Numerical Modelling of the Micromechanical Behaviour of Catastrophic Long Run-Out Rock Avalanches

Abstract

Long runout rock avalanches occur in steep mountainous areas, typically due to the effects of heavy rain, freeze-thaw cycles or seismic shaking among others. The runout is renowned for travelling a large horizontal distance in comparison to the vertical fall height and the extent is largely determined by the volume of source rock. The material dynamically disintegrates during runout depositing angular fragments surrounded by rock flour and preserving the original stratigraphy.

The propagation mechanism of long runout rock avalanches has been debated for over a century. The majority of the mechanical theories suggested to explain the long runout behaviour may focus on a particular aspect noted from one or several events, however, do not properly explain the angularity of the grains within the deposit or the possible internal behaviour of the avalanche.

This thesis aims to investigate the fragmentation theory of rock avalanche propagation from a soil mechanics perspective. The rapid application of load and high speed shearing is postulated to cause the dynamic fragmentation of debris, where the rapid movement of fragments and fines reduces effective stress and therefore friction, increasing mobility. Material then moves rapidly until all available kinetic energy has been dissipated or no further dynamic fragmentation can occur. The potential influence of multiple dynamic fragmentation events occurring at once provides useful information for the prediction and extent of rock avalanches, along with micro scale behaviour of rock under rapid loading and high speed shearing for mining purposes.

Discrete Element Modelling (DEM) via the use of PFC3D has been utilised to undertake oedometer and shear box testing of idealised samples. These tests are used to represent dominant mechanisms that occur in the two key periods of a long runout rock avalanche — the fall (modelled by high strain rate oedometer testing) and the runout (modelled by high speed shear box testing). Synthetic and fully calibrated bonded particle models are used to investigate the response of rock boulders under these conditions. The calibrated materials of sandstone, weak chalk and extremely weak chalk were chosen to represent typical large and small scale long runout events.

Numerical oedometer testing reveals that the application of high strain rates normal to the ground surface produces fast and significant breakage along with a noticeable response in kinetic energy and a reduction in mobilised friction. The additional kinetic energy remains available in the system for a longer period of time than that produced by semi-static strain rates. The high strain rate oedometric tests suggest that dynamic fragmentation occurs under fast loading rates. It is plausible that dynamic fragmentation in a sturzstrom due to rapid loading at the transition point from fall to runout can enhance mobility.

High speed shear box testing indicates a significant rise in normal and shear stresses resulting in intense crushing and dilation rather than dynamic fragmentation. Kinetic energy produced from breakages is quickly dissipated through dilation and does not remain in the system long enough to influence mobility. The majority of the shearing is likely to occur in a small zone at the base of the debris. The minimal mixing of layers during the shear testing and substantial dilation supports the observed preservation of stratigraphy and increase in debris volume seen at the majority of sturzstrom sites.