The HS2 Chiltern Tunnel is a 10-mile twin-bore tunnel beneath the Chiltern Hills, running from the Colne Valley near the M25 to South Heath in Buckinghamshire. It is the longest tunnel on the HS2 route and took nearly five years to build.
This guide covers the tunnel's route, construction method, portal design, environmental impact, and the contractors who delivered it.
The HS2 Chiltern Tunnel is a 10-mile (16km) twin-bore tunnel beneath the Chiltern Hills, stretching from the Colne Valley near the M25 to South Heath in Buckinghamshire. It is the longest tunnel on the entire HS2 route between London and the West Midlands.
Twin-bore means two parallel tunnels running side by side, one carrying northbound trains and one carrying southbound. Each bore holds a single track. High-speed rail projects typically use twin-bore designs because they allow safe evacuation through cross passages and make maintenance easier without shutting down the whole line.
The tunnel passes through chalk geology beneath the Chilterns Area of Outstanding Natural Beauty (AONB), which is a nationally protected landscape. Civil engineering works wrapped up in January 2026 after nearly five years of construction. The tunnel sits within Contract C1, one of HS2's Main Works Civils Contracts covering a 21.6km section of the line.
A surface route through the Chiltern Hills was never really an option. Building above ground would have destroyed ancient woodland, cut through communities, and triggered fierce political opposition. The AONB designation, which protects the landscape for its natural beauty, made tunnelling the only path forward.
Even then, the tunnel kept growing. During parliamentary scrutiny of the HS2 Bill, the tunnel was extended 2.6km north in August 2015.
This extension replaced a planned cut-and-cover section and saved around 12 hectares of woodland, including Farthings Wood. Every extension reduced environmental and political risk, but it also added cost.
The contract numbers tell the story clearly. When tenders were first invited, Lot C1, covering both the Chiltern Tunnel and the Colne Valley Viaduct, was estimated at £800m to £1.3bn. The contract was awarded to Align JV in 2017 at £1.6bn.
By the time the Oakervee Review examined HS2 in 2019–2020, the civils programme had escalated by roughly £6bn across all lots. The review flagged spoil disposal from the Chiltern Tunnel as a major cost driver. Deep cuttings on either side of the tunnel produced enormous volumes of chalk spoil that could not be reused directly.
HS2 found a creative solution: using 3 million cubic metres of chalk to create new chalk grassland habitat on the Colne Valley Western Slopes. Clever, yes, but it added both cost and complexity to an already stretched budget.
The tunnel runs from its south portal near the M25 in the Colne Valley to its north portal at South Heath, tucked between Great Missenden and the village. The entire route passes beneath the Chilterns AONB.
Five ventilation and emergency access shafts sit along the route, each serving a different purpose:
Chalfont St Peter: First shaft south of the tunnel midpoint
Chalfont St Giles: Shaft with a headhouse designed to look like agricultural buildings
Amersham: Central shaft location along the route
Little Missenden: Shaft serving the northern section of the tunnel
Chesham Road: Final shaft before the north portal
The shafts range between 35m and 65m deep, each with an 18m diameter. Grimshaw Architects designed the surface headhouses to blend into the surrounding landscape. At Chalfont St Giles, the headhouse is intentionally disguised as farm buildings, so it does not stand out against the rural backdrop.
For a broader view of where the Chiltern Tunnel fits within the full HS2 route, the HS2 Route Map provides an interactive overview of all stations and tunnel sections.
Two tunnel boring machines (TBMs) named Florence and Cecilia excavated the twin bores. Built by Herrenknecht, each machine weighed around 2,000 tonnes. A TBM is essentially a self-contained underground factory: it bores through rock, lines the tunnel with concrete segments, and grouts them into place, all in one continuous operation.
Crews of 17 operatives worked each shift, supported by over 100 people at the main compound near the M25. On average, the machines advanced 16 metres per day. Florence broke through at the north portal on 27 February 2024, and Cecilia followed on 22 March 2024.
The numbers give a sense of scale:
112,350 concrete segments lined both tunnels
16,050 rings formed from those segments
38 cross passages between 15m and 20m long, spaced 500m apart, connecting the two bores for emergency evacuation
5 adits linking the tunnels to the ventilation shafts above
A typical TBM drive covers 5–6km before the machine is removed or repositioned. This 16km drive pushed well beyond normal limits and required technical innovations never before used in the UK. The machines launched from a site near the M25 in May 2021 and worked continuously for nearly three years.
The south portal near the M25 features something new for UK rail: a porous portal. This is a curved, tapered concrete structure with perforated walls designed to reduce the pressure wave, sometimes called "tunnel boom," that forms when high-speed trains enter a tunnel. Without a porous portal, trains entering at 200mph would create a sonic boom effect that disturbs nearby communities.
Portal | Location | Key features |
|---|---|---|
South | Colne Valley, near M25 | Porous portal structure, calcareous grassland roof, dark steel louvres with aluminium fins |
North | South Heath, near Great Missenden | Hidden low into landscape, earth-coloured zinc cladding, calcareous grassland roof |
Both portals are topped with calcareous grassland roofs, a habitat native to the Chilterns. Ancillary buildings at each portal house mechanical and electrical equipment, wrapped in breathable louvre facades for natural cooling.
The north portal sits hidden low into the landscape between Great Missenden and South Heath. It is only partially visible from a nearby footbridge.
Over 20 hectares of new woodland, shrubs, and wildflowers were planted around the portal to blend the railway into the countryside. Species typical to the Chilterns, including Beech, Oak, Yew, Holly, and Dog Rose, were chosen for the planting scheme.
Chalk excavated during tunnelling was combined with recycled concrete and aggregates to create new habitat at the Colne Valley Western Slopes. The project reused 3 million cubic metres of chalk to establish 138 hectares of chalk grassland, woodland, and wetland.
The restoration work included 65,000 trees and shrubs from 32 local species, plus 4km of new footpaths, cycling routes, and bridleways. On paper, it sounds like a net positive. However, the Chilterns Conservation Board has stated that HS2 will have "a significant and lasting negative impact" on the Chilterns National Landscape.
Concerns remain about sinkholes that have appeared above the TBM route at Hyde Heath. Environmental groups have also raised questions about the impact on the chalk aquifer and the River Misbourne, one of nine internationally important chalk streams in the Chilterns.
Align JV delivered the Chiltern Tunnel under Contract C1. Align is a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. The contract was awarded in 2017 at £1.6bn, covering the full 21.6km C1 section, which includes both the tunnel and the Colne Valley Viaduct.
Grimshaw Architects designed all visible infrastructure: portals, headhouses, and ancillary buildings. Knight Architects and Atkins contributed to concept design, while RWDI provided groundborne noise and vibration assessments. Herrenknecht manufactured the two TBMs.
The four Main Works Civils Contracts (MWCCs) together represent the bulk of HS2 Phase 1 civils spend, each structured as a framework agreement with a joint venture:
SCS Railways (Skanska, Costain, Strabag): £3.3bn for Euston and Northolt Tunnels
Align JV (Bouygues, Sir Robert McAlpine, VolkerFitzpatrick): £1.6bn for Chiltern Tunnel and Colne Valley Viaduct
EKFB JV (Eiffage, Kier, Ferrovial, BAM Nuttall): £2.3bn for the section from Chiltern Tunnel north portal to Long Itchington Wood
BBV JV (Balfour Beatty, Vinci): Northern section into Birmingham
Now that civil engineering is complete, the tunnel is moving into fit-out and rail systems. This phase brings new procurement across the procurement lifecycle for mechanical and electrical systems, track installation, overhead electrification, signalling, communications, and ongoing maintenance contracts.
HS2 is one of the largest sources of public sector procurement in the UK, with an estimated 400,000 supply chain opportunities across Phase 1. Contracts are published through Find a Tender, HS2's own supplier portal, and through the Tier 2 joint ventures who subcontract significant work packages.
Stotles aggregates all of this into one platform. The HS2 Ltd buyer profile on Stotles shows open tenders, recent contract awards, upcoming expiries, and procurement patterns. The EKFB JV buyer profile covers subcontracting activity on the adjacent section north of the Chiltern Tunnel.
Tip: The HS2 Route-wide Framework pre-procurement notice covers upcoming works worth £500m–£600m across the entire route. Suppliers tracking this pipeline notice can engage early, before formal tenders are published.
