London’s ‘Super Sewer’ finishes underground construction

A Sarens gantry installs a 1,200-tonne concrete cover to the Thames Tideway Tunnel in London, UK. (Image: Sarens)

Sarens – a Belgium-based heavy-lift, engineered-transport, and crane rental services company – announced the completion of work, noting it was the contracting firm to place the 24-metre-diamater concrete lid at the Abbey Mills Pumping Station near Stratford, East London, UK.

“The long-awaited moment has arrived,” said Sarens. “Works to construct… London’s Super Sewer, including its tunnels and shafts, are now fully complete after installation of the final piece of the puzzle, a precast concrete circular cover weighing 1,200 tonnes and [measuring] 24m in diameter.”

The cover slab Sarens installed – as part of a joint venture with UK-based CBV – caps a shaft around 70m deep.

The company used a purpose-built SPMT-mounted gantry crane, which it used to lift and move the cover slab into place during an operation that lasted five hours.

“The cover slab is the heaviest piece handled on the Thames Tideway Tunnel project – even surpassing the lifting of Tideway’s six tunnel boring machines in the early stages,” said Sarens.

The next steps include a test of the system and running live storm sewage through the new infrastructure.

“But the capping of the Abbey Mills Pump Station shaft marks an absolutely critical milestone for the Tideway project and for London,” said Sarens.

Gantry assembly for Super Sewer took four weeks

While the cap assembly lasted just five hours, the preparation for the project’s ‘hat’ lasted nearly one month, said Sarens.

“The assembly of the SPMT gantry, which included main beams weighing 100 tonnes each, took four weeks,” confirmed the company. “[Fifty] trucks brought all the equipment to site.”

Working in cramped conditions, Sarens said cooperation among contractors and stakeholders was key.

“Coordination of the deliveries was critical as there was not room on site to store materials meaning they had to be incorporated into the gantry as they arrived at site,” said Sarens. “A CC2800 crawler crane was used to assemble the Gantry. Due to the very limited site space, this crane was selected; it could assemble the entire gantry from one position.”

Robert Frost, Sarens’ engineer, said of the final lift, “This operation to lift, transport and place the cover slab was a complex operation, and required careful collaboration between various teams working on this project.”

What is London’s Super Sewer? 

The Super Sewer is a 25 km-long, 7.2m diameter, sewer tunnel designed to dramatically reduce sewage pollution in the central London River Thames. The scheme is a joint venture between main contractors Costain, Bachy Soletanche, and Vinci working for Tideway; the company building the super sewer for Thames Water.

The project was announced nearly one decade ago with a price tag of approximately £4.2 billion (US$5.2 billion). As of 2024, it’s estimated to cost $6.2 billion.

The goal of the civil megaproject is to capture, store, and transport raw sewage and rainwater that overflows into the River Thames estuary. The initiative, Sarens said, is meant to improve the health of the river.

“In a typical year, tens-of-millions-of-tonness of storm sewage spill into the River Thames, but once fully operational… the new infrastructure will reduce those spills almost completely,” said the company.

Measuring 7.2-m wide, the tunnel runs majority under the Thames from just south of Ealing, West London, to the Stratford, East London, region. A second tunnel, named the Lee Tunnel, will connect the Abbey Mills Pumping Station to Beckton Sewage Treatment Works near Newham.

“The shaft at Abbey Mills is the point at which the Super Sewer connects to the Lee Tunnel, which will take the sewage flow out of London to the Beckton sewage works for treatment,” explained the company.

The Thames Tideway Tunnel is planned to be commissioned and fully operational in 2025.