Slope instabilities on perennially frozen and glacierised rock walls: multi-scale observations, analyses and modelling

Abstract

Slope failures from steep bedrock slopes have occurred in mountain areas throughout time.
This is a consequence of the topography, geological characteristics, intense freeze-thaw activity
and oversteepened slopes from glacier erosion. However, during the past decades, an increased
number of periglacial rock avalanche events have been recorded in the European Alps and
other high mountain ranges which are thought to be related to permafrost degradation and glacier
shrinkage, indicating the potentially serious hazard related to slope instabilities originating
from high-mountain faces.
The primary aim of this study is an interdisciplinary investigation of topographic, geological,
cryospheric and climatic factors influencing high-mountain rock slope stability in view of the
ongoing climatic change. The investigation of slope instabilities in high-mountain faces must
account for the large variety of factors and processes and also consider the difficult conditions
for data acquisition. The objectives of this study, where detachment zones of recent periglacial
rock avalanches in the European Alps are investigated based on a multi-scale approach, can be
divided in (a) the investigation and modelling of slope instabilities on periglacial high-mountain
faces in order to better understand the different factors and processes leading to a slope failure,
and (b) the application of different data acquisition and investigation techniques to test their
suitability for steep faces in complex and difficult high-mountain terrain.
The implemented approaches consist of 1) a GIS-based statistical multi-factor analysis of
detachment zones over the entire Central European Alps based on a rock avalanche inventory,
2) a GIS-based multi-factor analysis and detailed remote-sensing-based time-lapse topographic
investigations of the Monte Rosa east face using LiDAR and digital photogrammetry, and 3)
geomechanical analysis and numerical slope stability modelling of the Tschierva rock avalanche
at the Piz Morteratsch.
This study has shown that in most cases a combination of several critical factors leads to a
slope failure and no specific single primary factor was distinguished. The two factors slope angle
and a pronounced discontinuity system are included in critical factor combinations at all
failure magnitudes. The change in a factor and the time scale of change are considered to be
more important than the individual factors. Rapid changes in a factor do not allow adequate
stress redistribution within a flank and therefore, the critical shear strength may be exceeded. A
large number of detachment zones were found to be located in areas with recent changes in
glaciation and near the lower limits of local permafrost occurrence. Glaciers, mainly influencing
the topography, and permafrost, mainly affecting the groundwater regime and geotechnical
characteristics of discontinuities, are currently the predisposing factors having the fastest
changes. However, slow processes such as progressive failure were also found to contribute to
slope instabilities.
The present study demonstrates the benefits of a multi-scale approach and the combined
application of conventional and novel techniques for the investigation of slope instabilities in
high-mountain terrain. Furthermore, the findings provide a fundamental basis for prospective
slope instability susceptibility analyses and subsequent hazard assessments.