Kérino tunnel. Vannes | France

Solving Traffic Issues in Vannes with a New Underpass Beneath the Harbour Entrance in France

The city of Vannes, on the Gulf of Morbihan, is Brittany’s fourth-largest conurbation. For some years it was plagued by an urban-transport problem in the port, where the interests of road vehicles and boats were in conflict at the Kérino swing bridge, with vehicles using it to cross the entrance to the harbour, and boats requiring it to be opened to leave or enter. Both vehicle and boat traffic were governed by tides and the position of the bridge which was opened 102 minutes per day, on average, to allow 7,500 boats through every year, compared to 15,000 vehicles crossing every day. The fluidity of traffic was thus severely affected.

The Kérino swing bridge has been replaced by an underpass beneath the entrance to the harbour. This enables road and water traffic to move fluidly simultaneously. The underpass comprises two lanes for vehicles and a separate lane for pedestrians and cyclists. It consists of a 153-m-long western approach ramp, a 248-m-long cut-and-cover tunnel section, and a 150-m-long eastern approach ramp.

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Typical cross-section Kérino tunnel
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Kérino tunnel
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A total of 1,061 tonnes of 3.5 to 16.5-m-long double PU 18 sheet piles and CU 18-2 box piles made from S 355 GP grade steel were used to build two cofferdams in the river. A gap was left around the cofferdams at all times during the three-year construction period so that boats could continue to leave and enter the port.

The first phase of works involved dredging the River Marle, after which work on the tunnel began on the west bank with the construction of a steel sheet pile cofferdam so that the cutand- cover tunnel could be excavated in the dry. The approximately 130-m-long, 30-m-wide three-sided cofferdam was built out from the riverbank, half on land and half in water. It was a composite pile wall made up of steel double PU 18 sheet piles and CU 18-2 box piles. The same method was also used on the east bank: here, the three-sided cofferdam was about 100 m long and 30 m wide, 70 % on land and 30 % in water. 

Work began in June 2013, with pile driving starting in October that year. The underpass was officially opened on June 24, 2016. 

The work was carried out under a 28-year public–private-partnership contract (3 years for construction, 25 years of maintenance). The total cost was estimated at 76 million euros. Some 14.5 million euros of this was funded by the Vannes metropolitan authority, the Morbihan general council, and the Brittany regional council.