Boring job makes exciting start
HS2 tunnelling is underway in the Chilterns, and Phil Marsh visits to find out more.
HS2 has released details about the Tunnel Boring Machines (TBMs) Cecilia and Florence, which have commenced work excavating the Chilterns to create the Up and Down lines respectively. Florence began operations on May 7 as Cecilia was still being assembled for a planned start date later in June. They will create two parallel 10-mile (16km) tunnels from the south portal at West Hyde, which is on the Hertfordshire/Buckinghamshire border north of Denham. The tunnels are also just to the north of the two-mile (3.3km) Colne Valley Viaduct that is contained within the same worksite. Together, this part of the HS2 project is known as Area C1, and is being worked by a joint venture between Bouygues Travaux Publics, Volker Fitzpatrick and Sir Robert McAlpine that totals 13.5 miles (21.6km) of high speed rail infrastructure.
The 136-acre Area C1 worksite has its own slip roads to and from the adjacent M25 motorway, which avoid the need to use local roads, and this is how many of the TBMs’ component parts were delivered to site.
Three-year job
The two TBMs are due to finish the twin 9.1m-diameter tunnels at the end of 2024, moving forward by about 15m a day. Each machine comprises six gantries (sections), including an emergency refuge chamber in Gantry 3 that can accommodate 12 people for up to 24 hours. They are essentially 2000 ton, 170m-long linear factories that will work around the clock every day of the year.
To maximise staff efficiency and working hours, a pop-up hotel has been built onsite with capacity for 50 TBM operatives, avoiding the need for travelling time to/ from other hotels or homes. At the start of June, around 750 people worked on site, but this will rise to a peak of 1200 when work expands to constructing five emergency tunnel vent shafts, the Colne Valley Viaduct, and other associated work. The Chiltern Tunnels will use more than 112,000 concrete segments within the 16,000 rings that form each of the tunnel linings (seven segments per ring), and all of these will be manufactured on-site. Around 300 segments had been produced by the end of May. Each has a barcode attached so that it can be checked for its date of manufacture, which of the 49 moulds was used, when it was installed
in the tunnel, and then also for ongoing examination once HS2 is open. Every 24 hours, 196 concrete segments (28 complete rings) will be installed.
TBM Florence, which will also be cutting the emergency connecting cross-tunnels, had a planned slow start to ensure the processes and machinery were working as expected. By May 28, 35 rings were in place and Gantry 6 was still in view at the tunnel mouth.
The six gantries
As the boring work is carried out, tunnel segments are put in place by Gantry 1, creating a structurally watertight ring to support the ground load. These TBMs have been specifically designed for the Chilterns’ chalk and flint mix, and are known in the trade as ‘Variable Density Type’ borers – these being the first of such used in the UK.
The TBMs run on rails at ground level, but underground road wheels on retractable axles are lowered to replace the traditional railway wheels.
The cutting edge comprises ten 350kW motors producing 20.1kNm of torque to turn the cutter head. Just behind these are the concrete segments, which are loaded onto an erector arm that places them into position as the tunnel walls. Close behind in Gantry 2 is a robot that removes the wooden spacers between the concrete segments and then inserts locking connection dowels.
Gantry 3 contains the emergency refuge chamber and welfare cabin, closely followed by a grout manufacturing area in Gantry 4. A power supply cable wheel and air compressors are contained within Gantry 5, while Gantry 6 houses the ventilation cassettes and slurry pipe extension system.
The excavated material passes along the TBM and is converted into slurry that will be reused to landscape the site. While standing next to Florence, it was possible to hear the flint rattling along the screw conveyor extraction pipe under the rails.
It is estimated that three million cubic metres of spoil will be dug out and placed to the south east of the tunnel mouths where a massive site is being prepared to accept the spoil – including ground drainage arrangements.
Much of the tunnelling will be below the water table, and so this was subject to careful planning to ensure the right processes are in place. Should a TBM hit a water-filled void, the ground can be stabilised by filling it with previously extracted slurry, which is then solidified around the TBM to create the stability. ■