IDM pressure measurements in a large-scale excavation test

Abstract

Inclinodeformeter (IDM) pressure measurements were carried out behind a retaining wall during an excavation test forming part of feasibility studies for a new underground railway station in the city of Lucerne. The strut loads measured during the excavation appeared to be considerably larger (up to 2·5 times) than those expected in the design. A possible scenario suspected for the development of such large forces was overpressure of the soil behind the slurry wall due to excessive jet-grouting pressures. IDM measurements were used for validation and quantification of this phenomenon. Using elasto-plastic numerical analysis, the lateral earth pressure changes and the soil stiffness were back-calculated based on IDM and local displacement measurements. The back-calculated soil stiffness was validated for each of the six soil layers against the in situ measurements obtained by cone penetration tests and Marchetti dilatometer. The excessive strut loads could be explained by the back-calculated pressure changes. For the excavation test, lateral earth pressure changes obtained by IDM measurements helped to improve understanding of the reaction of the retaining wall to installation processes, to provide an early detection of potential problems and to prevent a possible excavation collapse.