Reversible rock-slope deformations caused by cyclic water-table fluctuations in mountain slopes of the Central Alps, Switzerland

Abstract

Within the framework of the Gotthard Base Tunnel Project in the Central Alps, Switzerland, geodetic monitoring networks were installed above the tunnel trajectory in alpine valleys. Natural ground-surface deformation recorded in the years prior to the tunneling excavation was seen to contain an unexpectedly large cyclical component of horizontal strain across the valleys, which was seasonal and appeared to be due to elastic processes. The strain is strongly correlated with snow melt and rainstorm precipitation, suggesting the implied rock-mass deformation is driven by changes in water-table elevation within adjacent mountain slopes. The horizontal strains are of the order of 1–2 · 10–5, which is close to the design limits that can be accommodated by hydropower arch dams in the study area. This study investigates these processes in detail and describes a new mathematical model (REROD), which is able to accurately reproduce and predict such natural rock-slope displacements. The model implements a transfer-function approach to predict the valley-crossing strains from rainfall and winter snow height data recorded at nearby meteorological stations. It has been used to estimate and remove the natural strain signal from the net recorded deformation so as to resolve the component due to tunneling.