Motorway underpass using geomembranes as a groundwater barrier FIGURE 2 Excavation by hydraulic excavators deck was placed on top of the permanent sheet piles. As a result, traffic could drive over the tunnel basin again within 48 hours. Underwater concrete made this tunnel section water-resistant in relation to the groundwater. As mentioned, a geomembrane conFIGURE 3 Unrolling prefabricated geotextile struction was used as a groundwater barrier for the tunnel ramps. The principle of this geomembrane construction is that it must be installed at a certain depth and ballasted with sand until the membrane construction is in equilibrium, i.e., the pressure on the geomembrane construction is greater than the water pressure under the geomembrane construction. The special thing about this project FIGURE 4 Pulling the geotextile across the water surface is that the geomembrane construction was installed by submerging (sinking) it into the underwater excavated pit. Also, the geomembrane needed to be fixed at a certain depth to the permanent sheet piles of the tunnel part. For the connection of the membrane construction to the permanent sheet 18 Geosynthetics | April May 2023 piles, a second temporary sheet pile in front of the permanent sheet piles was needed. The space between the sheet piles (10 feet [3 m]) was dewatered and excavated. On the bottom of this temporary pit, a concrete beam with a clamping construction regarding the geomembrane was built against the permanent sheet piles. The three-layer geomembrane construction was placed and clamped watertight to the concrete beam in this temporary pit. The pit was filled with water and the three-layer geomembrane construction was pulled up to the water surface by a small winch system. At this point, the ramp excavation started using hydraulic excavators and a GPS to dig to the right depth (Figure 2). The sand was placed in the depot so it could be used as ballast sand after the installation of the three-layer geomembrane construction. After excavation, the temporary sheet piles were removed to create one