Geographic-information-system-based topographic reconstruction and geomechanical modelling of the Köfels rockslide
Authors: Zangerl Christian , Schneeberger Annemarie , Steiner Georg , Mergili Martin 
Published: August 2021

Abstract. The Köfels rockslide in the Ötztal Valley (Tyrol, Austria)
represents the largest known extremely rapid landslide in metamorphic rock
masses in the Alps. Although many hypotheses for the trigger were discussed
in the past, until now no scientifically proven trigger factor has been
identified. This study provides new data about the (i) pre-failure and
failure topography, (ii) failure volume and porosity of the sliding mass, and
(iii) numerical models on initial deformation and failure mechanism, as well
as shear strength properties of the basal shear zone obtained by
back-calculations. Geographic information system (GIS) methods were used to
reconstruct the slope topographies before, during and after the event.
Comparing the resulting digital terrain models leads to volume estimates of
the failure and deposition masses of 3100 and 4000 million m3,
respectively, and a sliding mass porosity of 26 %. For the 2D numerical
investigation the distinct element method was applied to study the
geomechanical characteristics of the initial failure process (i.e. model
runs without a basal shear zone) and to determine the shear strength
properties of the reconstructed basal shear zone. Based on numerous model
runs by varying the block and joint input parameters, the failure process of
the rock slope could be plausibly reconstructed; however, the exact geometry
of the rockslide, especially in view of thickness, could not be fully
reproduced. Our results suggest that both failure of rock blocks and
shearing along dipping joints moderately to the east were responsible for the
formation or the rockslide. The progressive failure process may have taken
place by fracturing and loosening of the rock mass, advancing from shallow
to deep-seated zones, especially by the development of internal shear zones,
as well as localized domains of increased block failure. The simulations
further highlighted the importance of considering the dominant structural
features of the rock mass. Considering back-calculations of the strength
properties, i.e. the friction angle of the basal shear zone, the results
indicated that under no groundwater flow conditions, an exceptionally low
friction angle of 21 to 24∘ or below is required to
promote failure, depending on how much internal shearing of the sliding mass
is allowed. Model runs considering groundwater flow resulted in approximately
6∘ higher back-calculated critical friction angles ranging from
27 to 30∘. Such low friction angles of the basal
failure zone are unexpected from a rock mechanical perspective for this
strong rock, and groundwater flow, even if high water pressures are assumed,
may not be able to trigger this rockslide. In addition, the rock mass
properties needed to induce failure in the model runs if no basal shear zone
was implemented are significantly lower than those which would be obtained
by classical rock mechanical considerations. Additional conditioning and
triggering factors such as the impact of earthquakes acting as precursors
for progressive rock mass weakening may have been involved in causing this
gigantic rockslide.