Author Pierre-Jean Pompée
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 100boreholes. 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 tunnellingconditions 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), showingtemporary fixed equipments (water pipes, ventilation duct diam 1600mm, power and radio communications cables, double track and walways...). Below : Tunneling progress on 29th of April 1990, 7 months before first undersea breakthrough, showing forecasted ("M"), and final (black balls) meeting points.
speed railway impose large radius curves, leading to drive 150 km of tunnels within 150 mm tolerances:40 mm for topography, 55 for TBM steering, 25 for segments manufacturing/erection (ring axis), 30 between surfaces of adjacent segments. TBMs, launched over 38 km distance, had to meet with precision !
Different systems in UK and France British maps are based on Mercator system, French maps on Lambert system. Sea level on both sides differed by 440 mm according to a 1958study, reduced at 300± 80 mm in 1987, measured at 360 mm after breakthrough. Joint mapping and levelling Mercator system was adopted, but based on the mid-Channel 1°30 East meridian. Satellite Global Positioning System (GPS, precision 5 cm for 50 km) was used and water movement studies reduced levelling uncertainty , leading to the RTM87 grid, and the NTM88 levelling ("Reseau" and "NiveauTransManche"). Final orientation
to the North required a gyrotheodolite with precise astronomical calibration and correction of all horizontal angles, locally different due to the Mercator system. Survey brough underground RTM87 was brought underground trough the inclined adits of the UK side and the Sangatte shaft in France through benchmarks set at the tunnels starting points with a precision of 0.6 mm(5 mm error would have meant 1.5 m at 15 km distance). Grid was relayed along TBMs journey with computer correction of temperature, pressure, hygrometry. After first breakthrough, a reference point was at least available for running tunnels survey.
High precision alignment
The tunnel alignment was a critical stage in the design process. It was later optimised using experience gained while...