Tideway floats largest structure ever to pass along the Thames

Utility Week speaks to the team behind a two-year project to float a 3,700-tonne culvert across the Thames – a very public engineering feat that was successfully carried out this month.

As the UK’s largest ever water infrastructure project, Tideway has already marked more than its fair share of engineering ‘firsts’.

One very public record was broken in the small hours of 31 August when the team behind London’s super sewer successfully floated the largest ever structure to take to water on the Thames at the Tideway Blackfriars site.

A 100 metre-long , 3,700-tonne culvert, which will connect to a combined sewer overflow (CSO) from the River Fleet, was transported 100 metres across the river at Blackfriars.

Aside from the logistical challenges of floating such an immense structure, the route took the team over the top of Waterloo & City line and under Blackfriars Bridge.

To add to the pressure, the whole Tideway project was derailed by the Covid lockdown, which has added nine months and £233 million to the overall Thames Tideway programme.

Utility Week spoke to project managers Peter Rouzel and David Izquierdo about the unique challenges presented by this project and how they overcame them.

The culvert, which is 6.5 metres high and the same in width, has been constructed over a two-year period in a cofferdam – a watertight enclosure that enables construction below the waterline. Ordinarily the cofferdam would have been constructed to span the length the culverts would need to cover to divert the CSO to a new shaft which will ultimately connect to the main Tideway tunnel. However, after analysis showed the potential for the cofferdam to cause movement to the river wall and utilities the innovative idea of floating the first culvert into place was hit upon.

Following its construction, tests were undertaken to ensure the culvert was watertight and would float. In the background designing was undertaken to model this short but complicated trip, using scale models.

The float could only be attempted under very particular weather conditions. An initial float date in May was set back by Covid meaning the end of August was the next available window.

Rouzel says: “We had quite a tight timeframe to work as we could only do the float between 30 August and 7 September. If we’d had bad weather all the way through that period we would have been looking at waiting another month.”

Luckily, the bank holiday weekend brought clement conditions and on the morning of Sunday 30 August the culvert began its slow journey across a distance that, for unfair comparison’s purposes, Usain Bolt could cover in 10 seconds.

The first stage was to flood the cofferdam and allow the tide to pick up the structure for an initial 15-metre move. This was conducted from 11.30am, when there was minimal traffic on the river.

The structure was then set down as the tide receded and left for 12 hours until the bulk of the move could be completed.

At just before midnight that day, the final 90 metres of distance was covered, taking the culvert to a concrete mattress acting as a prepared riverbed. The culvert was locked in place via a wheel underneath that fits into a vertical rail, called a catcher, on the mattress. Currently the structure is ballasted by a series of butterfly valve which allow water in. This is required until the structure is complete.

The procedure involved inching under the bridge using a guide-wheel on the side of the structure, with a specially designed GPS system pinpointing the exact point it would lock in.

Even after the final resting place was located (at c 12.45am on the Monday) the team was still working against the clock. The culvert was now sitting in front of the CSO outflow and parts of a bulkhead needed to be removed to avoid any obstruction. The flow will now be taken into the culvert and turned east through a doorgate.

Izquierdo says: “It went really smoothly and we finished exactly when we planned to do. It’s amazing because it’s a really short time but there are years and years of planning leading up to it. It’s a great achievement for all the different teams that worked on it.”

Now it is in place there is a further two years’ work to complete the structure, which was kept as slim as possible to enable the float. Its height will eventually reach 9.5 metres, with the addition of the roof structure and landscaping, while cladding also needs to be added. The extra weight will mean that the culvert no longer needs to be ballasted by water.

The cofferdam will now be dried out to continue construction on the adjoining culvert, which will connect to the Low level 1 Bazalgette sewer which is within the river wall. The online shaft works will recommence once the tunnel has been completed

Rouzel explains: “The Blackfriars project is unique in that it’s the only site where a shaft will break through the tunnel. That was decided because this project was so complex we wanted to de-link it from the wider work.

“Half of the shaft is done already but we need to wait for the tunnel to be completed

“When the tunnel goes through to Chamber’s Wharf then that tunnel team come back and they use foam-filled concrete to fill a section underneath our shaft. Then we will go down with spray concrete lining and break through, do the base slab, come back up with the secondary lining and vortex structures and then up to the capping beam.”

The Blackfriars project is due to be ready for commissioning at the end of 2023, when landscaping work will be undertaken on what will be known as Bazalgette Embankment.

Rouzel and Izquierdo are clearly passionate about the product and justifiably proud of their achievements. What can we learn from it for other major infrastructure projects?

Rouzel says: “There are so many unique elements to this because we were finding solutions to problems no one has ever faced before. The challenge of not being able to construct it in the dry, of crossing the Waterloo & City line. There was the cutting rig that was created to cut the tubular piles that form the eastern cofferdam.

“A lot of this will be of value to other parts of Tideway or other projects. But the unique set of challenges at Blackfriars presented unique solutions.”