Forensic analysis of rockfall scars

Abstract

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Controls on the geotechnical response of sedimentary rocks to weathering

“…Figure 14 illustrates this process, with each 'new' failure event weakening the rock, so that the next stage of brittle fracturing requires a lower magnitude of stress to act as a trigger for further failure. This multi-stage failure processes may be represented as 'step-wise' fracture through multiple rock bridges or partial fracturing through an individual rock bridge, where fracture represents an initial failure stage of the stress-strain graph (Brideau et al, 2009;Eberhardt et al, 2004b;de Vilder et al, 2017). For final failure to occur, only a low magnitude stress perturbation may be required (Figure 14) due to the critical concentration of micro-cracks and accumulated damage within the rock (Main, 2000).…”

“…Conceptual stress-strain diagram of the stages and drivers of weathered brittle rock failure, displaying initial micro-crack initiation and propagation thresholds for intact rock (adapted from Eberhardt et al, 1998). damage accumulation in Figure 14) to occur before the final loss of strength and collapse (de Vilder et al, 2017). [Colour figure can be viewed at wileyonlinelibrary.com] concentrations within the slope (Brain et al, 2014).…”

“…In such a scenario, even though topographic stress is not a control on the rate of weakening, as we have determined from our experimental datasets, it may therefore control the location of rock failure. The time-scales over which such deformation could be observed is also dependent on the exact rock bridge attributes within an incipient failure mass, with the amount of deformation increasing for larger rockfalls (Kromer et al, 2017b), reflecting the breakage of multiple rock bridges (de Vilder et al, 2017). Monitoring observations have revealed that external precipitation triggering events can initiate extended periods of increased and accelerated deformation indicating a critical damage threshold has been reached in the rock, with previous observations showing that in some settings that this can accelerate to final failure within a week (Carlà et al, 2018;Kromer et al, 2017a;Loew et al, 2016).…”

“…Durgin, 1977;Fookes et al, 1988;Hencher and McNicholl, 1995;Migon, 2010;Thomson et al, 2014;Yatsu, 1988) and its importance for rock slope stability has been demonstrated in numerical and analogue studies (e.g. Selby, 1980;Hoek, 1983), but such schemes do not sufficiently consider weathering-induced strength degradation of intact rock bridges that critically influence shallow rock slope failures and rockfall activity, which are our focus here (de Vilder et al, 2017;Jennings, 1970;Kemeny, 2005). Engineering classifications provide descriptive insight into the nature of weathering along discontinuities (e.g.…”

“…Moses et al, 2014). Firstly, few of these studies consider changes in rock strength at a scale relevant to rock slope failures, particularly for small and shallow rockfalls where stability is controlled by one critical rock bridge (de Vilder et al, 2017). Firstly, few of these studies consider changes in rock strength at a scale relevant to rock slope failures, particularly for small and shallow rockfalls where stability is controlled by one critical rock bridge (de Vilder et al, 2017).…”