Railways Illustrated

Twin-bore HS2 sisters Florence and Cecilia

- Phil Marsh

THE THREE-YEAR HS2 Chiltern tunnelling project commenced on May 7 when Tunnel Boring Machine Florence started cutting the line from the south portal. Sister TBM Cecilia is scheduled to commence boring the other line by the end of June and both are expected to progress northwards by 15 metres every 24 hours, taking three years to complete the project if working 24/7. HS2 Area 1C includes this 16km tunnel and the 3.7km Colne Valley Viaduct, totalling 21.6kms of highspeed rail infrastruc­ture and is a Joint Venture between Bouygues Travaux Publics, Volkerfitz­patrick and Sir Robert Mcalpine. The two TBMS will carve out around 3m3 of spoil from the 9.1m diameter bores, including the connecting evacuation cross tunnels and five ventilatio­n shafts for smoke or fume extraction in case of an emergency. They were specifical­ly designed for the Chilterns’ chalk and flint mix and are known as Variable Density Type borers.

Testing the TBM procedures meant a slow and steady start by Florence, with 35 concrete rings in place after three weeks, leaving 16,008 to go! Each ring is made up of seven segments, all manufactur­ed on-site, and by the start of June, 300 segments had been produced using the 49 moulds, which have to cure for 28 days before being used. Each segment carries a barcode used for quality monitoring during manufactur­e, installati­on and maintenanc­e once the line opens. Each TBM is 170m long, weighs 2000 tons and is a mobile production line, with six linked gantries running on rails above ground and rubber wheels undergroun­d using a system of retractabl­e axles. Gantry No 3 contains the emergency 24-hour safety refuge with food, water and oxygen for up to 24 people, plus a mess facility.

Each day’s work has been calculated to install 196 concrete segments, creating a ground-load structural support. Excavated material is removed by a screw conveyor and pumped into the site for reuse in a major rewilding project. The potential effects of tunnelling under the water table were carefully investigat­ed and subsequent­ly planned by HS2, Affinity Water, and the Environmen­t Agency. Should a water-filled void be struck it will be stabilised using extracted slurry to fill it, which will then solidify.

 ??  ?? Tunnel Boring Machines Cecilia (left) and Florence (right) on June 1, with the latter already disappeari­ng into the Chilterns. (Phil Marsh)
Tunnel Boring Machines Cecilia (left) and Florence (right) on June 1, with the latter already disappeari­ng into the Chilterns. (Phil Marsh)

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