A new underground railway, named M2, is planned to be constructed within the next decade in Lausanne, the fifth densest populated city of Switzerland. In 1994, a preliminary geological report of the quarters chosen by the administration had shown the general conditions and features of the lithology of interest. The involved geology constitutes alpine molassic bedrock, an overlaying quaternary glacial fill and an overburden of loose soil. The choice of the corridor had to consider the exact thickness of these geological units. It was necessary to find out whether the best effective cost ratio would be an easier, but deeper, tunnel in a favourable lithology (Bedrock) or the technically more difficult tunnel in a superficial lithology (Glacial fill). Nearly 750 existing well loggings over the city, conducted for geological superficial surveys, test pits and geological maps were used to build a map of the depth to bedrock. Its surface was positioned with a precision of 3 to 5 m. The depth is varying between 2 and 15 meters. Once the corridor was chosen, the needed precision was increased. The requested precision in depth determination was about 1 meter. For this reason, more drillings were executed in 1999 and 2001 along the chosen corridor. The free availability of all data by the Road Service of the canton Vaud was an opportunity for testing a micro-gravity survey, integrated by other geophysical techniques, along the 6 km metro corridor.


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