Multi-stage structural and kinematic analysis of a retrogressive rock slope instability complex (Preonzo, Switzerland)

Abstract

Large catastrophic rock slope failures are difficult to predict because the underlying mechanisms causing slope accelerations are difficult to study under in-situ conditions. For slope failures with compound basal rupture planes, not only fracture propagation and slip in the basal rupture surface, but also within the landslide body control the displacement evolution and time-to-failure. The Preonzo instability complex in southern Switzerland failed several times since 2002 and offers a unique opportunity to study these mechanisms under in-situ conditions. The largest failure at Preonzo occurred in May 2012 and was conditioned by large slope parallel (NNE-SSW) fracture zones, which are intersected by NE and SW and E-W striking faults creating diverging lateral boundaries. Whereas in the head scarp region oblique and flexural toppling along pre-existing fracture sets is the prevailing mechanism, sliding of either planar or wedge type is the preferred kinematic mode facilitated by the new discontinuities developed within the basal rupture surfaces. These dominating mechanisms, leading to a classical rock slope collapse, could only be revealed in retrospect. We compare fracture patterns in stable ground (tectonic and unloading fractures) with new fractures related to rock slope failure observed on basal rupture surfaces and in the head scarp. Simplified mechanical and kinematic analyses show that old pre-existing tectonic fractures cannot accommodate substantial deformations under the in-situ stress conditions and propagate/connect through curved wing cracks forming at high angles. This situation favors the development of new nearly slope parallel fractures, which are interpreted as synthetic P-shears along localized shear zones. Fractographic markings on these fractures indicate a continuous and relatively fast formation, which presumably has taken place during a period of about 60 years prior to catastrophic failure. This detailed description of the multi-stage evolution of slope failure at Preonzo represents a unique data set for future numerical studies of progressive failure in crystalline rocks.