To create two road tunnels under an existing motorway in Raalte in the Netherlands, Genap submerged two prefabricated geomembrane panels to create a groundwater barrier along the tunnel ingress and egress for each ramp, i.e., a total of four prefabricated panels for each tunnel. The motorway could not be closed to traffic for more than 48 hours so the contractor had to tunnel under it. The two panels for each ramp are about 32,500 ft² and were unrolled and welded together on site to create a panel of about 65,000 ft² for each ingress and egress ramp. The groundwater barrier system consists of a 40 mil thick PVC geomembrane underlain and overlain by a cushion geotextile. After the underwater excavation of the ingress and egress ramps (see upper left photo), the geomembrane was pulled across the water with rope hems (see upper right photo) and then submerged (sunk) to create the groundwater barrier system. After the geomembrane was pulled across the water body, the nonwoven geotextile was pulled across the water with rope hems (see lower left photo) and then submerged to create the groundwater barrier system. The cushion geotextile was prefabricated in five panels instead of two because of the thickness of the material. These five panels were stitched together on site before pulling it across the water surface to be submerged. These five panels created a cushion geotextile of the same area, 65,000 ft², as the geomembrane. The underlying cushion geotextile was already installed before pulling the geomembrane across the water. After performing a geo-electric leak location survey that showed the geosynthetic construction was 100% watertight, the ramps were partially backfilled with ballast sand. The tunnel area was dewatered to permit construction of the ingress and egress ramps to the tunnel after it was dewatered (see lower right photo). The tunnel was open for traffic and the groundwater barrier system is maintaining a dry area even after dewatering ceased. This project shows that a geosynthetic groundwater barrier system is effective, which obviates the use of significant concrete along and below the ramps. In addition, the groundwater barrier allowed the soil along the sides of the ingress and egress ramps to be seeded to grow grass.
Submerging geosynthetics is possible and not known to many contractors; however, coordination between different construction activities is important and should be worked out before construction commences.
Seaming the individual rolls of geomembrane at the confined site (see upper left photo) was not possible because of the level of water tightness required, time available, and site access and logistical constraints. The use of prefabricated geomembrane panels also resulted in only one seam or weld being performed on site, which reduced construction time, costs, and equipment needed. In addition, the prefabricated panels were created in the exact shape of the ingress and egress ramps, which resulted in no waste generation on site.