Future of Tunnelling | Complex Tideway sewer connection makes progress at Blackfriars

Tunnel boring machine (TBM) launches and breakthroughs usually grab the headlines when it comes to delivery of new tunnel projects. But completion of the main drives is usually followed by equally complex and challenging work.

London’s new £4.5bn, 25km Tideway sewer is no different – while the TBMs may have finished their work last year, another two years of technically difficult work is still needed to bring the tunnel into operational use.

Since construction started in 2016, public concern about sewage discharges to watercourses in the UK has heightened. Tideway’s completion in 2025 will go a long way to improving water quality in the River Thames.

The tunnel runs from Acton in west London to Abbey Mills pumping station in east London and will intercept discharges from 34 combined sewer overflows (CSOs) before they enter the Thames. One of these is the Fleet Main CSO which has the dubious title of being one of London’s largest sewage discharge points.

Work at Blackfriars will divert the 521,000m³ a year of untreated sewage discharge from the Fleet Main CSO, as well as some of the discharge from Bazalgette’s Low Level 1 sewer – which runs below the Victoria Embankment – into the super sewer.

However, until the tunnel is operational, considerable temporary works are needed so that these CSOs can still discharge to the Thames while also being readied for the final connection into the Tideway system.

We needed to get the shaft started to prevent Blackfriars impacting on the tunnelling programme

Getting to this stage of the work at Blackfriars has been complex for main contracting joint venture Ferrovial Laing O’Rourke. It also called for considerable collaboration between contractor and client.

The challenges are partly due to the need to connect through the Grade II listed river wall structure and into Bazalgette’s original sewer, built in the 1870s. The stability of the wall during the work plus the sensitivity of a gas main running under it was also a major consideration. Another complication was the location of the Fleet Main CSO’s discharge point directly under Blackfriars [road] Bridge with limited headroom above.

At Blackfriars the Tideway tunnel runs below the river foreshore, just in front of the existing river wall and well below the current outlet for the Fleet Main CSO and the Low Level 1 sewer. The main focus of work at Blackfriars has been to connect those sewers to the main tunnel via a 22m diameter, 54m deep shaft.

The original plan was to carry out shaft construction using diaphragm wall techniques behind the safety of a twin walled cofferdam running the full length of the site. But this placed too much load on the river wall and the gas main. As a result, the plan had to undergo a major rethink as delivering the work at Blackfriars on time was critical to the overall Tideway project.

“You can’t complete Tideway unless we complete Blackfriars,” explains Tideway project manager for Blackfriars Bridge Foreshore Peter Rouzel. “This is why, when Blackfriars got really challenging, you couldn’t get rid of Blackfriars from the project as then Tideway wouldn’t achieve its aim of cleaning up the Thames.

“However, we needed to get the shaft started to prevent Blackfriars impacting on the tunnelling programme, but we also needed time to consider the engineering challenges of the wall and gas main.”

New approaches

The rethink resulted in three different approaches across the site – a western cofferdam, an eastern cofferdam and an innovative floated culvert.

The shaft was constructed in the twin walled western cofferdam using secant piling at the top followed by jet grouting and sprayed concrete lining to take it to the full depth as there was not enough room for diaphragm wall equipment within the western cofferdam area.

The eastern cofferdam was a “lighter” construction to avoid loading on the original river wall. It was formed from a series of 800mm diameter steel tubular piles with sheet piles between them to form a combi wall with additional ship protection at the front to allow the main CSO connections to be built.

Making the river level connection between the Fleet Main CSO and the new Tideway shaft required a 3,700t, 100m long precast concrete culvert structure that was floated into position onto a prepared section of the riverbed.

The structure was cast within the “dry dock” of the eastern cofferdam and floated from there into position. The floated structure reduced the load on the river wall compared with constructing a permanent cofferdam. The floating operation also removed the normal lifting needed to position such a structure. This worked with the lack of headroom below the bridge.

The floated culvert was flooded to position it on the riverbed but later pumped dry once further permanent concrete structures had been cast on top to weigh it down. A bulkhead prevents the CSO discharge from entering the Tideway works, while
a curved gate in the structure allows the CSO to keep discharging into the river until the Tideway tunnel is ready.

Even in dry weather there are flows of around 900litres/s in the Low Level 1 sewer

Although preparation work started at the Blackfriars Bridge Foreshore site in 2017, it was not until the floated culvert was positioned in September 2020 that the tricky process of making the connections could get underway.

The shaft and its breakthrough into the tunnel have now been completed, as have the two steel vortex structures that control the flow from each sewer into the tunnel and the cast insitu concrete culverts that connect the CSOs to the vortex structures. Rouzel refers to the shaft as a “concrete cathedral” due to its scale. Work to connect the CSOs was still progressing when NCE visited the site in May.

The connection into Bazalgette’s Low Level 1 sewer is almost ready with temporary works in place to allow for business as usual to continue until Tideway is ready.

Breaking into the 2.5m diameter Bazalgette sewer was a painstaking task and called for a 21m long steel liner, formed from 410 sections small enough to be brought into the sewer via manholes, to be installed first to allow the sewer to continue to operate while the connection was made.

Rouzel says that a temporary bulkhead was installed at Temple, just upstream from Blackfriars, to reduce the flow in the Low Level 1 sewer, reducing the risk of the work. “Even in dry weather there are flows of around 900litres/s in the Low Level 1 sewer,” he explains.

Removing Bazalgette's wall

With the liner in place, a row of 31, 23.9m long, 650mm and 750mm diameter secant piles was installed behind it to support the ground and allow an 18.5m section of Bazalgette’s granite block river wall to be removed. This allowed the stone built sewer and cable subway above the sewer to be demolished, exposing the steel liner.

“Museum of London staff were on site with a watching brief while we undertook this work,” says Rouzel. “The top three courses of stone [from the listed river wall] had to be numbered and tagged so that they could be put back once the connection had been completed.”

Reconstruction of the wall and a new cable subway structure have just been finished and temporary bulkheads are currently obstructing the opening in the cast insitu concrete structure that will link the old sewer with the new.

The only question Rouzel must now resolve there concerns the removal of the steel liner – it could either be left in place with the section in front of the connection chamber removed or it could all be removed completely.

Rouzel, more than most people, is hoping for good weather next spring as the work to make the Fleet Main CSO connection will call for some prolonged periods of dry weather.

Connection completion

“If it rains and there is flow through the Fleet, then we will have to remove all of our equipment, wait for the flow to stop and then assess the area before starting work again,” he says. He is aiming for the connection to be completed “no later than the first half of 2024”.

The exact timing depends on the commissioning of the tunnel as the flow can only be turned from the river and into the tunnel when the tunnel is ready to receive it. The finishing work involves removing the temporary bulkhead and permanently closing the curved gate, which will also act as shuttering for the casting of the permanent river wall. The wall will then be clad to match the rest of the new structure.

Before the work on the Fleet Main CSO is completed, a number of other milestones must be passed on site.

Many of these are key to getting it ready to become a new 6,000m² “micro park”, to be named Bazalgette Embankment, once Tideway is finished.

The final section of cast insitu river wall inside the western cofferdam was expected to be cast in late May.

The wall features 50mm reinforcement bars as it will act as ship impact protection for the park and Tideway’s assets below.

There is also a 2.5m gap between the inner side of the wall and the shaft and culvert structures, which Rouzel says will be backfilled with “soft” material to avoid the river wall overloading Tideway structures.

The river wall structure is being faced with 173 prefabricated panels with a 50mm granite covering that has been sourced from the same quarries that Bazalgette used for his river wall.

Another key change on site will come later this year when the cofferdams that have protected the site since 2017 are removed.

Rouzel says that the landscaping work on Bazalgette Embankment is likely to be complete by the end of 2024, but it will not open to the public until 2025 when the tunnel is opened. 

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