ABSTRACT
Västlänk is a railway tunnel through Göteborg city centre in Sweden. Tunnelling, a cut and cover procedure with wire cutting in competent rock and traditional excavation in soft clay, goes through the driven precast piles of an 8-floor building whose occupancy cannot be interrupted. The existing piles, designed just for vertical loads, would not resist lateral thrust, given by clay on steep rock, mobilized by excavation. For these reasons a retaining structure, to be executed on one side of the building, is combined with the underpinning of the existing plinths with new micropiles to excavate till adequate depth for the execution of post-tensioned reinforced concrete beams which will support the building above the tunnel for the long term. Outside the tunnel footprint the underpinning micropiles will be permanent, while those above the tunnel, once the load of the building is transferred to the new beams, will be cut for the railway tunnel to run through. This paper focuses on work and construction phases and the underpinning itself. New micropiles are executed close to existing plinths and pre-loaded by flat jacks against steel clamps with friction contact to columns faces. Once micropiles heads are embedded in new reinforced concrete cubes, with post-drilled shear connectors to existing plinths, the formers are clamped to plinths by post-tensioning bars. At this point the system has the capacity to withstand the horizontal thrust mobilized by excavation. This underpinning procedure faces all the typical issues when the purpose is barely to retrofit a building foundation system, plus some specific issues, given that it’s quite uncommon to run a tunnel not under but exactly through a piled foundation.
