I apply a forensic, multidisciplinary approach that integrates engineering geology field investigations, engineering geomorphology mapping, long-range terrestrial photogrammetry, and a numerical modelling toolbox to two large rock slope failures to study their causes, initiation, kinematics, and dynamics. I demonstrate the significance of endogenic and exogenic processes, both separately and in concert, in contributing to landscape evolution and conditioning slopes for failure, and use geomorphological and geological observations to validate numerical models. The 1963 Vajont Slide in northeast Italy involved a 270-million-m3 carbonate-dominated mass that slid into the newly created Vajont Reservoir, displacing water that overtopped the Vajont Dam and killed 1910 people. Based on literature, maps and imagery, I propose that the landslide was the last phase of slow, deep-seated slope deformation that began after the valley was deglaciated in the Pleistocene. Field and air photograph observations and stream profiles provide the context of Vajont Slide. The first long-range terrestrial digital photogrammetry models of the landslide aid in characterising the failure scar. Analysis of the failure scar emphasises the complexity of the failure surface due to faults and interference between two tectonic fold generations, influencing failure behaviour. Observations of the pre- and post-failure slope and interpretation of numerical simulations suggest a complex three-dimensional active-passive wedge- sliding mechanism, with two main landslide blocks and five sub-blocks in the west block, separated by secondary shear surfaces. The 1959 Madison Canyon Slide in Montana, USA, was triggered by an M = 7.5 earthquake. A 20-million-m3 rock mass descended from the ridge crest, killing 24 people and blocking Madison River to create Earthquake Lake. Marble at the toe of the slope acted as a buttress for weaker schist and gneiss upslope until the earthquake undermined its integrity and triggered failure. Rock mass characterisation, long-range terrestrial digital photogrammetry, and kinematic analysis indicate that the lateral, rear, and basal release surfaces formed a hexahedral wedge-biplanar failure. Dynamic numerical modelling suggests topographic and damage amplification due to ridge geometry and pre-existing tension cracks. Analysis of the case studies highlights the complexity of large, catastrophic rock slope failures, their causes, and their evolution from incipient failure to disaster.
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