Milestone as Chiltern HS2 Tunnelling Is 90% Complete

Florence and Cecilia, the giant tunnel boring machines that are being used to hollow out the Chiltern Tunnels for HS2, are 90% of the way through their journey.

Florence and Cecilia, the tunnel boring machines that are being used to hollow out the Chiltern Tunnels for the HS2 scheme, are 90% of the way through their journey. New Civil Engineer reports that the 2,000 tonnes machines have been working their way through the 16km journey for the past two years and are set to complete next year.

The Chiltern Hills present one of the most geographically and technically challenging sections of the HS2 route between London and Crewe. The twin-bore tunnel will have a combined length of 20 miles to allow for the passage of southbound and northbound trains. It is the longest tunnel on the route, running from the M25 to South Heath in Buckinghamshire.  

The names of the tunnel boring machines, Florence and Cecilia, were suggested by students at Meadow High School in Hillingdon and The Chalfonts Community College. They were inspired by the founder of modern nursing, Florence Nightingale, who lived locally, and also the pioneering astronomer and astrophysicist, Cecilia Payne-Gaposchkin.

So far, the machines have excavated approximately 2.8M.m3 of chalk and flint, and are designed to make adjustments for the geology of the area. Each machine is operated by a crew of 15 workers who work in shifts and are supported by a large surface team, who oversee the progress and the operations. 

The tunnels are lined with concrete segments as the machines progress, which are sealed into place with grout. Along the way a couple of setbacks have been experienced as a sinkhole occurred in November. These form when water erodes the chalk and it replaced with a less dense material. 

HS2 Ltd head of delivery Mark Clapp said the team is “making great progress in the Chilterns, with 90% of the tunnel excavation now complete. That’s an incredible engineering achievement and I look forward to the breakthrough, next year.”

Align underground construction director Didier Jacques added: “With our first TBM Florence having reached our fifth shaft at Chesham Road and our second TBM Cecilia due to reach the shaft shortly, this a great achievement for not only the tunnelling team, but also the supporting teams on the surface at the south portal, manufacturing the concrete segments required to line the tunnels and processing the spoil from the tunnels.”

“We are looking forward to continuing the good progress with the TBMs, which are due to complete their drives early next year.”

Four shafts have been excavated along the tunnel route to provide ventilation and emergency access, and work is ongoing to complete the headhouses that will provide surface access points. These will be carefully designed to blend into the surrounding countryside.

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