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Tunnelling

Tunnelling was a major engineering challenge, with the only precedent being the undersea Seikan Tunnel in Japan, which opened in 1988. A serious health and safety risk with building tunnels underwater is major water inflow due to the high hydrostatic pressure from the sea above, under weak ground conditions. The tunnel also had the challenge of time: being privately funded, an early financial return was paramount.

The objective was to construct two 7.6-metre-diameter (25 ft) rail tunnels, 30 metres (98 ft) apart, 50 kilometres (31 mi) in length; a 4.8-metre-diameter (16 ft) service tunnel between the two main ones; pairs of 3.3-metre (10 ft 10 in) -diameter cross-passages linking the rail tunnels to the service one at 375-metre (1,230 ft) spacing; piston relief ducts 2 metres (6 ft 7 in) in diameter connecting the rail tunnels 250 metres (820 ft) apart; two undersea crossover caverns to connect the rail tunnels, with the service tunnel always preceding the main ones by at least 1 kilometre (0.6 mi) to ascertain the ground conditions. There was plenty of experience with excavating through chalk in the mining industry, while the undersea crossover caverns were a complex engineering problem. The French one was based on the Mount Baker Ridge freeway tunnel in Seattle; the UK cavern was dug from the service tunnel ahead of the main ones, to avoid delay.

Precast segmental linings in the main TBM drives were used, but two different solutions were used. On the French side, neoprene and grout sealed bolted linings made of cast iron or high-strength reinforced concrete were used; on the English side, the main requirement was for speed, so bolting of cast-iron lining segments was only done in areas of poor geology. In the UK rail tunnels, eight lining segments plus a key segment were used; in the French side, five segments plus a key. On the French side, a 55-metre (180 ft) diameter 75-metre (246 ft) deep grout-curtained shaft at Sangatte was used for access. On the English side, a marshalling area was 140 metres (459 ft) below the top of Shakespeare Cliff, the New Austrian Tunnelling method (NATM) was first applied in the chalk marl here. On the English side, the land tunnels were driven from Shakespeare Cliff—the same place as the marine tunnels—not from Folkestone. The platform at the base of the cliff was not large enough for all of the drives and, despite environmental objections, tunnel spoil was placed behind a reinforced concrete seawall, on condition of placing the chalk in an enclosed lagoon, to avoid wide dispersal of chalk fines. Owing to limited space, the precast lining factory was on the Isle of Grain in the Thames estuary, which used Scottish granite aggregate delivered by ship from the Foster Yeoman coastal super quarry at Glensanda in Loch Linnhe on the west coast of Scotland.

On the French side, owing to the greater permeability to water, earth pressure balance TBMs with open and closed modes was used. The TBMs were of a closed nature during the initial 5 kilometres (3 mi), but then operated as open, boring through the chalk marl stratum. This minimised the impact to the ground, allowed high water pressures to be withstood and it also alleviated the need to grout ahead of the tunnel. The French effort required five TBMs: two main marine machines, one mainland machine (the short land drives of 3 km (2 mi) allowed one TBM to complete the first drive then reverse direction and complete the other), and two service tunnel machines. On the English side, the simpler geology allowed faster open-faced TBMs. Six machines were used; all commenced digging from Shakespeare Cliff, three marine-bound and three for the land tunnels. Towards the completion of the undersea drives, the UK TBMs were driven steeply downwards and buried clear of the tunnel. These buried TBMs were then used to provide an electrical earth. The French TBMs then completed the tunnel and were dismantled. A 900 mm (35 in) gauge railway was used on the English side during construction.

In contrast to the English machines, which were given technical names, the French tunnelling machines were all named after women: Brigitte, Europa, Catherine, Virginie, Pascaline, Séverine.

At the end of the tunnelling, one machine was on display at the side of the M20 motorway in Folkestone until Eurotunnel sold it on eBay for £39,999 to a scrap metal merchant. Another machine (T4 "Virginie") still survives on the French side, adjacent to Junction 41 on the A16, in the middle of the D243E3/D243E4 roundabout. On it are the words "hommage aux bâtisseurs du tunnel", meaning "tribute to the builders of the tunnel".