|Location||United Kingdom (Buckinghamshire, Hertfordshire)|
|Coordinates|| (north portal)
|OS grid reference|
|System||High Speed 2|
|Start||West Hyde, Hertfordshire|
|End||South Heath, Buckinghamshire|
|Work begun||13 May 2021|
|Traffic||High-speed passenger trains|
|Length||16.04 km (9.97 miles)|
|No. of tracks||2|
|Track gauge||1,435 mm (4 ft 8+1⁄2 in) standard gauge|
|Electrified||25 kV 50 Hz AC|
|Operating speed||320 km/h (200 mph)|
|Width||9.1 metres (30 ft)|
The Chiltern Tunnel is a high-speed railway tunnel currently under construction in Buckinghamshire and Hertfordshire, England, and will upon completion carry the High Speed 2 (HS2) railway line under the Chiltern Hills. The twin-bore tunnels, which are 16.04 km (9.97 miles) long, will be the longest on the HS2 line. Each tunnel will also have additional 220 m (720 ft) entry and 135 m (443 ft) 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 is to be largely performed by a pair of tunnel boring machines (TBMs), is advancing at an average speed of 15 m (49 ft) per day; it is anticipated that it will be complete in March 2024. As of September 2023[update], the first TBM had excavated a total distance of 13 km (8.1 miles).
The construction of the High Speed 2 (HS2) railway is to involve numerous major civil engineering works along its intended route. Construction periodical New Civil Engineer has described the Chiltern Hills as "HS2’s biggest and potentially most challenging site". It has been anticipated that, of the 102 km (63 miles) of tunnels that are to be bored to accommodate the line, around 16 km (9.9 miles) will be bored for the Chiltern tunnel alone.
The design of the Chiltern 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 2.6 km (1.6 miles) north; this revision has allowed for the replacement of a separate planned cut-and-cover tunnel, as well as avoiding the destruction of around 12 hectares (30 acres) of woodland such as Farthings Wood. During 2017, Contract C1 (Central 1), which covers the 21.6km section of the line that the Chiltern tunnels 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.
The basic configuration of the Chiltern tunnel will consist of a pair of parallel bores, which are to be excavated by a pair of tunnel boring machines (TBMs). Each TBM weighs around 2,000 tonnes, has a length of 170 m (560 ft), 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 will have 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 will be active at a time while within close proximity 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 will be 16.04 km (9.97 miles) long, with the northern portal near South Heath. At their deepest point, they will be 90 m (300 ft) below ground level; the internal diameter of each bore will be 9.1 m (30 ft). The completed tunnel will have a total of forty cross passages linking the two separate bores and five 18 m (59 ft) diameter shafts used for both ventilation and emergency access purposes between 35 m (115 ft) and 65 m (213 ft) deep. These are located (in order from South to North) at Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden  and Chesham Road - the latter of which is just 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, and after several iterations the design is at its final approval stage.
The tunnel will have porous portals (a form of tunnel entrance hood) that extends for up to 220m 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.
In July 2020, work was completed on a 17 m (56 feet) 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 has been constructed on site; this site will also be 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. During March 2021, the first section of fibre-reinforced concrete tunnel lining was cast; this is produced onsite at a purpose-built facility near to the southern portal, which is capable of manufacturing up to 49 sections at a time. A total of 56,000 precisely-engineered sections will be required to complete each tunnel.
Excavation commenced on 13 May 2021, with completion estimated around March 2024. During the following month, it was reported that the first TBM, which was being operated around the clock, had been advancing at a typical speed of 15 m (49 ft) per day. Each TBM is operated by a crew of 17 working in 12-hour shifts, while surface logistics and support for the boring work is provided by over 100 people on the surface. During June 2021, the second TBM commenced boring as well. The two TBMs were named Florence (after Victorian social reformer and nurse Florence Nightingale) and Cecilia (after astrophysicist Cecilia Payne-Gaposchkin) following a national vote on names suggested by local schools.
The first stage of the drive (from the south portal to the Chalfont St Peter ventilation shaft) 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 and Little Missenden in August the same year.
The chalk that has been excavated during the boring of the tunnels is to be used to create a nature reserve covering 127 hectares (310 acres) in the vicinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to 3,000,000 cubic metres (110,000,000 cu ft) 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 small 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 3m to 4m deep, near Shardeloes Lake, Amersham in May 2023, although an investigation has been launched by the Environment Agency into the causes of the ground collapse.
As of 19 September 2023[update], TBM Florence, the first machine to be launched in May 2021, had progressed a distance of 13.126 km (8.156 miles) from the south portal – 82% of the bore length. TBM Cecilia, the second machine to be launched in June 2021, had completed 12.709 km (7.897 miles) – 79% of the bore length. Both TBMs are currently south of the Buckinghamshire village of Hyde Heath, having passed under the A413 road close to Little Missenden. Both tunnel boring machines are expected to break through at the North portal at South Heath in early 2024.
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