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08Oct92 UK: CHANNEL TUNNEL SPECIAL - FRENCH DIG TOOK SMALLER BITES.

The ambitious construction of a large open arch 35 m below the seabed was considered too risky for the French crossover chamber.Ground conditions on the French side were expected to be more fractured and fissured than on the British side, so the fear of collapse or excessive movement of the roof of the chamber was too great.The solution to the cavern construction conundrum was to use a variation on the multiple drift excavation method used on the British side.'Basically what we did was excavate the French crossover in smaller bites,' says Colin Kirkland. 'The running tunnels drove through first, as opposed to the British method, and the chamber was then formed around those two main bores.'A series of small tunnels was then bored in an arch around the main drives.These mini-tunnels were filled with concrete to form the arch, allowing excavation of the chamber below the new roof.Although this method was successful the big disadvantage was that it had to be built late in the construction programme, once the running tunnels had passed the crossover site 12 km from the French coast.Construction of the second crossover was completed only last November, almost a year after the first tunnel breakthrough.