Fragmentation in the Val Pola rock avalanche, Italian Alps

Abstract

[1] Grain size data from the deposit of the 1987 Val Pola rock avalanche (central Italian Alps) are compared with data concerning rock avalanching, rock fragmentation, and comminution. The Weibull distribution fits a small part of the entire particle-size distribution of debris samples, with a mean value of the curve shape factor of 0.54 ± 0.28. This is typical of multiple comminution, or fragmentation with much shearing. A fractal distribution fits over a greater size range. Computed fractal dimensions range between 1.3 and 3.2 within the deposit, with average values of about 2.6-2.7. These values cover the range between the theoretical values of the plane-of-weakness model (1.97) and the pillar-of-strength model (2.84) and are close to the theoretical value for the constrained comminution model (2.58). These suggest that both texturally mature and immature deposits are present and that more than a single comminution process acted during the rock avalanche motion. Variation of the grain size distribution within the deposit and grain size segregation show as trends in the fractal dimension and arise from variation in the fragmentation process. A variety of different physical and empirical laws suggest that 1-30% of the energy expended in the rock avalanche was consumed in fragmentation.

Dynamical stress in force chains of granular media traveling on a bumpy terrain and the fragmentation of rock avalanches

“…The evident discrepancy between observed and predicted final grain size shows, unsurprisingly, that only a small fraction of the available energy goes into the comminution process and that the fragmentation in a rock avalanche might be dynamically different from that occurring in a comminution device. Similar conclusions have been drawn by Crosta et al [4] for the recent Valpola landslide in northern Italy. In any case, it is concluded that the energy estimates are not informative on the physics of fragmentation, and one has to consider the kinetics of the fragmentation process.…”

“…The expression can also be transformed as the ratio of two incomplete gamma functions. Table 2 shows the results of the calculation compared to data taken from Crosta et al [4]. According to the measurements of ref.…”

“…According to the measurements of ref. [4], at a depth of 60-70 m, the median value of diameter distribution D 50 ranges between 0.6 and 15 mm depending on the position in the rock avalanche deposit. The above coupled formulas show that at these depths, particles of diameter smaller than 70 mm will not be broken by static force chains.…”

Simulation of the erosion process of landslide dams due to overtopping considering variations in soil erodibility along depth

“…Most landslide dams are highly heterogeneous along depth and along the run-out direction (Dunning, 2006;Crosta et al, 2007;Chang et al, 2009;Evans at al., 2007), which is a crucial factor in determining the failure time and outflow hydrograph (Costa and Schuster, 1988;Casagli et al, 2003). The capability of existing models to consider the variations in soil erodibility along depth has yet to be improved.…”

A methodology to obtain the block size distribution of fragmental rockfall deposits

“…Complete inventories including deposited blocks of all sizes are difficult to obtain. Several researchers mainly working on rock blasts, have used photoanalytical techniques to obtain the fragment size distribution (Crosta et al 2007). However these provide results only for the block area and not for the volume.…”