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Piles support permanent access to French tourist destination

Spie Fondations has just completed piling work for a new viaduct to connect the iconic Mont St Michel in Northern France to the mainland for the first time.

The new bridge will allow islanders and tourists to travel across the tidal mudflats in all but the highest of tides.

Although the construction of the 760m bridge the biggest change to island life since the tidal causeway was built in 1876, the bridge is part of a plan to preserve the island as an island and prevent the bay from silting up.

Spie’s work involved constructing 142 plunge columns to 30 to 35m below ground level with a 1200mm diameter during low tides to support the bridge. Using a Casagrande B300 the rotary bored piles were installed under bentonite slurry through saturated sands and silt and were founded up to 18m into the underlying shales. 

The new bridge is being formed from 300 precast concrete unit, so the plunge columns had to be positioned with millimetre accuracy.

According to Spie, the other challenge was getting the right concrete mix for the plunge columns which would also rapidly achieve an initial set before the next tide came in. A special blend of cement was selected to achieve a low heat of hydration to prevent micro-cracking in the aggressive saline environment.

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