HS2 S1-S2 Northolt Tunnel Shafts Dewatering

Project overview

The Northolt Tunnel is a key segment of the HS2 high-speed railway project, connecting West Ruislip to Old Oak Common via an 8.4-mile (13.5 km) tunnel. The western section of the tunnel spans 5 miles (8 km) from West Ruislip Portal (WRP) to the Green Park Way Ventilation Shaft (GPWVS) in Greenford, following the Chiltern Main Line. Along this route, multiple ventilation shafts, including South Ruislip Vent Shaft (SRVS) and Mandeville Road Vent Shaft (MRVS), are crucial for tunnel operations and safety.

Client

Skanska Costain Strabag (SCS JV)

Project owner

HS2 Ltd

WJ was engaged to provide dewatering solutions to support excavation and ensure groundwater pressures remained stable at the tunnel portal and ventilation shafts.

Project Scope and Challenges for the Northolt Tunnel Shafts Project

Given the challenging hydrogeological conditions along the tunnel alignment, WJ was tasked with designing and installing dewatering systems to ensure safe excavation. Key challenges included:

  • Managing groundwater pressures within Chalk strata at depths of up to 70m.
  • Dewatering the Lambeth Group sands and Harwich Formations using ejector systems.
  • Implementing in-shaft dewatering at GPWVS to facilitate sprayed concrete lining (SCL) construction.
  • Overcoming site constraints and sewer discharge limitations through groundwater recharge wells.
WJ’s Innovative Approach

WJ delivered a tailored dewatering solution that included:

 

  • Deepwell Systems: Installed to depressurise Chalk strata at all shaft sites, ensuring stable excavation conditions.
  • Ejector Dewatering Systems: Implemented at all sites except SRVS to manage water levels within the Lambeth Group sands and Harwich Formations.
  • In-Shaft Dewatering at GPWVS: A specialised system was deployed to achieve full drawdown within the Lambeth Group sands, enabling safe SCL construction.
  • Recharge Wells for Sustainability: Abstracted water was reinjected into the Chalk aquifer, reducing environmental impact and cutting costs associated with sewer discharge.
Innovative TBM Reception System

The two earth pressure balance tunnel boring machines (TBM’s) Sushila and Caroline, excavating the western section of the Northolt tunnels, will arrive in a specially designed reception chamber, or “can,” filled with foam concrete—an innovative solution developed to counter the site’s high groundwater pressure. The reception chambers allow the TBMs to maintain their pressure during arrival. Sealant is applied to the tunnel lining to prevent water ingress before the chamber is depressurised. Once complete, the TBMs are opened and lifted out of the ground.

The TBMs working on the Northolt Tunnel will be extracted via the two shafts at GPWVS, making the site a pivotal part of HS2’s construction journey.

A Key Contribution to HS2’s Success

WJ’s groundwater management solutions ensured safe and efficient excavation across multiple tunnel shafts, allowing TBMs Sushila and Caroline to be constructed and launched from WRP without delays. By integrating recharge wells, WJ also provided significant cost savings, preventing excessive sewer discharge fees and supporting sustainable water management.

The WJ Difference

At WJ, we provide more than just water management—we deliver tailored, site-specific solutions that support complex infrastructure projects. Our involvement in the HS2 Northolt Tunnel project highlights our ability to combine technical expertise, sustainability, and cost efficiency in high-profile engineering works.

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