Dismantling large underground machinery has always presented challenges, but for the High Speed 2 (HS2) rail project, the task of removing massive Tunnel Boring Machines (TBMs) demanded a safer and faster approach. Working in confined shafts with heavy equipment, heat, and fumes exposes crews to considerable risks. Instead, Mammoet developed a bespoke gantry system to lift the biggest TBM sections to the surface in single operations, reducing time and improving safety.
The HS2 project, linking Birmingham and London, required the extraction of four Herrenknecht-manufactured TBMs from the Northolt Tunnel section in west London. Normally, TBMs are dismantled underground and removed piece by piece, a process that can take weeks. Mammoet, collaborating with Skanska Costain STRABAG JV (SCS JV) and Herrenknecht, proposed an alternative: lifting the cutter head, front shield, and middle shield in one move.
Time Savings in Urban Conditions
Each TBM section was removed in a single day rather than several weeks. The 8.4-mile twin Northolt Tunnels required four TBMs to excavate from both ends before meeting at the Green Park Way vent shaft, the only site where extraction was possible. With shafts 35 meters above the tunnels, Mammoet faced the challenge of lifting out components weighing up to 900 tonnes near live rail lines and dense civil works.
Darren Watson, Sales Director at Mammoet, explained:
“One option was to split and disassemble the TBMs inside the shaft. But that would have meant people working in congested spaces with ventilation issues, while a large crane would have restricted other works on site. Instead, we proposed a gantry system to bring lifting operations to ground level.”
Engineering the Gantry
The gantry design process started nine months before execution, followed by five months of detailed engineering and one month of construction on site. While much of the equipment existed, modifications and fabrication were required to assemble a configuration strong enough to handle loads of up to 900 tonnes.
The gantry was mounted on a hydraulic skidding system, enabling movement between the two vent shafts. An equalizer beam with a built-in swivel allowed loads to be rotated remotely without the need for manual handling. This eliminated the use of taglines and kept operators clear of suspended components.
Sam Ellwood, Lead Engineer at Mammoet, noted:
“Our solution allowed us to be ‘hands off.’ Everything could be remote controlled, which made for a safer operation.”
This maneuverability was essential at one shaft, which extended beneath a nearby railway track. Unlike the underground HS2 line, the gantry’s skidding track could not run parallel. As a result, mid-lift turning was required to orient the TBM sections correctly for skidding and transfer.
Lifting and Transporting
Once lifted, each section was placed onto 32 axle lines of Self-Propelled Modular Transporters (SPMTs), equipped with a steel cradle for transit. The SPMTs carried the loads to a designated disassembly area where climbing jacks lowered them for dismantling at ground level. This avoided working at height while removing motors, hydraulic systems, and other internal components.
The operation required extensive planning. According to Richard Dexter, Managing Director at Herrenknecht International Ltd:
“The teams had to coordinate lifting in twelve-hour windows during weekend railway possessions. The total works involved four 24-hour lift and carry operations, supported by months of engineering, testing, and community engagement.”
Looking Ahead
The project highlights how custom engineering can enhance safety and reduce project timelines. Mammoet’s gantry solution enabled early access for follow-on construction activities in all four tunnels, saving several months of work.
Both Mammoet and Herrenknecht view the collaboration as a model for future TBM extractions. The gantry system, designed for confined environments, may also serve in other heavy lifting projects where conventional cranes are not practical.
