Groundforce Shorco has supplied an innovative sheet piling and structural support solution for the installation of an Air Treatment Unit (ATU) at Tideway’s Deptford Church Street site in the south London borough of Lewisham.
Located next to the historic St Paul's Church in Deptford, this is an important element of the £4.4 billion Thames Tideway Tunnel project, a 25km combined sewer running mostly under the tidal section of the River Thames across London to capture, store and convey 95 per cent of the raw sewage and rainwater that currently overflows into the Thames.
At Deptford, a joint venture comprising Costain, Vinci Construction Grands Projets and Bachy Soletanche is currently installing the ATU, which will control and treat air exhausted from the sewer tunnel. The ATU is being installed alongside a massive, 60m-deep, 16m-diameter shaft that will carry effluent from street level to the main sewer tunnel below.
“When the sewer is operating, air will build up inside and needs to be vented,” explains Freddie Gorman, section engineer. “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 60m-deep 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.
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, as Groundforce area manager Arpad Nagy explains:
“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.”
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 main reason why we were awarded this project is the engineering support, communication and cost saving on formwork,” says Arpad. “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.”
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, yet rewarding.
“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 impossible to install the steel sheet piles just with the excavator-mounted vibrating hammer, therefore a pre-augering method had to be adopted.
Section C provided an engineering challenge due to the proximity of the existing shaft.
“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” explains senior engineer Ross Hope.
Freddie Gorman is pleased with progress so far. “We like to use companies that can offer a design service and hire the equipment - Groundforce is one of those. Their equipment and service has been excellent, and Arpad has been really helpful throughout.”
The job now moves 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,” says Arpad.