Inside the 55m deep shaft at Tideway’s Blackfriars site in London.

Tideway’s plans to clean up the River Thames and increase London’s sewer capacity have led to the involvement of formwork and scaffolding supplier Peri on many of its sites along the 25km main tunnel – or super sewer as it is known. This includes the 55m deep Blackfriars combined sewer overflow shaft, which will store and transfer sewage flows to the main tunnel. 

The shaft at Blackfriars introduced a new set of challenges for Peri compared to other shafts on the project. This is because the Blackfriars shaft intersects the main tunnel, so all structures were on the critical path for the overall project

Aside from the two vortex tubes that will control the flow of sewage down the shaft, a large aspect of the construction work in the shaft is the 1m thick secondary lining which will encase the waterproof membrane and strengthen the structure. 

Jumpform technology, in the form of a hybrid rail-guided climbing and single-sided climbing system from Peri has been used to help the formwork accommodate the changing shape of the shaft, which was formed by a combination of piling and jet grouting, and included tunnel connection structures.

For the proposed construction sequence, the customer required a concrete pressure of 40kN/m2. To achieve this, Peri’s engineers created special angled wall shoes that were compatible with the shaft’s circular geometry and capable of withstanding tension loads of more than 200kN.

Completing the work without modifying or removing the formwork from the wall during the process was made more challenging by the changing profile of the shaft. To further minimise operations on site, engineers ensured that special filler plates remained attached to the shutters throughout the climb. 

The first structures engineers had to accommodate during the secondary lining works were two 7.3m diameter drums situated opposite each other at the bottom of the shaft. The drums are temporary structures that will form openings from the shaft to the main tunnel once the foamed concrete currently sealing them has been broken down. 

The structure of each drum was formed using eight prefabricated shutters and Variokit components which enabled the whole system to retract from the concrete for easy striking. Due to the limited space inside the shaft, all shutters were prefabricated at Peri’s depot in Rugby and delivered to site ready to be joined together, minimising equipment and labour required. 

Installing the drums was a challenge as it required the structures, weighing 7.5t each, to be manoeuvred from a horizontal position on the ground to a vertical one ready to be erected within the shaft. 

Each drum was slung and lifted by crane before being tilted to one side where one set of crane hooks was repositioned on the other side to support its vertical descent into the shaft. 

It was important to keep the drums securely in position during the initial secondary lining pours. Engineers accounted for the self-weight of the drums by designing a cantilevered moving frame which comprised an 8t counterweight to overcome the uplift pressure of the concrete being pumped in. Six ties were installed from the bottom of the temporary structure through to the base slab to also counteract the uplift.

Despite the challenges, collaborative working and creative engineering have proved successful in the delivery of the secondary lining work so far.

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.

Sign in or Register a new account to join the discussion.