Modelling Fragmentation in Rock Avalanches

Abstract

The physical description of rock masses travelling down a slope is a complex problem, involving bouncing, rolling, sliding, flowing, fracturing and/or combinations of these. Modelling serves as a valuable tool to study these systems which are rarely monitored at high resolution in nature. Often, granular models (e.g. loose sand) are used to study rock avalanches in experimental simulations. For such granular models, one has to assume that the rock mass disintegrates instantaneously after detachment and that fragment size does not reduce further during the movement. We present a new method that overcomes this limitation by simulating dynamically fragmenting gravitational mass movements.

We have developed a material that fails in a brittle manner at lab scale conditions. The material is produced by cementing sand with gypsum (anhydrite) or potato starch, which allows controlling the shear strength over a wide range. Experiments are performed by releasing the material down a slope and monitoring with a digital camera at frequencies of 50 or 250 Hz. Two techniques are used to quantify the experimental results: particle image velocimetry which quantifies the surface velocity field, and optical image analysis to derive geometric (e.g. fragment size distribution) and mechanical properties (e.g. basal friction) of the model.

Preliminary results from the experiments illustrate the different dynamics of the gravitational mass movement as a function of shear strength.