TUNNELS AND TUNNELLING
Achieve in 5 years 3 tunnels of more than 50 km each (including 38 km under sea):
- 2 running tunnels 7, 60 m internal diameter
- 1 service tunnel 4, 80 m internal diameter.
GEOLOGICAL CONDITIONS – TUNNEL ROUTE – DESIGN OF THE TUNNEL BORING MACHINES (TBMs)
Prior to the design of the route of the tunnels, survey campaigns were carried out at sea and on land. Their results led to the following recommendations:
- Avoid aquifer and fractured grey chalk layer as well as the inflating Gault clay layer.
- Keep the route of the tunnels in the blue chalk layer which is homogeneous, watertight and has a strength of about 80 bars.
- Ensure as far as possible a blue chalk cover of 5 meters.
In a typical section, tunnels are located about 100 meters under the sea level with a coverage of 50 m of land and 50 m of water.
Keeping the tunnel route continuously in waterproof blue chalk on its whole length allowed the use of conventional TBMs working in dry conditions, with light segment rings erected directly on the natural ground, at the back of the TBM.
The very difficult geological conditions encountered in the first kilometers under sea with severe ground folds and faults, as well as crossing areas of grey chalk under land and sea tunnels, were decisive for the choice of the TBMs.
In order to cross these aquifers chalk marl layers without ground injection, the TBMs were designed as submarine resistant to hydrostatic pressure up to 11 bars, with heavy and waterproof segment rings erected inside the shield skirt.
THE TUNNEL BORING MACHINES (TBMs)
These machines are of open type, light and fast.
Significant difficulties resulted from their limited water-sealing in several zones, where unexpected water ingresses were encountered.
These machines could not have reached the French coast.
These machines are of closed type, ensuring earth pressure containment; they are heavier and slower.
However, they can work at higher speed in open mode in areas of waterproof chalk marl.
The annular gap between the ground and the rings erected inside the shield skirt of the TBM requires an immediate filling with cement mortar completed with a grout bonding.
These machines were capable of reaching Dover.
ELEVEN TBMs IN TOTAL:
6 for the British side to dig 3 tunnels under sea and 3 tunnels underground.
5 for the French part: 3 to dig 3 tunnels under sea but 2 only for the 3 tunnels underland, since the running tunnel TBM started from the Sangatte shaft to dig the South running tunnel and then turned back from the Terminal towards the shaft, drilling the North running tunnel.