24 - Randa:

Summary

Abstract

In this chapter we summarize integrated investigations carried out at the Randa rock-slope instability in Matter Valley, Switzerland, between 2000 and 2010. We present a 3D geometric and structural model of the current instability, which consists of 5–6 million m³ of crystalline rock. We also document the complex kinematic behavior and discuss the driving factors for observed slope movements. We show that both the May 1991 failure and the current instability are bounded laterally by the same large-scale fault, and at the base by a planar or stepped rupture surface daylighting at the contact with the Randa orthogneiss. The spatial distribution of current displacements indicates toppling in the upper section of the instability between 2200 and 2400 m asl and sliding in the lower area between 1900 and 2200 m asl. Continuous displacement time series from the surface and deep boreholes show increasing deformation rates when ground surface temperatures decrease in fall and a decrease after snowmelt in spring as the rock warms. We have not detected displacement signatures related to heavy rainstorms or snowmelt. Mapping of the locations of springs and recordings of borehole pore water pressure demonstrate locally perched groundwater in open fractures, and a low regional groundwater table located at or below the basal rupture surface. Numerical modeling results support the hypothesis that thermo-mechanical coupled deformation resulting from annual temperature changes and critically stressed fractures in a complex topography is the primary mechanism driving deep-seated displacements at Randa.