Contributions of Rock Mass Structure to the Emplacement of Fragmenting Rockfalls and Rockslides: Insights From Laboratory Experiments

Abstract

Rockfalls and rockslides often occur in mountainous areas, and they may develop into rock avalanches because of fragmentation. A series of laboratory experiments were conducted to study the contributions of rock mass structure to the emplacement of fragmenting rockfalls and rockslides. In these experiments, we considered the process of breakable analog blocks with different structures sliding along an inclined plane, impacting at the kink point with a horizontal plane where deposition occurs. The results show that the initial geometrical subdivision (i.e., the rock mass structure) of the source rock can greatly influence the impact of the fragmentation process and total runout, while the degree of fragmentation controls the travel distance of the center of mass. The occurrence of transversal discontinuities enhances the momentum transfer efficiency from the rear to the front part of the rock mass. A negative correlation between the apparent friction coefficient (linked to the total runout) and equivalent friction coefficient (linked to the center of mass runout) was found, which appears to be induced by fragmentation. We proposed a new fragmentation-spreading model to describe this negative correlation. This simple physical model supports the importance of fragmentation in rock fragment trajectories and the runout of rockfalls and rockslides. Fragmentation is an energy-sinking process that will shorten the runout of the center of mass. Thus, we suggest that impact fragmentation does not fully account for the long runout of large rockfalls and rockslides.

Short communication: Runout of rock avalanches limited by basal friction but controlled by fragmentation

“…We compare our experimental results in Fig. 3 with data from nine rock avalanches reported by Locat et al (2006) that show no clear volume dependence of run-out. This feature makes this data set ideal for testing whether a scaleindependent process is operating besides dynamic basal weakening.…”

“…Recently, the process of dynamic fragmentation has received increased attention from the research community, and much progress has been made in our understanding of its role in the dynamics of rock avalanches (Locat et al, 2006;Imre et al, 2010;Bowman et al, 2012;Pudasaini and Miller, 2013;De Blasio and Crosta, 2015;Haug et al, 2016;Zhao et al, 2017Zhao et al, , 2018Lin et al, 2020;Gao et al, 2020;Knapp and Krautblatter, 2020). Firstly, one may expect that the finer the material is, the more flow-like the behaviour will be, increasing its mobility and allowing the rock mass to spread more easily (Locat et al, 2006;Wang et al, 2017;Zhao et al, 2018). Secondly, models of fragmenting rockslides suggest that dynamic fragmentation actively increases the spreading (Bowman et al, 2012;De Blasio and Crosta, 2015;Lin et al, 2020).…”

Variations in runout of rock avalanches controlled by fragmentation, not basal friction

“…Only recently, the process of dynamic fragmentation received increased attention and progress occurs in our understanding of its role in the dynamics of rock avalanches (Locat et al, 2006;Bowman et al, 2012;De Blasio and Crosta, 2015;Haug et al, 2016;Zhao et al, 2017Zhao et al, , 2018Lin et al, 2020;Gao et al, 2020;Knapp and Krautblatter, 2020). Firstly, one may expect that the finer the material, the more flow-like the behavior, increasing its mobility and allowing the rock mass to spread more easily (Locat et al, 2006;Wang et al, 2017;Zhao et al, 2018).…”

“…Firstly, one may expect that the finer the material, the more flow-like the behavior, increasing its mobility and allowing the rock mass to spread more easily (Locat et al, 2006;Wang et al, 2017;Zhao et al, 2018). Secondly, models of fragmenting rockslides suggest that dynamic fragmentation actively increases the spreading (Bowman et al, 2012;De Blasio and Crosta, 2015;Lin et al, 2020). However, fragmentation has also been shown to consume energy (Haug et al, 2016;Zhao et al, 2017;Lin et al, 2020), potentially at a cost to the runout length.…”

“…Secondly, models of fragmenting rockslides suggest that dynamic fragmentation actively increases the spreading (Bowman et al, 2012;De Blasio and Crosta, 2015;Lin et al, 2020). However, fragmentation has also been shown to consume energy (Haug et al, 2016;Zhao et al, 2017;Lin et al, 2020), potentially at a cost to the runout length. Clearly, the integrated effect of fragmentation on the runout dynamics of rock avalanches remains to be specified.…”