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Construction of A11 highway: design and monitoring of the dewatering systems

Since the beginning of 2013, the consortium Via Brugge is working on the new A11 highway, which will connect the existing N31 in Bruges with the N49 in Westkapelle (Belgium). At the moment, this is the largest DBFM-project for roads in Flanders. The length of the total road is 12 km, of which several kilometers are constructed in tunnels. As the groundwater level is very shallow in the area, the tunnel sections have to be temporarily dewatered to enable dry excavation. AGT is responsible for the design of different dewatering systems. In this design, the surrounding environment has to be taken in account, such as the heterogeneity of the underground, the balance between sweet and salty groundwater layers, the presence of infrastructure sensitive to subsidence and agriculture dependent on the availability of sweet groundwater. Both the design and the monitoring of the dewatering have to comply with restrictions imposed by the Project to avoid negative impacts on the environment. The dimensioning of the dewatering systems was based on hydrogeological parameters, estimated from double pumping tests, also executed by AGT. The influence of the dewatering on groundwater levels is monitored in piezometers. Any changes in the sweet and salty groundwater balance are monitored by regularly measuring with a borehole EM-probe.