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Alan Myers Channel Tunnel's UK crossover chamber

Tunnels

I will never forget the feeling of immense pride the day before the first big running tunnel TBM broke through into the Channel Tunnel's UK crossover chamber. It was near the end of August 1990 - I was contractor TML's hand tunnel manager and had much to do with the crossover.

The size of the British excavation was simply breathtaking, especially after two and a half hours spent travelling through the small service tunnel to reach it.

Built by NATM following that method's successful use in the Channel Tunnel- related Shakespeare underground development, the crossover primary lining was 22m wide, 15.5m high and the chamber was 160m long with a total face area of 257m2. Some 48,000m3 of muck was excavated, 3,500m3 of shotcrete was sprayed, and 13,000m3 of concrete batched and poured, in the crossover, using purpose designed equipment.

The size of the cross-section meant construction started with side wall drifts. Concreting the inverts generated stifling heat. We then built the crown heading across the top of the side walls and the dumpling excavation between them, breaking out the inner shell of the sidewall drifts.

It took nine months to complete the primary lining and a further six to finish the secondary lining.

There was one hiccup. During construction of the crown heading, a crack suddenly developed in the lining along both sides of the chamber over a length of 15m.

The cover above the tunnel to the seabed was only two diameters and when water gushed in through the crack and the crown settled 150mm there was concern, but nowhere to run. Water pressure had built up above the crown of the chamber which the lining was not designed to support.

The solution was to drill through the lining in a regular pattern throughout the whole of the crossover to allow water to drain. It cost us three weeks at a critical stage of our programme.

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