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.

中文翻译：

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基于地理信息系统的 Köfels 滑坡地形重建与地质力学建模

Ötztal 山谷（奥地利蒂罗尔）的 Köfels 滑坡代表了阿尔卑斯山变质岩体中已知的最大的极快速滑坡。尽管过去讨论了许多触发因素的假设，但直到现在还没有确定科学证明的触发因素。这项研究提供了关于 (i) 预破坏和破坏地形，(ii) 滑动体的破坏体积和孔隙率，以及 (iii) 初始变形和破坏机制的数值模型以及剪切强度特性的新数据。通过反算得到的基底剪切带。地理信息系统 (GIS) 方法被用于重建事件之前、期间和之后的斜坡地形。比较所得的数字地形模型，得出 3100 和 40 亿米的破坏和沉积质量的体积估计³，分别为 26% 的滑动质量孔隙率。对于 2D 数值研究，应用了独特的元素方法来研究初始破坏过程的地质力学特征（即模型运行时没有基础剪切区）并确定重建的基础剪切区的剪切强度特性。基于通过改变块体和节理输入参数进行的大量模型运行，可以合理地重建岩石边坡的破坏过程；然而，无法完全再现岩石滑坡的确切几何形状，尤其是在厚度方面。我们的结果表明，岩块的破坏和沿着向东适度倾斜的节理的剪切是地层或岩崩的原因。渐进式破坏过程可能是由于岩体的破裂和松动而发生的，从浅层向深层区域推进，特别是通过内部剪切带的发展以及块体破坏增加的局部区域。模拟进一步强调了考虑岩体主要结构特征的重要性。考虑强度特性的反算，即基底剪切带的摩擦角，结果表明，在无地下水流动条件下，摩擦角异常低，为 21 至 24^{需要 ∘}或以下才能促进失效，具体取决于允许滑动质量的内部剪切量。考虑地下水流的模型运行导致大约 6 ^∘更高的反向计算临界摩擦角，范围从 27 到 30 ^∘. 对于这种坚固的岩石，从岩石力学的角度来看，基底破坏带的摩擦角如此之低是出乎意料的，即使假设有高水压，地下水流也可能无法触发这种岩石滑坡。此外，如果没有实施基础剪切带，在模型运行中引起破坏所需的岩体特性明显低于通过经典岩石力学考虑获得的岩体特性。额外的调节和触发因素，例如地震作为岩体逐渐弱化的先兆的影响，可能与造成这种巨大的滑坡有关。