Uncertainty of geotechnical conditions – a case study of metro tunnel construction underneath the existing road tunnel
In the paper, description of a case study concerning construction of a metro tunnel underneath the existing road tunnel is presented. The metro tunnel will be built in a very complex geotechnical conditions (various soft Quaternary soils and high water pressure due to the vicinity of the Vistula river). The conventional method (NATM) was chosen for the construction of the metro tunnel. In the first stage of investment process, e.g. building permit and tender design stage, the primary soil investigation was performed. In the subsequent investment stages – technical design stage and during construction – complementary soil investigations were performed and the results occurred to differ significantly. Locally, instead of the stiff Pliocene clays, pockets of loose fine and silty sands carrying water under pressure were found. That caused substantial problems in the construction process of the metro tunnel.