ABSTRACT
ABSTRACT: The construction of the A73 highway (the Netherlands) involved the construction of a 2.4 km long, 2*2 lane tunnel to mitigate the impact of traffic on the city of Roermond and to cross the 1 km wide valley of the river Roer. Given the ground conditions (mainly dense sand and gravel), required cross section and tunnel depth, an immersed tunnel appeared to be the most economic solution to cross the valley. However, a facility to permit precasting of the RC tunnel elements needed to be established. The small depth and width of the Roer prevented transportation of tunnel elements by use of the river and dewatering was not permitted due to the associated environmental impact. The analysis of the geotechnical data indicated the presence of a 5 m thick local loam layer over a length of 350 m along the tunnel alignment, located at the eastern part of the Roer valley. This impervious layer offered the opportunity to excavate a 350 m long dock along the axis of the tunnel between temporary anchored sheetpiles that were installed into the loam layer. Two precast tunnel elements of approximately 158 m long could be constructed in this dock. A trench was excavated between and temporary sheetpiles were installed 5 m outside the future tunnel location, thus permitting the transportation and immersion of a total of four such tunnel elements in two installments from the dock to their final location. The remaining part of the tunnel was constructed in what was previously the dry dock after the immersion of the elements. This paper describes how the concrete structure was designed and specified (concrete weight and tolerances) with allowances for the specific geotechnical and hydraulic conditions (water depth, concrete weight and freeboard, water level management in the trench, excavation depth and uplift of the impervious layer etc.). The paper describes the behavior and provisions of the concrete structure on the gravel bed foundation, which was preferred to sand flow to minimize the risks of liquefaction as the area is mode-rately seismic.
