Abstract:
For the construction of the tunnel for the new Berlin S21 light rail line, a construction pit had to be built
below the crossing B 96 federal road. For this purpose, the construction pit M’ has been built beforehand as
a preliminary structure for the later S21 Berlin. A crucial element of this structure is a 2.50 m deep reinforced
concrete deck slab which carries the B 96. During the later construction of the S21, it became clear that the
existing preliminary structure could not be used without major modifications. Consequently, the
construction pit design was gradually upgraded – still with the aim of retaining the basic concept of a trough
structure with a jet-grouting cut-off. As construction progressed, for reasons of stability it became necessary
to develop a fundamentally new concept with an underwater concrete base replacing the jet-grouting cutoff. This paper describes the planning and construction of a cut-and-cover structure with an underwater
concrete base – a layout that brings with it the difficult task of underwater excavation and concrete placing
without direct accessibility from above. Special emphasis is placed on describing the innovative approaches
in connection with a remotely controlled underwater dredger from the Norwegian offshore industry for
excavation works. Furthermore, the sophisticated underwater technologies for concrete placing in the
confined surroundings are described.