Detailing Concrete Structures of a Top-Down Built Urban Tunnel Using Diaphragm Walls

Abstract

The existing railway viaduct that cuts the city of Delft into two halves is being replaced by a new 2,4 km long four-track railway tunnel including an underground railway station and an underground parking space. The article focuses on the tunnel part situated in Phoenix street, as this is the part where the new tunnel will be realized close (3,00 m) to historic buildings. Owing to scarcity in (working) space and the constraint to ensure free car/train and tramway traffic during the whole construction period, a phased top-down building method using diaphragm walls is applied. Apart from providing an introduction explaining the geotechnical calculation methods and applied monitoring and mitigating measures (prestressed struts, etc.) to secure the neighbouring buildings from unacceptable damage, the article also treats concrete-related aspects. The first point of interest is the dimensioning of the longitudinal reinforcement to avoid unacceptable crack widths in the slabs poured between (zero shrink) diaphragm walls. The calculations performed, comprising different crack models, execution details, and the results obtained are shown. The second point of interest is the modelling of the joints between the horizontal slabs (roof and floor) and the diaphragm walls. In the first phase of the work, completely fixed joints were modelled, resulting in heavily reinforced joint sections and thus risks of possible bentonite inclusions. Investigation methods used to check the quality of the diaphragm walls before excavation are discussed briefly. Owing to the problems encountered, the structural model of the second phase tunnel part was modelled using hinged connections between the diaphragm wall and slabs. The impact of this alternative design on the concrete work in the second and first phases is explained.