The construction of the A73 highway from Venlo to Maastricht (the Netherlands) involved the construction of the 2.4 km long, dual two lane Roertunnel near the city of Roermond. The Roertunnel consists of a somewhat deeper part crossing the quite small River Roer and its 1 km wide flood Valley, continued by a 1.4 km long shallow tunnel alongside the southern part of Roermond. The water levels in the River Roer vary from low water (summer conditions) to high water levels and possibly flooding of the Valley (spring conditions). Environmental conditions were severe, prohibiting amongst other for dewatering. The project was tendered the Ministry of Public Works and Water Management (Rijkswaterstaat RWS as a design and build contract. The tender documents included a reference design developed by the client for the crossing of the Roer Valley. This design consisted of a cast in situ piled cut and cover tunnel constructed in a building pit formed by temporary sheet piles and closed off at its bottom by a unreinforced concrete slab supported by tie down piles. This concept was designed to avoid dewatering of the building pit. In view of the ground conditions, an immersed tunnel was a more cost effective solution compared to the proposed reference design. However, an immersed tunnel appeared at a first glance not to be feasible as the small depth and width of the River Roer does not allow transportation of tunnel elements over the River from any remote dry dock, and as a dry dock in the vicinity of the tunnel location could not be created as dewatering was not allowed. The careful analysis of the geotechnical data indicated the local presence of a 5 m thick silt layer of low permeability over a length of 350 m along the tunnel alignment at the eastern side of the Roer Valley. This offered the opportunity to create a dry dock of length of about 350 m along the alignment of the tunnel by means of sheet piles reaching into the silt layer, avoiding any dewatering. The dry dock has been used for the construction of the four immersed elements of a total length of about 600 m in two batches. Finally the dock was used to construct the remaining part of the crossing of the Valley as a cast in situ tunnel. A gravel bed foundation consisting of nine berms per tunnel segment has been preferred above sand flow to mitigate the risk of weakening of loose sand in case of earthquake. The paper describes the concept and the various aspects of the design of the immersed tunnel constructed in a dry dock along the tunnel alignment. A section of the paper is devoted to the interaction of structural design and gravel bed design.
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