Introduction

The relocation of the Guildford Sewage Treatment Works (STW) represents one of Thames Water Utilities Ltd’s most significant infrastructure programmes in recent decades. The move facilitates the development of the transformative Weyside Urban Village, a mixed‑use regeneration scheme delivering around 1,500 homes, commercial units, and enhanced community amenities.

As a core contractor to the BAM‑Enpure Joint Venture (BEJV), Joseph Gallagher Ltd (JGL) delivered the complex tunnelling and shaft works required to transfer wastewater from the existing STW to the new facility. This involved constructing seven deep shafts and 1.375km of 1500mm ID tunnel, forming the backbone of the new wastewater conveyance system. JGL’s involvement was critical in enabling the long‑term strategy for wastewater management in Guildford and in unlocking the regeneration of the Slyfield area.

Project Overview

The new STW required a gravity-fed transfer tunnel, situated entirely within the London Clay strata between 10.6m and 15.7m below ground levels. This tunnel intercepts flows at the southern end of the existing works and directs them to the new inlet shaft on the STW site.

The works were further constrained by the site's history as a former landfill, requiring sensitive construction methods and stringent environmental controls. This environment demanded the highly specialist shaft sinking, tunnelling, and pipe-jack expertise for which Joseph Gallagher Ltd is widely recognised.

Scope of Works Delivered by Joseph Gallagher Ltd

Shaft Construction

JGL designed and constructed seven jacked caisson shafts positioned strategically along the tunnel alignment. These included shafts needed for:

TBM launch operations
TBM reception
Intermediate tunnelling drives
Future sewer system tie-ins
Access and operational needs

Key shaft characteristics

Quantity: 7 shafts
Diameter range: 4m to 12.5m internal diameter
Depth range: 6m to 20m deep, depending on location and function
Construction type: Segmental caisson shaft construction
Launch/reception pits: 7.5m ID for both launch and reception shafts

Shafts were constructed through a combination of made ground, historic landfill boundaries, and London Clay, requiring rigorous controls on ground gas, odour, stability, and vibration.

Tunnel Construction – Pipe‑Jacked and Microtunnelling Works

JGL delivered 1,375m of 1500mm ID tunnel using a combination of Micro Tunnel Boring Machine (MTBM) operations and hand‑dig shield methods.

The tunnel works included:

Five separate drives of varying lengths
One curved tunnel drive, which required advanced guidance systems
Intermediate jacking stations to control thrust forces on long drives
A closed‑face MTBM, chosen for ground stability and water tightness
Precision alignment to ensure gravity flow capability throughout the system

The tunnel alignment demanded exceptional control to maintain the gradient, navigate London Clay stiffness variations, and avoid displacing surrounding landfill materials.

Hand‑Dug Final Connection Section

The final metres of tunnel required a high‑precision hand‑dig shield to safely connect into the existing Guildford Main:

Connection to a 10m - deep live-flow brick and concrete chamber
Temporary works to stabilise the excavation
Controlled excavation to avoid disturbing continuous wastewater flows

This represented one of the highest‑risk sections of the tunnelling programme and highlighted JGL’s expertise in confined‑space tunnelling.

Additional Pipeline and Structural Works

In parallel with tunnelling and shaft sinking, JGL delivered:

600mm ID auger‑bored connection (30m)
450mm ID precast concrete pipeline – 40m open-cut section
Installation of ductile iron pipework
Installation of precast concrete cover slabs on shafts ranging from 4m to 12.5m in diameter
Casting of structural benching and reinforced concrete (RC) walls within shafts
Installation of galvanised access equipment for long-term operational maintenance

These works form the permanent structural and operational components of the wastewater conveyance system.

Challenges and Technical Innovations

Confined Construction Environment

Working within one of Guildford’s busiest zones required meticulous traffic planning, phased works, and tightly controlled access to minimise disruption.

Working Adjacent to Historic Landfill

Although tunnelling occurred in London Clay, many shafts were close to historic landfill boundaries. JGL adhered to strict protocols in:

Ground gas management
Vibration limits
Odour and dust control
Avoiding mobilisation of landfill materials

Variable Ground Conditions and Deep Excavations

The shafts spanned multiple geological horizons, with depths of up to 20m:

Water control systems were required during sinking
Segmental caisson construction mitigated settlement risks
Clay behaviour varied significantly between shafts, requiring adaptive methodologies

Technical Precision for Future Network Integration

Three additional assets—Bowers Farm Main, Stokes Road syphons, and Abbotswood syphons - required future connections. Achieving millimetre‑level accuracy was critical to facilitating future tie‑ins with minimal operational disruption.

Contribution to the Overall Project Vision

Joseph Gallagher Ltd tunnelling and shaft package from the central infrastructure element enabling the relocation of Guildford’s STW. Without this deep conveyance system:

Flows cannot be transferred
The existing STW cannot be decommissioned
The Weyside Urban Village regeneration cannot proceed

JGL’s safe and efficient delivery has:

Reduced environmental impacts in a sensitive former landfill setting
Maintained programme momentum for BEJV and Thames Water alike
Provided long‑term, durable infrastructure essential to Guildford’s wastewater strategy
Demonstrated industry‑leading tunnelling excellence

Conclusion

Joseph Gallagher Ltd has delivered a technically complex, environmentally sensitive, and safety‑critical tunnelling package that underpins the success of the wider Guildford STW relocation programme. Their expertise in shaft sinking, MTBM tunnelling, road crossings, and high‑risk hand‑mined connections has ensured the reliable transfer of flows to the new facility, enabling the regeneration of Slyfield and supporting Guildford’s future growth.