Integration of bored tunnel and conventional technologies in the north-south railway crossing below the Antwerp city centre

The Antwerp north-south railway link is the main project of the northern part of the Belgian high-speed network. Its most important structure is the bored tunnel section with a length of 1225 m under a densely populated area of the old city centre. This section comprises two single track bored tunnels with an internal diameter of 7.30 m. During the design phase, as well as during the boring of the tunnel tubes, constant care was taken to limit subsidence in urban area. This required the combination of classical underground technologies with the actual boring process itself. The starting section of the tunnel has a low cover depth, less than half the tube diameter, and at this location a block of houses had to be under crossed. To protect these buildings from settlements, a pipe-jacked roof was built. In addition, the pipes allowed the execution of grout injection before and during the crossing of the TBM. At the end of the boring section, an existing metro station is situated right above the TBM arrival shaft. The station was underpinned by ''sheeted trench'' walls and jacked to keep its initial level. The shaft was also equipped with the main electrical and mechanical devices, such as ventilation equipment. In addition, before reaching the arrival shaft, a 15 storeys hotel building had to be under crossed by the TBM with a very low soil cover. Grout injections were carried out in order to block any further displacements apart from those directly related to the bore front. In a central location of the bored tunnel section, a shaft reaching from the surface to the tunnels must allow the evacuation of the passengers and personnel and give access for the fire brigade, in case of emergencies. This shaft is situated at the exact location of existing tramway tracks, which had to be kept operational at all times. Hence the shaft was built by the ''sheeted trench'' method. The sheeted trench method is a typical excavation method for vertical walls, consisting of the succession of manual excavation phases and the placing of precast concrete shuttering plates, as frequently used in Belgium in confined work areas, where a preliminary dewatering of the soil is possible. Special care was taken for integrating structurally the shaft walls and the bored tunnel linings. At two intermediate locations both tunnel tubes had to be connected by cross-passages. The passages consist of jacked reinforced concrete tubes, the tunnel lining containing locally steel segments replacing the normal concrete ones. The authors intend to show how the selection of the techniques as well as the use of advanced TBM technologies resulted for this project in minimal subsidence in urban area. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.