Secondary lining work across Tideway’s sites is ramping up after the project reached the halfway mark for lining the full 25km super sewer in spring.
This stage of works comes after tunnelling and primary lining work was completed on the super sewer in April this year, after four years of construction.
The now complete sewer tunnel runs from east to west London and is around 70m at its deepest. Construction has been taking place at 24 sites across the city.
While the primary lining is formed of precast segments that form a ring, the secondary concrete inner shell is poured in situ by a machine called a shutter. Shutters are moved into position, concrete is pumped around the edge and set – before the shutter is moved along to create another section of secondary lining.
A joint venture of Ferrovial Construction and Laing O’Rourke is delivering tunnelling works at Tideway’s Kirtling Street, which is the central point of the super sewer.
The team finished secondary lining the west tunnel, which goes 5km towards Fulham, in September and completed the east tunnel, which runs 7.6km underneath central London, in May this year.
This has involved a total of 1,000 pours across the 12.6km stretch of tunnels from Carnwath Road to Chambers Wharf. Four shutters were used on the central section, working concurrently on the west and east tunnels. Up to 300m3 of concrete was batched daily during the operation and around 75,000m3 was poured in total.
The poured secondary concrete lining is 250mm and the primary lining is 350mm thick. This gives the tunnel an internal diameter of 7.2m. It is required to give the super sewer extra strength and to create a smooth surface over which the flows from the sewer network can travel once the tunnel is operational.
As well as completing the lining works six months ahead of schedule, the thickness of the lining was reduced, which in turn reduced the concrete needed in the central section by 15,000m3 – creating a saving of approximately 20,000t of CO2.
The team is now working on secondary lining the 30m in diameter and 55m deep shaft at Kirtling Street, which is the biggest across the whole project because it was the only shaft which launched two tunnel boring machines.
It has begun to lay a waterproofing membrane against the shaft walls to protect the diaphragm wall that forms the shaft.
As explained by Ferrovial tunnel project manager Ignacio Tognaccini-Sainz, this work is crucial to prevent the wastewater going through the wall during the functioning of the sewer.
He continued: “What we're doing is creating this membrane and installing it to create a waterproof layer so that it makes the shaft and the whole tunnel itself totally independent from the ground. We don't want to get our sewage water introduced into the aquifers because ultimately that will mean polluting the environment.”
The team is now concrete lining the shaft walls using a moving formwork system. This allows them to pour concrete in “lifts” starting at the bottom of the shaft. Once installed, the lining will be around 1.1m thick.
Tognaccini-Sainz added: “The whole formwork system is backed by a series of hydraulic power backs that essentially are going to allow us, by pressing a few buttons, to get the whole formwork raised. So, you pour one lift, and then you wait for the concrete to cure and then you strike it and then you move it up to the next level.”
Over the next few months, the team will work with Tideway, the company delivering the scheme, to inspect the whole tunnel, after which the structure will be certified.
It has already inspected the concrete regularly for any holes or cracks since completing the tunnel lining drive, especially as heavy plant was brought back through the tunnels which could damage the concrete.
The team has a specification in place that determines which cracks it should repair and which not to.
Tideway’s Carnwath Road Riverside site in Fulham has passed a milestone with the completion of secondary lining of its 100m long west tunnel section in August.
Meanwhile, secondary lining of the 5km long central tunnel section to Kirtling Street is 92% complete. The west main tunnel broke through into the central section in July 2021. The team has now started to take the shutter system out of the tunnel to allow it to cast the final 13 sets of shutters.
Secondary lining of the Frogmore connection tunnel, a 1.1km connection tunnel linking Wandsworth to the main super sewer, was completed in June last year.
Balfour Beatty’s Thames Tideway Tunnel joint venture with Bam Nuttall and Morgan Sindall (BMB) is delivering the tunnelling works at the Fulham site, which is one of the main TBM drive sites. There are a total of three tunnels at the site which cover a total distance of 7km.
BMB’s Carnwath Road Riverside section manager John Clifton explained how the site team had adhered to the highest quality standards when lining the tunnel.
He said: “The secondary lining provides the lifespan which is 120 years. On such a big project that’s quite unusual. The normal lifespan would be around 60 years. That’s why there’s such a high spec and extensive testing.”
Indeed, material testers regularly test the concrete, which is pumped from the top down into the tunnel, for quality issues.
The high specification requirements also extend to the tunnel design itself, which stipulates that each primary lining tunnel segment weighs 4.3t. Every tunnel ring is then made up of eight individual segments and these have a 35mm lead allowing the rings to follow the curves of the tunnel alignment.
In fact, the tunnel segments are unique in that each one has its “own identity”. Clifton continued: “We know exactly where it is in the tunnel, its exact position, so if there are any quality control issues, we’ll be able to identify them.”
The segments are also “unusual in that they don’t have any bolts on them. Instead, there’s a round plastic dowel that hold them together.”
“This makes the logistics of delivering the segments in place much easier because they don’t have a left-hand and right-hand bolt.”
Logistics have been the main challenge during the secondary lining works at the Fulham site. The concrete is transported into the tunnel using small trains or “bullet” cars and is then put into the shutters. Each bullet holds 130m3 of concrete, each train has two bullets, and two trains fill one shutter.
The team normally completes 51m of tunnel lining about every 48 hours, and according to Clifton, they are “on schedule”.
The Fulham team has shared its knowledge with the Thames Tideway east team at Chambers Wharf, which this week celebrated a milestone as the shutter for tunnel secondary lining was moved into the main tunnel.
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