Chester’s drainage infrastructure needed future-proofing but the city’s ancient walls posed a challenge
Chester Northgate is a new market and leisure destination, and the biggest development Chester has seen in decades. The multi-million pound redevelopment is at the heart of Chester's One City plan to deliver a sustainable, future-proof city. The existing drainage scheme was at capacity and new drainage infrastructure was needed. However, designing a tunnel within the city’s historic walls was a first for Cheshire West and Chester Council. With no previous tunnelling experience in their team, the Council had to call in the experts.
Working together with the Council, the OTB Engineering project team navigated complex engineering, geological and archaeological challenges and deftly liaised with local and national stakeholders to secure the requisite consents in record time. They delivered a quality, value engineered project that is on track to complete in early 2022 and has already saved the Council 5% of its project cost.
Working alongside Vinci and Donegan, OTB Engineering was appointed by Cheshire West and Chester Council to provide NEC3 contract project management, project supervision and technical advice to the council on tunnelling issues.
Originally, the design included eight shafts and a tunnel with an internal diameter of 1.0m to 1.2m with an outfall into the River Dee running through the historic town. When the OTB Engineering team looked at the proposed route, they could see that they could bring their wealth of experience to bear to make improvements that would clearly benefit the Council and the people of Chester.
Value engineering design within the city walls
Working within this historic city setting, there was a unique range of site constraints to deal with including made ground, glacial till, Sherwood sandstone, services (gas, electricity, drainage, sewers, telecommunications), man-made obstructions, archaeology, proximity of adjacent buildings and structures, potential for settlement and an underpass. OTB Engineering was able to value engineer the design to work despite these complexitie
Excavation of sandstone in underpinned shaft
Four calculated design upgrades that paid off
OTB Engineering thrives on challenge and this project was no exception. Four design recommendations changed the fortunes of the project for the better:
Altering the route of the drainage system
Eliminating one of the shafts
Adjusting the tunnel alignment
Reducing the carbon footprint of the scheme by re-designing the shaft construction methodology and so reducing the amount of concrete used
The original route started at the Princess Street/St Martins Way junction, followed St Martins Way and Nicholas Street to Grosvenor Roundabout, then tracked along Grosvenor Road and down Castle Drive into the River Dee.
However, there was an existing Welsh Water critical asset running down Castle Drive. OTB advised amending the route and taking out one of the shafts.
Chester City Walls close to tunnel crossing point
Removing the shaft reduced the overall project risk. But it brought significant challenges that needed to be swiftly overcome if the benefits were to be realised. The new route would take the tunnel under the historic City Walls, which is designated as a scheduled ancient monument. There was nervousness that the work might damage the wall. Archaeologists would need to be onsite in case the work unearthed important Mediaeval or even Roman finds. And getting the right permissions in time could risk the project running behind schedule and running over budget.
To carry out this change to the design and tunnel under the historic walls required Scheduled Ancient Monument consent. Permission was required from the Secretary of State, Historic England and the landowners - the University of Chester. There was some concern at the time about whether the approval would be granted soon enough to remain on programme. Thanks to the hard work of all the parties involved, the whole process took two weeks instead of six months.
In addition, OTB recommended altering the vertical tunnel alignment. This ensured the tunnel would be within the sandstone as it passed beneath the City Walls making the tunnel boring process easier.
OTB Engineering began its work in February 2020 and is set to conclude in March 2022. The project has remained on programme and thanks to OTB’s input, the council made a saving on the Off-site drainage project cost of around 5%.
1.2m Mini Tunnel Boring Machine (MTBM) at shaft S3