Liquefaction of alpine lake sediments: Predictive simulations and post-event validation at Spitallamm Reservoir

¹ Terraqua Engineering AG, Zollikerberg, Switzerland
² OST – Eastern Switzerland University of Applied Sciences, Rapperswil, Switzerland
³ Ben-Gurion University of the Negev, Be’er Sheva, Israel
⁴ Schläpfer & Partner AG, Zürich, Switzerland

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Fine-grained lacustrine sediments in alpine reservoirs may exhibit pronounced undrained instability when subjected to rapid hydraulic unloading. In this paper, static liquefaction denotes a monotonic, non-seismic undrained instability in which excess pore-pressure generation causes the mean effective stress to collapse towards zero, followed by rapid post-peak strength loss and flow-like deformation. Such stress paths can occur during reservoir drawdown or sudden depressurization of a buried hydraulic opening, even though fully drained dewatering would increase effective stress. This paper presents a documented case history from the Spitallamm reservoir (Switzerland), where the emptying of a disused tunnel inlet beneath the former lakebed triggered a collapse of saturated silty sediments. Prior to the operation - and before any direct geotechnical characterisation of the sediments was available - numerical simulations using PLAXIS (small strain, NorSand model) and ABAQUS (large strain, CEL) were conducted based on grain-size classification and literature parameters. The models predicted progressive loss of effective stress and failure by static liquefaction. Post-event CPTu, Ball-cone, and laboratory tests on undisturbed samples confirmed pronounced post-peak softening and liquefaction behaviour, validating the pre-event predictions. The results confirm that failure was governed by stress-path-dependent undrained instability rather than low peak strength, highlighting limitations of strength-based assessment for contractive lacustrine soft soils.