Geoscience Reference
In-Depth Information
The design of the Channel Tunnel (completed in 1994) involves three tunnels, one
for each of the two traffic directions, and a smaller one for emergency and maintenance
work. The tunnels are excavated as far as possible within the lowest unit of the lower
Chalk, known as the Chalk Marl (Fig. 143). This material consists of a mixture of clay
minerals and fine-grained calcium carbonate, making it largely impermeable to water
flow, yet plastic enough to suit the tunnelling machinery. It is also not so brittle that it
contains cracks and fractures that would cause collapse and flooding by groundwater.
Excavation took place simultaneously from both France and England and took
about three years. Spoil from the tunnels on the English side was used to construct a
1 km by 300 m extension to the coastline just west of Shakespeare Cliff (
d1
), most of
which is now used as the Samphire Way Country Park.
The decision was also taken to locate the tunnel mouths very well inland, some
2.5 km north of the coast in central Folkestone (Fig. 144). The route of the tunnels
crosses the coastline below Shakespeare Cliff (
d1
), some 2 km from the Western Docks
of Dover and almost 10 km from the tunnel mouths north of Folkestone. This 10 km
of tunnelling below land was felt to be preferable to locating the tunnels closer to the
coastal cliffs, which had proved to be extremely unstable in the past, particularly along
the 3 km coastal stretch just northeast of Folkestone known as the Warren (
d2
). Cliff
erosion of the soft Early Cretaceous Gault overlain by the Late Cretaceous Chalk in
this area has caused continuous land-slipping here. The decision to construct the first
railway line to Dover across the Warren required frequent movement and re-cutting of
the line, culminating in a major landslip in 1915 which carried away a complete train
(Fig. 145).
