The recent extraction of tunnel boring machines (TBMs) from the
Northolt Tunnel project presented a formidable challenge, particularly given the urban setting and proximity to live railway lines. Dutch heavy lifting specialist
Mammoet, working in close partnership with
Skanska Costain STRABAG JV and TBM manufacturer
Herrenknecht, devised a bespoke gantry solution that transformed the approach to the TBM removal.
Traditionally, dismantling such large machinery underground involves significant safety risks due to confined spaces, heat, and fumes from cutting equipment. Mammoet’s strategy was to eliminate these hazards by shifting the dismantling process above ground.
Darren Watson, sales director at Mammoet, said: “Different approaches were considered at the start. One option was that the TBMs would be split and disassembled within the shaft and the component parts lifted out by a crane. However, this would have meant people working down there in a very congested space, with ventilation issues to overcome and a large crane taking up a significant portion of the compound area above, interfering with other works on site.”
Instead, Mammoet proposed a custom-built gantry system capable of lifting the cutter head, front, and middle shield of the TBMs in a single operation. This approach not only reduced the time required for extraction — from weeks to a single day — but also significantly improved safety by minimising underground work. The gantry was mounted on a hydraulic skidding system, allowing it to move between two vent shafts located 35m above the tunnels. It featured an equaliser beam with an inbuilt swivel, enabling remote-controlled rotation of the lifted components.
“Sam Ellwood, lead engineer at Mammoet, explained: “Normally, if that was any other piece of equipment, you would have to attach another system and exert external force onto the load to turn it. We created a system that could be remote controlled for ease of operation. This made for a safer operation.”
Safer dismantling of internal partsThe gantry’s manoeuvrability was critical, especially given that one of the shafts undercut a nearby railway track, requiring a mid-lift turning operation to align the TBM section for skidding. Once lifted, the gantry positioned the TBM sections over 32 axle lines of self-propelled modular transporters (SPMTs), which transported them to a disassembly area on site. There, climbing jacks lowered the components to ground level, allowing for safer dismantling of internal parts such as hydraulic rams and drive motors.
The entire extraction programme was a complex feat of engineering and coordination. Richard Dexter, managing director of
www.herrenknecht.com/en Herrenknecht International Ltd, said: “The two — and, more widely, four — site teams had to erect and commission the system at the start of the year, and then, several months later, coordinate lifting in 12hr windows during weekend railway possessions. The total works needed four 24-hour lift and carry operations. These were, in turn, based upon four week-long preparations, two month-long assemble and disassemble periods and eight months of intense engineering, design, planning, structural analysis, materials and weld testing — as well as possessions and community engagement.”
The success of the operation demonstrates Mammoet’s ability to deliver tailored engineering solutions that reduce time, cost, and risk. Mr Watson added: “Herrenknecht and Mammoet were aligned in our goals and worked closely together. As a team we wanted to remove the load in the largest piece possible, removing the need for size reduction, and extracting the TBM in as little time as possible.”
Looking ahead, Mammoet sees potential for the gantry system to be deployed in other projects requiring heavy lifting in confined or urban environments. Mr Dexter said: “It has been one of the smoothest real-time demonstrations of relationships developed over many years, brought into play to solve a considerable challenge. Together, all of the teams yielded large time savings for the project, in the form of early access for follow-on tasks of several months in each of the four tunnels.”