The interior of the Chiltern Tunnel looking out to the south portal.

Chiltern Tunnel

The Chiltern Tunnel is the longest tunnel on the HS2 route between London and Crewe. The twin-bore tunnel is 10 miles (16 km) long and will carry passengers under the Chiltern Hills.

Two Tunnel Boring Machines (TBMs), Florence and Cecilia, are being used to create the tunnel. The two enormous 2,000 tonne machines were delivered to their launch site at the Chiltern Tunnel South Portal, next to the M25 in Buckinghamshire, meticulously reassembled, tested and commissioned by an expert team of tunnelling engineers, before they started work on the high speed rail project’s first tunnels.

The two TBMs that are being used to create the ‘twin bore’ Chiltern Tunnel were launched in the summer of 2021. TBM Florence completed her drive in February 2024 and TBM Cecilia completed her drive shortly after in March 2024.

Watch our webinar on building the Chiltern Tunnel

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HS2 tunnelling in Buckinghamshire

Infographic map showing a below ground view of the HS2 tunnels in Buckinghamshire.

Tunnel construction

The enormous 2,000 tonne machines were built in a factory by world-leading German tunnelling specialists Herrenknecht. After completion the machines were disassembled before beginning their long journey to England. They were reassembled at our contractor’s main site to the west of London, just inside the M25. On arrival at site they were assembled by the entrance to the tunnels, where a specially prepared head wall allowed them to safely begin their excavation.

Designed specifically for the geology of the Chilterns, each machine is a self-contained underground factory, digging the tunnel, lining it with concrete wall segments and grouting them into place as it moves forward. A crew of 17 operatives keep the machines running, working in shifts and supported by over 100 people on at the main compound, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Each of the separate northbound and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete wall segments – which are all being made at the south portal of the tunnel, next to the M25.

Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels and used for landscaping around the south portal site. Once construction is complete, this will help create around 90 hectares of wildlife-rich chalk grassland habitats. Chalk grassland used to be widespread across the hills of south east England and are considered habitats of international conservation significance with just 700ha left across the Chilterns.

Chiltern Tunnel facts and figures

Diameter of cutter head. – 10.26m
Internal diameter – 9.1m
Length machine – 170m
Weight machine – 2000 tonnes
Number of segments required – 56,000 for each tunnel
Number of rings placed – 8,000 per tunnel
Weight of a segment – 8.5 tonne average
Dimensions of the segments – 2m x 4m
Volume of material excavated along the length of the Tunnel excavation – 3 million m3.
All the spoil has been processed through the STP, which has 24 filter presses. The largest facility of its type operating in Europe.
Average number of operatives working on machine each shift – 17.
Average number of support staff working on each shift – 30.
Average distance travelled each day – 16m.
Best production figures (Oct 2023): 44m in one day, 232m in one week, 842m in one month.
Difference in elevation between start point and end point is 110m.
1205 was named Florence after Florence Nightingale, the founder of modern nursing, a name chose by the students from Meadow High School in Hillingdon and selected following a public vote.
TBMs include innovations that have never been included on TBMs before – Krokodyl/Dobydoo.
Semi-continuous boring, as the TBMS do not need to pause to install rings.
5 shafts, 38 cross passage and 4 adits.
Track slab has been laid a few 100ms behind each TBMs so MSVs can pass. This improves productivity and efficiency, especially relating to servicing the TBMs and the cross passage construction.
Chalk from the tunnels is being used to create a 127ha of chalk grassland at the South Portal.
As we process the spoil on site this has removed the need for thousands of truck movement on local roads, saving an estimated 12,000 tonnes of CO2

Cross passage construction

In August 2022, construction of HS2’s longest tunnels begun a new phase with the start of work on thirty-eight cross passages linking the northbound and southbound tunnels.

An expert team of miners used a remotely controlled excavator to break out of and excavate from one running tunnel to the adjacent tunnel. As the excavation is progressed the ground is supported using a sprayed concrete lining (SCL).

Delivered by HS2’s main works contractor, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – each passage is between 15 and 20 metres in length.

Once the team had completed the SCL lined tunnel, a water-proof membrane was installed followed by a secondary concrete lining constructed by placing concrete behind formwork installed in the cross passage.

Once the passageway is complete, the safety doors can be installed at either end. Each running tunnel has an emergency walkway that allows passengers to safely evacuate the train and walk through the cross passages into the other tunnel where they can be rescued on a passenger train. Access for emergency services is from either the portal or one of the five ventilation/emergency access shafts.

Slide 1 of 8: TBM Cecilia completes her 10-mile dig with spectacular breakthrough at Chiltern Tunnel North Portal.

Slide 2 of 8: TBM crew wave through the cutter head after the breakthrough of Cecilia.

Slide 3 of 8: TBM Florence completes her 10-mile dig with spectacular breakthrough at Chiltern Tunnel North Portal.

Slide 4 of 8: TBM crew wave through the cutter head after the breakthrough of Florence.

Slide 5 of 8: Chiltern tunnel curving out of sight.

Slide 6 of 8: South portal of the Chiltern tunnel after launch of the TBMs .

Slide 7 of 8: Chiltern tunnel with crosspassageway visible to the right.

Slide 8 of 8: Aerial shot of the south portal site.

Transforming the site

The largest project in our Green Corridor programme – Chilterns South Portal Chalk Grassland Project – will see the transformation of what is now an HS2 construction site into one of the largest areas of new chalk grassland in the Chiltern hills.

Chilterns South Portal Chalk Grassland Project