Applying the lost shield method by freezing to connect bored tunnels to Rokin station on the North/South metro line, Amsterdam

Abstract

The North/South metro line in Amsterdam (the Netherlands) is a challenging tunnel project in an urban setting, where the ground is made up of soft clay and peat layers, and most buildings are founded on traditional wooden piles. The two 7m diameter tunnel boring machines (TBMs) (named Noortje and Gravin) were unable to enter metro station “Rokin” because the necessary depth had not yet been excavated due to delays. To avoid a programme disruption, the TBMs commenced from the northern start shaft in March 2010 and were permanently parked just outside Rokin station (northern side). The backup trains of the northern TBMs were retrieved through the already built tunnels and were used a second time by the two additional southern TBMs (named Molly and Victoria). To stabilise the remaining soil between the diaphragm wall and TBM, the excavation chamber of the TBMs was filled with grout. The two additional TBMs commenced in May 2011 from the southern start shaft to construct the southern part of the bored tunnels. By parking the TBMs outside Rokin station the tunnelling works were separated from the construction works in the station, resulting in a more efficient programme. The tunnel connections on both sides of the station were constructed using the “lost shield method”, see Figure 1, which involves freezing the subsoil and leaving the dismantled shields in the ground as part of the tunnel. Figure 1. Overview start (left) and final situation (right) After completing the two tunnel connections at the north end of the station using this method, optimisations were developed in order to speed up the process of connecting the south end of the station to the tunnels. The TBMs at the south end will arrive after freezing lances have been installed. Since all four TBMs are parked outside of the station, additional structures are required to connect the tunnels to the station. The design and construction process as well as the lessons learnt from these challenging connections of Rokin station are discussed in this publication. World Tunnel Congress 2013 Geneva Underground – the way to the future! G. Anagnostou & H. Ehrbar (eds) © 2013 Taylor & Francis Group, London ISBN 978-1-138-00094-0