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Dewatering design for residential project in De Panne, Belgian coast located between protected dunes and habitat directive areas

For the construction of the new rest and nursing home Duinenzee in De Panne (Be), AGT Consultants have recently successfully completed the dewatering design and the dewatering study for their Real Estate client De Wielingen Development NV.

The project is environmentally complex because of the very permeable dune sands, surrounding habitat directive areas, nature protected dune areas, dune pans, tidal activity and the salt gradient in the groundwater. Because of the location of the project in relation to these areas, there were restrictions on the permissible pumping rate and anti-interference measures had to be designed in order to avoid adverse effects on the environment. A test drilling was carried out on the site and an extensive groundwater monitoring well network was installed after. From these drillings and the monitored groundwater levels at rest in the various layers in the subsoil, a uniform presence of a less permeable strong clay containing sand was found between 11 to 22 meters below surface. The construction pit, with dimensions of 45 m by 60 m and an excavation depth of 7 m below surface, was therefore conceived within water-retaining Cutter Soil Mix walls set to 15 m-mv. The dewatering was designed with shallow depth wells in combination with reinfiltration of all the extracted water, distributed over the northern coastline and the southern habitat directive area.
Based on a 3D modeling, the dewatering concept was evaluated and the effects on the environment were budgeted.

The monitoring of the dewatering showed that the concept is very successful: the flow rates stay well below the permissible limits and all water can perfectly be infiltrated in the provided areas. Through online monitoring of the water levels of the monitoring wells, the groundwater level is monitored continuously and not any negative influence of the dewatering system has been observed. The monitoring takes place in consultation with the Environmental Department of De Panne Municipality, and Flemish Nature and Environmental Protection authorities (ANB and VMM) and will be maintained for approximately 2 months after the shutdown of the dewatering system.

The project is a textbook example of collaboration between the design office and the construction team, with one common goal of combining and optimizing the technical feasibility of a project with special attention to maximum protection of the environment and the surroundings.

 

Excavation pit: 

Infiltration zones: