Joseph Gallagher Ltd
Guildford Sewage Treatment Works
Segmental caisson shaft under construction at Guildford STW, with precast concrete rings installed to form the permanent shaft lining for the new wastewater transfer system.
Project Overview
The relocation of Guildford Sewage Treatment Works (STW) is a major Thames Water infrastructure project that will facilitate the Weyside Urban Village regeneration scheme, delivering approximately 1,500 homes alongside commercial and community facilities.
To transfer flows from the existing treatment works to the new STW, Thames Water required a new gravity-fed wastewater conveyance system. Joseph Gallagher Ltd (JGL), working for the BAM-Enpure Joint Venture (BEJV), was appointed to deliver the specialist shaft sinking, tunnelling and associated civil engineering works forming the core of the transfer network.
The completed system comprises 1.375km of 1500mm internal diameter tunnel and seven deep shafts, providing the infrastructure needed to support the relocation of the treatment works and the future redevelopment of the Slyfield site.
Scope of Works
The transfer tunnel was constructed entirely within London Clay at depths ranging from 10.6m to 15.7m below ground level. The tunnel conveys wastewater from the southern end of the existing STW to a new inlet shaft at the relocated facility. JGL’s scope included:
- Design and construction of seven segmental caisson shafts.
- Construction of 1,375m of 1500mm ID tunnel.
- MTBM and pipe-jacking operations.
- Hand-dug shield tunnelling works.
- Installation of associated pipelines and structural elements.
- Future connection provisions for additional wastewater assets.
The shafts ranged from 4m to 12.5m in diameter and up to 20m in depth, serving as launch and reception points, intermediate drives, maintenance access locations and future tie-in structures.
Segmental caisson shaft installation at Guildford STW, forming part of the new wastewater transfer tunnel.
Construction Challenges
The works were delivered within a constrained urban environment adjacent to historic landfill boundaries, requiring careful environmental management and construction planning. Key challenges included:
- Working near historic landfill sites.
- Managing deep excavations in variable ground conditions.
- Constructing beneath a major public highway.
- Maintaining precise tunnel alignment for gravity flow operation.
- Safely connecting to existing live wastewater infrastructure.
Ground gas monitoring, vibration control measures and strict dust and odour management procedures were implemented throughout construction. Segmental caisson construction techniques were used to minimise settlement risks and maintain shaft stability.
Tunnel Construction
JGL constructed the tunnel using a combination of Micro Tunnel Boring Machine (MTBM) and hand-dug shield methods across five separate drives, including one curved alignment requiring advanced guidance systems and continuous monitoring.
Closed-face MTBM technology was selected to maintain ground stability and control groundwater ingress while achieving the precise tolerances required for long-term gravity flow performance. Intermediate jacking stations were incorporated on longer drives to manage thrust loads and maintain installation efficiency.
A particularly challenging section involved the final connection into the existing Guildford Main sewer. The connection was completed using a hand-dug shield excavation into a live-flow chamber approximately 10m below ground level, requiring temporary works, careful excavation sequencing and continuous flow management.
Guildford STW wastewater transfer network
Outcome
The completed shafts, tunnel and associated infrastructure form the backbone of the new wastewater conveyance system serving the relocated treatment works.
The scheme provides a durable and future-proof wastewater transfer solution, enabling the decommissioning of the existing STW and unlocking the wider Weyside Urban Village regeneration project. In addition, the new infrastructure has been designed to accommodate future network connections, supporting Guildford’s long-term wastewater strategy and future growth requirements.
Conclusion
Joseph Gallagher Ltd successfully delivered a technically complex tunnelling and shaft construction package that is fundamental to the Guildford STW relocation programme. Through the construction of seven deep shafts and 1.375km of gravity tunnel, JGL provided the critical infrastructure required to transfer flows to the new treatment works, supporting both Thames Water’s long-term operational objectives and one of Guildford’s most significant regeneration projects.
For More Information, contact us at https://www.josephgallagher.co.uk/
