Deep open excavation in soft plastic ground in Salzburg, Austria

Probably one of the most difficult ground condition pertaining to the design and construction of deep excavation in built-up areas is exhibited when a relatively shallow stiff layer of soil (gravel and sandy gravel) overlies a deep layer of soft plastic soil (silt and clayey silt), especially, when the latter is prone to liquefaction. This is the case for the Quaternary deposits within the basin of Salzburg in Austria. In this case the choice of the foundation system and of the construction process as well as of the support system for the open excavation is very much dependent on the short and longterm deformation behaviour of the soft plastic subsoil, apart from the size and the founding depth of the structure and the vicinity of adjacent buildings on account of potential damage during the construction phase. In this paper the problem of deep open excavations using the diaphragm wall technique with its associated deformations is highlighted in respect of the time-dependent construction activities, the bulk excavations and the relevant support system. Two case histories are presented, each employing a different method for the temporary supports of the retaining structure. In all instances a careful monitoring of deformations by means of various measurement techniques during all phases of construction has been made. Most revealing is the evaluation of the monitoring during the construction of a short stretch of an underground railway line in the subsoil of Salzburg. Finally, reference is also made to a 3D-FEM analysis which compares measured results with a calculated prediction.