Engenharia - eurotunel

CHANNEL TUNNEL TUNNELS CONSTRUCTION
Author Pierre-Jean Pompée

Scope
Within the 5 years following contract award, 3 parallel tunnels, 50.5 km each, had to be driven and lined with high-strengh precast segments : 38 km undersea, 3.2 underland in France, 9.3 km underland in UK. 2/3 of the total had to be excavated from UK side, and 1/3 from French side where ground conditions were unfavourable. 6 drives (total 12), 3 to the sea and 3 to the land have been conducted from each of the two tunneling sites, located as close as possible to the sea : Shakespeare Cliff (west of Dover) on the UK side, and Sangatte (west of Calais) in France. The service tunnel (ST) has a internal diameter of 4.8 m (excavated diam.5.8 m). The two running tunnels (RT) are at 30 m distance, lined diameter is 7.6 m (excavated diam.8.8 m). They are connected together by 194 piston relief ducts (every 250 m, diam 2 m), and by 270 cross passages (every 375 m) and 210 technical rooms (all diam 3.3 m) to the service tunnel, requiring special cast iron segments at each junction.

Geology and alignment
Many geotechnical investigations have been carried out across the Channel since 1875, including over 100 boreholes. Total information available including final investigation after contract award and geophysical survey undertaken in 1988, (specifically for crossing beneath the Fosse Dangeard and crossovers sites),comprised: - 120 marine boreholes and about 1,000 km of geophysical (seismic reflection), traverse lines, - adits and short lenght of tunnels excavated during previous attempts. Optimum tunnelling conditions are within the Chalk Marl, clayey carbonate mudstone, weak (20 to 200 bars), homogeneous rock, believed to be mainly impermeable, with any water restricted to discontinuities such as faults or fissures concentrated on French side. It is a tunnelling medium adequate for TBMs.
Above : View of a running tunnel on French side at the beginning of the tunneling phase (1988), showing