Influence of weathering and pre-existing large scale fractures on gravitational slope failure: insights from 3-D physical modelling

Abstract

Abstract. Using a new 3-D physical modelling technique we investigated the initiation and evolution of large scale landslides in presence of pre-existing large scale fractures and taking into account the slope material weakening due to the alteration/weathering. The modelling technique is based on the specially developed properly scaled analogue materials, as well as on the original vertical accelerator device enabling increases in the "gravity acceleration" up to a factor 50. The weathering primarily affects the uppermost layers through the water circulation. We simulated the effect of this process by making models of two parts. The shallower one represents the zone subject to homogeneous weathering and is made of low strength material of compressive strength σ l . The deeper (core) part of the model is stronger and simulates intact rocks. Deformation of such a model subjected to the gravity force occurred only in its upper (low strength) layer. In another set of experiments, low strength (σ w ) narrow planar zones sub-parallel to the slope surface (σ w <σ l ) were introduced into the model's superficial low strength layer to simulate localized highly weathered zones. In this configuration landslides were initiated much easier (at lower "gravity force"), were shallower and had smaller horizontal size largely defined by the weak zone size. Pre-existing fractures were introduced into the model by cutting it along a given plan. They have proved to be of small influence on the slope stability, except when they were associated to highly weathered zones. In this latter case the fractures laterally limited the slides. Deep seated rockslides initiation is thus directly defined by the mechanical structure of the hillslope's uppermost levels and especially by the presence of the weak zones due to the weathering. The large scale fractures play a more passive role and can only influence the shape and the volume of the sliding units.

Composite faults in the Swiss Alps formed by the interplay of tectonics, gravitation and postglacial rebound: an integrated field and modelling study

“…3b) because these processes can explain the large variations of the scarp height along strike of the composite faults. As suggested by three-dimensional physical models, deep-seated landslides provide a feasible mechanism for the formation of uphill-facing scarps (Bachmann et al 2004). Furthermore, the observation that displacements are largest when the fault crosses a ridge on the valley flank agrees with the fact that toppling mechanisms act most strongly on the bedrock at ridges (Fig.…”

Deep-seated failure propagation in a fractured rock slope over 10,000 years: The La Clapière slope, the south-eastern French Alps

“…Such deformations lead to progressive movement lasting hundreds to thousands of years (Chigira, 1992;Bovis and Evans, 1996;Crosta and Agliardi, 2002;Brückl and Parotidis, 2005;Jomard, 2006). Studies of the initiation and evolution of deep-seated slope deformations are usually conducted through: (1) laboratory tests on rock samples (Boukharov and Chanda, 1995), (2) physical modelling (Bachmann et al, 2004(Bachmann et al, , 2006, (3) numerical modelling (Agliardi et al, 2001), and (4) field work for landform observations, qualitative mapping and geophysical measurements (Lebourg et al, 2005;Jomard et al, 2007). The laboratory tests usually show a deformation in three phases: (a) a slow initial step with a low and constant deformation rate, (b) an exponential increase of the deformation rate and (c) rupture.…”

Landslides and Rockfalls

“…Tertiary movement represents a period in which strain weakening becomes progressively dominant, leading to complete rupture surface development and thus to final failure (Petley, 2004). The nature of the tertiary movement phase in landslides is primarily dependent on the basal deformation process, but is complicated by, for example, slope topography and discontinuities (Bachmann et al, 2004).…”