Extremely Energetic Rockfalls

Abstract

Extremely energetic rockfalls (EERs) are defined here as rockfalls for which a combination of both large volume and free fall height of hundreds of meters results in energy larger than about 80 GJ released in a short time. Examples include several events worldwide. In contrast to low energy rockfalls where block disintegration is limited, in EERs the impact after free fall causes immediate release of energy much like an explosion. The resulting air blast can snap trees hundreds of meters ahead of the fall area. Pulverized rock at high speed can abrade vegetation in a process of sandblasting, and particles suspended by the blast and the subsequent debris cloud may travel farther than the impact zone, blanketing vast areas. Using published accounts and new data, we introduce physically based models formulated on analogies with explosions and explosive fragmentation to describe EERs. Results indicate that a portion of the initial potential energy of the block is spent in rock disintegration at impact (typically 0.2%–18%), while other sources of energy loss (air drag, seismic, sound, and ground deformation) are negligible; consequently, more than 80% of the potential energy is converted to kinetic energy of the fragmented block (ballistic projection, shock wave, sand blast, and dust cloud). We also propose simple estimates for the flow of the dust cloud associated with an EER and its long settling time. The areal extent of the affected zone is estimated from the energy balance and an empirical power law relationship.

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

“…There are many empirical/semiempirical and simplified physically based models describing the propagation of a failure mass, and these models have been applied to attempt the estimation of the travel distances of natural failure masses (De Blasio et al, 2018; Okura et al, 2000; Pastor et al, 2014; Strom & Abdrakhmatov, 2018). However, few analytical models consider the effects of fragmentation on the runout of the fragmenting failure mass (De Blasio et al, 2018; De Blasio & Crosta, 2015; Jaboyedoff et al, 2005; Ruiz‐Carulla et al, 2017). Here, we introduce the “fragmentation‐spreading model” proposed by Davies et al (1999), which is modified based on our experimental results, to describe the phenomenon of our experiments and explain the inverse relationship of the friction coefficient in the framework of the fragmentation process.…”

“…However, Davies, McSaveney, & Reznichenko (2019) and Davies, Reznichenko, & McSaveney (2019) inferred that only very small part of input energy is taken up as surface free energy in a newly created fragment surface (Savvova et al, 2015; Zdziennicka et al, 2009). For practicality, De Blasio et al (2018) proposed a semiexperimental method by which to obtain the fragmentation energy loss from the fragment size distribution, which indicated that, typically, 0.2–18% of the potential energy is consumed by fragmentation. The spreading coefficient due to fragmentation ( S f ) strongly depends on the energy consumption efficiency of fragmentation ( K f ).…”

“…Only if we can clearly infer K f during fragmentation process can we then calculate S f based on the energy conservation law. Thus, the simplified analysis method used in De Blasio et al (2018) and our fragmentation‐spreading method are still useful in practice with appropriate parameter calibration based on experimental and field data sets.…”

“…Rockfalls and rockslides are always associated with rock mass disintegration and dynamic fragmentation, which are the critical conditions for the evolution of rock avalanches (Crosta et al, 2007; Hewitt et al, 2008; Corominas et al, 2012; Ruiz‐Carulla et al, 2017; De Blasio et al, 2018). Compared with the block size in the source area, the particle volume of the rock mass can be reduced by up to 15–18 orders of magnitude after intense fragmentation during movement (Locat et al, 2006; De Blasio, 2011).…”

“…This theory has also been supported by preparing an approximated energy budget for a real rock avalanche (Davies et al, 2019). Rhodes (1998) also reported that the traditional method for fragmentation energy calculation overestimates the fracture energy dissipation (De Blasio et al, 2018) Thus, fragmentation is an important phenomenon throughout the rock mass movement spectrum and requires further study.…”