MECHANISED TUNNELLING/HS2 | TECHNICAL
Left: The 3 million m3 of chalk
excavated by the TBMs will be used to landscape the area once the construction site is decommissioned
potentiometers to divide the thrust between hydraulic rams. The system allows the TBM pilot to choose a position of centre of thrust and then hydraulic cylinders calculate the pressure required for the direction. This was done through a touch screen and removed
some of the headaches for the pilots, says Jacques. The continuous boring removed the stop-start of
installing the rings, helping Align to achieve some record-breaking advances. TBM Florence exceeded expectations, achieving an advance of 42m in a day; 226m in a week; and 842m in a month.
Jacques says the approach required adjustment by
the TBM crews until they became familiar and confident with the new automated processes. Project developer HS2 Ltd’s carbon reduction target
was a driver for further innovation on the contract. Align cut the volume of concrete needed for four of the tunnel’s five shafts by optimising the design so that the D-walls – which normally would have been temporary works with permanent walls constructed inside them – were designed and constructed as permanent works. This reduced the diameter from 31m to 17m. Caisson construction was used for the Chesham Road
intervention shaft. The design of the tunnel walkways was also reviewed
to cut the amount of material required. To reduce carbon emissions on energy and fuel, Align
agreed a deal with National Grid that the power for the TBM would be generated by nuclear and renewables in equal measure, and vehicles on site were fuelled by HVO (hydrogenated vegetable oil) rather than diesel where possible. Align introduced a polymer to replace the use of lime
to process the slurry at the slurry treatment plant. Introducing the polymer was a big step, says Jacques,
and it had the bonus of being safer, as well as more environmentally friendly. The tunnel cuts across groundwater source protection
zone 1 (SPZ1) and passes under the M25 motorway, the River Misbourne and the Chiltern railway. All but the last 200m of the drives were in chalk, with flint present, and 30% in dry chalk where the tunnel sits above the water table. Prior to tunnelling, all the features along the
alignment were mapped and safe stopping zones identified. That did not, however, prevent some surface depressions, or dissolution features, forming along the route.
July 2024 | 13 Left:
Cutterhead of HS2 Chiltern Tunnel TBM ‘Florence’ dismantled after successful completion of long drive
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