CHILTERNS The 16.1km-long Chiltern Tunnel is the longest underground section of the HS2 line and tunneling was undertaken by two 10.26m-diameter TBMs - ‘Florence’ and ‘Cecilia’ - boring the twin tubes. The contractor is Align JV, which comprizes


Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. The operate as an ‘integrated project team’ (IPT). ‘Florence’ broke through at the north portal in February


2024 followed by ‘Cecilia’ weeks later, in late March. They were variable density Herrenknecht machines, launched in mid-2021. Innovations outside the machines saw D-walls for


four of the tunnel’s five shafts being designed and built only as permanent works. Elimination of a two-step approach, with first temporary works, radically reduced the shaft diameters from 31m to 17m. The project also includes long concrete shell


extensions to the driven tunnels to run beyond the bored portals. With ventilation portholes along one side, they are designed to help manage airflow and minimize, or eliminate, the possibility of ‘sonic boom’ shocks created by the high-speed trains. Visually, they are interesting structures in their own right.


Top: TBMs ‘Florence’ and ‘Cecilia’ bored Chiltern Tunnel’s twin tubes over 2021-2024 Above: Portal extensions on Chiltern Tunnel to minimize ‘sonic boom’ risk from high-speed trains


26 | Winter 2025


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53