Chiltern Tunnel
The Chiltern Tunnel is a high-speed railway tunnel in Buckinghamshire and Hertfordshire, England, which will carry the High Speed 2 railway line under the Chiltern Hills. The twin-bore tunnels, which are long, are the longest on the HS2 line. Each tunnel also has additional entry and exit perforated concrete portals to reduce sudden changes in air pressure and subsequent noise.
A contract for the tunnel's construction was awarded in 2017; preparatory work commenced during the following year. In May 2021, it was announced that excavation had commenced. The boring process, which was largely performed by a pair of tunnel boring machines, advanced at an average speed of per day; both TBMs completed their drives by March 2024. Civil engineering work on the tunnel was completed by January 2026.
History
The construction of the High Speed 2 railway involves numerous major civil engineering works along its intended route, with construction periodical New Civil Engineer describing the Chiltern Hills as "HS2’s biggest and potentially most challenging site". Of the of tunnels that are to be bored to accommodate the line, around were bored for the Chiltern Tunnel alone.The design of the tunnel has been subject to changes during the planning stages of the project. During August 2015, it was announced that the tunnels would be extended north; this revision has allowed for the replacement of a separate planned cut-and-cover tunnel, as well as avoiding the destruction of around of woodland such as Farthings Wood. During 2017, Contract C1, which covers the 21.6km section of the line that the Chiltern Tunnel comes within, was awarded to the Align JV joint venture, comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. The two tunnel boring machines were supplied by Herrenknecht and were delivered to the site in December 2020.
Design
The basic configuration of the tunnel consists of a pair of parallel bores, which are to be excavated by a pair of tunnel boring machines. Each TBM weighs around 2,000tonnes, has a length of, and has been specially customised to suit the local geology, which primarily consists of chalk and flint. Staff are conveyed between the surface and the TBM using people carriers, which had up to a one-hour transit time towards the later years of the boring. While both TBMs are intended to be operational simultaneously throughout the majority of the work, due to sensitivities surrounding the M25 motorway, only one TBM was active at a time while near to this key trunk route.The southern entrance to the tunnels is aligned with the nearby Colne Valley Viaduct and is near the M25. The pair of tunnels are long, with the northern portal near South Heath. At their deepest point, they are be below ground level; the internal diameter of each bore is.
The tunnel has a total of 38 cross passages, between and long, spaced apart, and 5 adits linking the two separate bores. The tunnel has five diameter shafts used for both ventilation and emergency access purposes between and deep. These are located at Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road - the latter of which is an intervention shaft for emergencies. These vents are all to be fitted with headhouses, produced through engagement with both the Chilterns AONB Review Group and Buckinghamshire Council. The surface elements of the St Giles shaft have been intentionally disguised as agricultural buildings. The initial design of the Amersham vent shaft headhouse was modified after being criticised due to its design being reportedly out of keeping with the location. Each tunnel has an emergency walkway that would allow passengers to evacuate and then cross into the other tunnel to be rescued on a passenger train.
The tunnel has porous portals that extend for up to beyond the end of the bored section in the hillside - this is to dissipate the pressure wave built up by trains at line speed, thus reducing the adverse effects of audible "sonic boom" in line with environmental commitments.
Construction
In July 2020, work was completed on a high headwall at the southern end, at, close to the M25 motorway. To accommodate workers during the construction phase, a temporary facility containing accommodation and various amenities was constructed on site; this site was also used to support the construction of the nearby Colne Valley Viaduct as well.During September 2020, final preparations for the arrival of the first of the two TBMs were made and both arrived on site in December that year. Names for the two machines were suggested by children from Meadow High School in Hillingdon and Chalfonts Community College, and Cecilia and Florence were chosen by a public vote. The names commemorate Buckinghamshire-born astronomer Cecilia Payne-Gaposchkin and Florence Nightingale. During March 2021, the first section of fibre-reinforced concrete tunnel lining was cast; this was produced onsite at the Align JV's purpose-built facility near the southern portal, which was capable of manufacturing up to 49 segments at a time.
Excavation commenced on 13 May 2021, with completion estimated around March 2024. Each TBM was operated by a crew of 17 working in 12-hour shifts, while surface logistics and support for the boring work was provided by over 100 people on the surface. During June 2021, the second TBM commenced boring. The two TBMs were named Florence and Cecilia following a national vote on names suggested by local schools.
The first stage of the drive was completed in March 2022, and the first of the cross passages between the main running tunnels were completed in August that year. The Chalfont St Giles ventilation shaft was reached in October 2022, Amersham in March 2023, Little Missenden in August, and Chesham Road in December of the same year.
The chalk excavated during the boring of the tunnels is to be used to create a nature reserve covering in the vicinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to of chalk slurry were provisioned onsite prior to excavation commencing. Approval for this scheme was issued in May 2021.
One of the tunnel bores was temporarily evacuated in May 2022 following a fire on a personnel transportation vehicle. No personnel were injured and the tunnel itself was undamaged after the fire was brought under control.
Tunnelling work was unaffected by the formation of a sinkhole, estimated to be deep and wide, near Shardeloes Lake, Amersham in May 2023, although an investigation was launched by the Environment Agency into the causes of the ground collapse. The resulting inquiry found that the local water supply was not contaminated, and the sinkhole was filled with of chalk in September 2023. Further work to ensure the hole is filled was completed in early 2024. Another sinkhole formed in November 2023 near Hedgemoor Wood, Hyde Heath, and two more followed in February 2024 adjacent to Frith Hill in South Heath, although tunnelling remained unaffected in both cases.
The cross-passages were excavated with a remote-controlled excavator and supported by a sprayed concrete lining, before a waterproof membrane and secondary concrete lining were applied.
Florence
TBM Florence broke through at the north portal in South Heath on 27 February 2024, with Cecilia following suit on 22 March. 112,350 concrete segments were used to line both tunnels, forming 16,050 rings.
On 19 January 2026, HS2 announced the completion of civil engineering works for the tunnel.