The Costain, Vinci Construction Grands Projets and Bachy Soletanche joint venture required excavation support to install an air treatment unit at Tideway’s Deptford Church Street site in Lewisham.

The air treatment unit (ATU) will control and treat air exhausted from London’s 25km long “super sewer” tunnel.

The Costain, Vinci Construction Grands Projets and Bachy Soletanche JV has installed the ATU alongside a 60m deep, 16m diameter shaft that will carry effluent from street level to the main sewer tunnel below.

Groundforce Shorco, in turn, supplied a sheet piling and structural support solution for the ATU excavation.

“When the sewer is operating, air will build up inside and needs to be vented,” explained section engineer Freddie Gorman. “We can’t just let it come straight out; it needs to be filtered. When the shaft is completed, it will be capped and any air trapped inside will be fed through the ATU, where it’s treated. It then goes into a ventilation chamber before being vented out into the atmosphere.”

The ATU excavation is located in a confined area adjacent to the main shaft and comprises three separate sections.

To support the excavation, Groundforce supplied KD6 sheets and Larssen sheet piles with its Mechshore, Maxi and Mega Bracing frames. Three levels of frame were installed, with the larger capacity Mega Brace frames located at the bottom to support the higher loads.

Groundforce area manager Arpad Nagy explained that the temporary works design was complicated by the fact that the three sections of the excavation, each a different size and depth, were constructed back to back.

“The frames work with and against each other and are interdependent. That means we couldn’t make any changes to one frame without affecting all of the others,” he said.

The steel sheets lining the excavation also act as the outer skin of the formwork required to pour the reinforced concrete walls of the ATU chamber.

“The original plan was to use formwork to construct the ATU walls, but by pouring the concrete against the sheet piles the project cost has been reduced,” Nagy added.

The complexity of the excavation, combined with varying ground levels and the proximity of the main shaft, meant the temporary works design process was challenging.

“The whole evolution of the temporary works design took around seven months and went through eight different versions,” says Arpad. The first design meeting took place in April 2021, and the first delivery of equipment to site was not until December 2021.

Another challenge for the site team was the extremely hard and stony ground conditions on the site. This made it difficult to install the steel sheet piles just with the excavator-mounted vibrating hammer, therefore a pre-augering method had to be adopted. A number of sheet piles still remain to be installed.

The proximity of the existing shaft caused further issues.

“We were tasked with designing a system to allow the removal of the sheets which were blocking the connection, as well as providing enough clearance to allow the concrete walls to be fully cast. We came up with using a trapezoidal frame in order to transfer loads to the existing capping beam, followed by a secondary temporary frame propping off the top of the capping beam in order to allow the removal of the frames below,” explained senior engineer Ross Hope.

The job has now moved onto a second phase – excavation to receive a long culvert that channels the filtered air from the ATU to the ventilation chamber.

“This excavation will be just as complicated, but we’ve learned a few things in phase one, so I’m sure we can provide a suitable solution,” Nagy said.

Located next to the historic St Paul's Church in Deptford, the ATU is an element of the £4.3bn Thames Tideway Tunnel project. It will form a combined sewer running mostly under the tidal section of the River Thames across London to capture, store and convey 95% of the raw sewage and rainwater that currently overflows into the Thames.

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