welded onto its outside which locked it perfectly into position once in place.
Tideway’s next specially made TBM arrived from Germany in July 2020. The new machine – named Selina after Bermondsey Medical Mission founder Dr. Selina Fox – was tasked with boring the final 5.5km stretch eastwards from Bermondsey to Abbey Mills Pumping Station near West Ham. In September, a 12-hour process lowered it into place to begin its drive.
The first shutters for the casting of the tunnels’ secondary lining were lowered into place in August 2020. The 240t, 34m-long shutter was lowered into the tunnel in four pieces. It then went on to line the main tunnel by casting a 250mm thick layer of concrete against the primary lining, which is made up of segmented concrete rings. The secondary lining protects the tunnels from abrasion, ensuring a 120 year design life.
Lowering the shutter to install secondary lining
TBM Ursula resumed its 7km drive in June 2020, but in September it hit an unexpected barrier: concerns that it might cause damage to Tower Bridge. The TBM had been working its way under many of the capital’s historic landmarks along the Thames during its drive from Battersea to Bermondsey. This forced Tideway to install “invar barcodes”, which would detect any unexpected movement on the bridge. Fortunately, Tower Bridge was not damaged as Ursula continued on its way.
The emergence of TBM Rachel from its 7km drive between Fulham and Acton marked the end of the main tunnelling on Tideway western section in September 2020.
Just weeks later, Ursula passed Tower Bridge, making it the 21st Thames bridge to be tunnelled under by Tideway. In December it finished its 7.6km journey, completing the main central tunnel of the sewer. Ursula remained underground until May 2021, when it was lifted out by a 1,800t marine crane and sent back to the Netherlands, where parts may be recycled for future TBMs.
Ursula completes the main tunnel of the central section
The completion of the main drives was by no means the end of tunnelling though, with many connection tunnels still to be completed and shafts to be filled.
December 2020 saw Annie being set off on its 4.5km journey to create a connection tunnel between Greenwich Pumping Station and the main sewer.
May 2021 saw Tideway celebrate the completion of its final shaft, the 64m deep shaft at the King Edward Memorial Park site in Wapping - the biggest of the 21 shafts on the project. The concrete pour into the shaft was carried out in July – a total of 780m 3 of concrete to form the slab at the bottom.
Work on Tideway's largest shaft at King Edward Memorial Park
TBM Charlotte had completed tunnelling on the Frogmore Connection Tunnel in February 2020, but it wasn’t until June 2021 that the secondary lining was inserted. This is because of the relative narrowness of the tunnel - an internal diameter of 2.5m - which made social distancing impossible.
Millicent completed a 75m connection tunnel at Carnwath Road in Fulham in July 2021. However, while the contactors BMB initially planned to pull the TBM out, the plan was changed at the 11th hour to bury Millicent as it would be more efficient and less distracting from other works.
Another 30m connection tunnel at Albert Embankment was completed by FLO in September.
Another enormous vortex tube was installed at Acton Storm Tanks in November 2021. The 18m long, 28t tube was lowered into place by a 200t mobile crane and is supported and horizontally braced on two levels.
December 2021 saw the concrete plugs at either end of the Blackfriars Bridge Foreshore broken out, signalling the next step in the secondary lining of the main tunnel.
Blackfriars Bridge Foreshore is broken through to begin secondary lining
The final connection tunnel on the project, a 4.5km tunnel between Greenwich and Bermondsey, was completed by Annie in April this year, after 16 months of boring.
Tunnelling for the project was announced as 100% complete at the end of April. London-based composer Rob Lewis livestreamed a performance from within the tunnels to celebrate the occasion.
Although tunnelling is complete, Tideway will not be operational until 2025. The remaining works include secondary linings throughout the 25km of tunnels, shoring up the connections between all the routes and testing.
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Tagged with: Bachy Soletanche Balfour Beatty BAM Nuttall Ferrovial Laing O'Rourke Morgan Sindall Tideway Vinci
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