HS2 has successfully completed the complex Tunnel Boring Machine (TBM) ‘Sushila’ extraction at the Green Park Way site in Greenford, West London. The massive machine was lifted from the ground on Saturday, March 15, following its completed tunnelling mission beneath the capital.
The engineering operation involved removing the TBM’s cutter head, front shield, and middle shield—weighing a combined 850 tonnes—in a single lift operation. Engineers employed a specialized 750-tonne gantry crane to extract the machine components from the ventilation shaft where the TBM had completed its journey.
The extraction marks the final phase of Sushila’s operational life on the Northolt Tunnel project, concluding its underground journey in December 2024. The TBM’s removal required precise planning and coordination to bring the massive components to the surface safely.
“Extracting a machine of this size presents significant engineering challenges,” explained Malcolm Codling, HS2’s Client Director for London Tunnels. “The teams had to carefully plan each aspect of the lift to ensure the structural integrity of both the machine and the shaft during the extraction process.”
How Tunnel Boring Machines Work
Tunnel boring machines like Sushila function as complete underground factories, combining multiple engineering systems in one massive apparatus:
- Cutterhead System: The front section features a rotating cutter head (9.84m diameter for Sushila) equipped with cutting tools and scrapers. As it rotates, it grinds through soil and rock, with the specific tools configured based on expected ground conditions.
- Pressure Balance: The machine maintains precise pressure at the excavation face to prevent ground settlement or water inflow. In Earth Pressure Balance machines like those used for HS2, the excavated material is mixed with conditioning agents in a pressurized chamber before removal.
- Segment Erection: Hydraulic arms place pre-cast concrete segments in a ring formation behind the cutter head. These interlocking segments form the permanent tunnel lining. Each ring typically consists of several segments plus a keystone.
- Propulsion: Hydraulic jacks push against the recently installed ring to advance the machine. This creates a continuous cycle of excavation and lining installation.
- Logistics System: A sophisticated conveyor system transports excavated material from the face to the rear of the machine and out of the tunnel. Simultaneously, new segments, grout, and other supplies are transported to the front.
- Guidance System: Laser-guided systems provide real-time position data, allowing operators to make minute steering adjustments to maintain the planned alignment with millimetre precision.
- Life Support: The machine contains systems for ventilation, power distribution, water management, and communications to support the underground workforce.
The extracted components represent only part of the entire TBM assembly, which has impressive specifications:
- Total TBM length: 160 meters
- Cutterhead diameter: 9.84 meters
- Total machine weight: Approximately 2,050 tonnes
The extraction process required several preparatory steps:
- Securing the TBM in position at the bottom of the shaft
- Separating the cutter head and shields from the remainder of the machine
- Installing lifting points and structural supports
- Positioning the gantry crane above the shaft
- Executing the lift with millimetre precision to avoid shaft wall contact
James Richardson, Managing Director of SCS JV (the joint venture of Skanska, Costain, and STRABAG), noted: “The extraction of TBM Sushila demonstrates the technical expertise of our engineering teams. Lifting components of this size from an underground position requires exceptional planning and execution.”
The extraction follows the successful completion of Sushila’s tunnelling mission. Launched from West Ruislip in October 2022, the TBM completed its 5-mile journey in December 2024, breaking through at the Green Park Way ventilation shaft.
During its underground operation, Sushila:
- Excavated over 1.2 million tonnes of earth
- Installed 4,217 concrete tunnel rings
- Advanced at an average rate of 16 meters per day
- Operated continuously with teams working around the clock
The TBM was the first of four machines to complete its section of the 8.4-mile Northolt Tunnel, which will eventually carry high-speed trains between HS2’s Old Oak Common station and West Ruislip.
The TBM extraction was planned to minimize community disruption, with operations coordinated to reduce noise and traffic impacts in the surrounding Greenford area. Throughout its tunnelling journey, excavated material was transported to designated areas west of the tunnel, eliminating the need for removal by road transport.
These disposal areas will be repurposed as wildlife meadows and wooded areas, forming part of HS2’s Green Corridor initiative, creating ecological benefits along the railway route.
The machine holds local significance and is named after Sushila Hirani, a local school teacher selected by a community school to receive this honour. After extraction, specific components may be refurbished for future tunnelling projects, while others will be recycled, aligning with HS2’s sustainability commitments.
This extraction marks an essential milestone in the Northolt Tunnel project. Three additional TBMs—Caroline, Emily, and Anne—are currently operating in the tunnel and will complete their journeys at Green Park Way in the coming months.
With the UK Chancellor confirming in October 2024 that construction of HS2’s Euston Tunnel would proceed, two additional TBMs, Karen and Madeline, are being assembled in the underground box at the Old Oak Common station site to continue tunneling eastward.
The completion of the Northolt Tunnel, expected by the end of 2025, will connect with the tunnel section from Old Oak Common to Euston, completing the London portion of the high-speed rail network.
“The successful extraction of Sushila brings us one step closer to delivering this transformative infrastructure project,” said Codling. “Each milestone demonstrates our commitment to building a transportation network that will serve generations to come.”
TLDR:
- HS2 successfully extracted the 850-tonne TBM Sushila from the Green Park Way site in Greenford
- A 750-tonne gantry crane lifted the cutter head, front shield, and middle shield in one piece
- The extraction follows the TBM’s completion of a 5-mile tunnelling journey in December 2024
- Sushila is the first of four TBMs to complete its section of the 8.4-mile Northolt Tunnel
- The tunnel will eventually connect HS2’s Old Oak Common station to West Ruislip
- Three additional TBMs continue to work on the Northolt Tunnel project
