Mammoet was called on to remove four tunnel boring machines (TBM) on the outskirts of London, UK. The TBMs, manufactured by Herrenknecht, had been used to excavate tunnels for High Speed 2 (HS2) – the rail project linking Birmingham and London.
In collaboration with Skanska Costain Strabag JV and Herrenknecht, Mammoet designed and built a customised gantry solution that enabled the removal of the TBM as a single piece. This method saved time and reduced dismantling operations within the shaft by bringing lift operations to the surface.
Four TBMs were used to excavate the 8.4-mile (13.5-km) Northolt Tunnel section of the HS2 route. The tunnel consists of two (twin) tunnels. In each tunnel, two TBMs were launched from the opposite ends of the route, digging towards each other until they met at a central point – the Green Park Way vent shaft site, which has two shafts.
The ventilation shafts are 35 m above the tunnels – where all the TBMs were extracted from once they had completed tunnelling. Mammoet’s intention was to remove the biggest components – the cutter head, front and middle shield – in a single lift so that they did not need to be taken apart.
The challenge was lifting them out, particularly on a site just a few metres away from two live railway lines, in a busy city and with other civil works taking place nearby.
Design of the custom-built gantry system began around nine months prior to execution. Once approved, it went into a detailed engineering phase of five months. The gantry was then constructed onsite over a one-month period.
While much of the equipment used to construct the gantry system already existed, it had never been assembled into this arrangement. There was also an element of fabrication required to realise its design and give it the ability to lift a maximum weight of 900 tonnes.
The gantry was mounted onto a hydraulic skidding system, which allowed it to move backwards and forwards so that it could be positioned over the openings of the two vent shafts. It was also designed with an equaliser beam containing an inbuilt swivel to bring everything to a central point. This enabled all components that were lifted to be rotated.
“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. This would require a lot of manual handling. We created a system that could be remote controlled for ease of operation,” said Sam Ellwood, lead engineer at Mammoet. “This avoided us having to use taglines and winches – and besides, we would have had to get close to the load to attach items to it. Our solution allowed us to be ‘hands off’, so that everything could be kept isolated. This made for a safer operation.”
Mammoet added that this manoeuvrability was of particular importance as the design of one of the vent shafts differed from the other. Beneath the ground, it undercut the nearby railway track.
Moreover, while the underground HS2 line runs parallel with this train line, the skidding track could not. Therefore, a slight turning operation was required, mid-lift, to put the TBM section in the right orientation for skidding.
Once each TBM section was successfully lifted out of the vent shafts, the gantry was moved and positioned over 32 axle lines of SPMTs, parked between the tracks of the skidding system.
Holding a steel cradle to support the sections during transit, the SPMTs then drove them to an area onsite for disassembly, lowering them to ground level for further dismantling using climbing jacks. This reduced the need to work at height when removing the hydraulic rams, drive motors and other ancillary equipment inside.
