Underground construction close call

Work started on a new E1.5bn 0.5km long north-south metro in Amsterdam in the Netherlands in 2004. The new line is designed to relieve pressure on tram services between the Buikslotermeerplein district, the city centre and the city's financial quarter in the south. The alignment of the twin tunnel line and its eight stations follows the street and canal system. The line will be a mixture of cut-and-cover and tunnel boring. The line will be tunnelled under Centraal Station using a method involving the installation of piled retaining walls beneath the station's basement floor. Diaphragm walls are being constructed at three stations by Cementation Foundations and Franki Geotechnics. The key challenges are associated with the tight urban location and with the soft ground. Walls are being installed within 2-3m of shop and house fronts. Cement is mixed into the top 12m of ground either side of the diaphragm wall trench to increase trench stability. The station diaphragm walls were designed to unusually tight tolerances of 1:200. The trenches were excavated using hydraulic grab rotators.