摘要：The Köfels rockslide in the Ötztal Valley (Tyrol, Austria)represents the largest known extremely rapid landslide in metamorphic rockmasses in the Alps. Although many hypotheses for the trigger were discussedin the past, until now no scientifically proven trigger factor has beenidentified. This study provides new data about the (i) pre-failure andfailure topography, (ii) failure volume and porosity of the sliding mass, and(iii) numerical models on initial deformation and failure mechanism, as wellas shear strength properties of the basal shear zone obtained byback-calculations. Geographic information system (GIS) methods were used toreconstruct the slope topographies before, during and after the event.Comparing the resulting digital terrain models leads to volume estimates ofthe failure and deposition masses of 3100 and 4000 million m3,respectively, and a sliding mass porosity of 26 %. For the 2D numericalinvestigation the distinct element method was applied to study thegeomechanical characteristics of the initial failure process (i.e. modelruns without a basal shear zone) and to determine the shear strengthproperties of the reconstructed basal shear zone. Based on numerous modelruns by varying the block and joint input parameters, the failure process ofthe rock slope could be plausibly reconstructed; however, the exact geometryof the rockslide, especially in view of thickness, could not be fullyreproduced. Our results suggest that both failure of rock blocks andshearing along dipping joints moderately to the east were responsible for theformation or the rockslide. The progressive failure process may have takenplace by fracturing and loosening of the rock mass, advancing from shallowto deep-seated zones, especially by the development of internal shear zones,as well as localized domains of increased block failure. The simulationsfurther highlighted the importance of considering the dominant structuralfeatures of the rock mass. Considering back-calculations of the strengthproperties, i.e. the friction angle of the basal shear zone, the resultsindicated that under no groundwater flow conditions, an exceptionally lowfriction angle of 21 to 24∘ or below is required topromote failure, depending on how much internal shearing of the sliding massis allowed. Model runs considering groundwater flow resulted in approximately6∘ higher back-calculated critical friction angles ranging from27 to 30∘. Such low friction angles of the basalfailure zone are unexpected from a rock mechanical perspective for thisstrong rock, and groundwater flow, even if high water pressures are assumed,may not be able to trigger this rockslide. In addition, the rock massproperties needed to induce failure in the model runs if no basal shear zonewas implemented are significantly lower than those which would be obtainedby classical rock mechanical considerations. Additional conditioning andtriggering factors such as the impact of earthquakes acting as precursorsfor progressive rock mass weakening may have been involved in causing thisgigantic rockslide.
