Analyses on granular mass movement mechanics and deformation with distinct element numerical modeling: implications for large-scale rock and debris avalanches |
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Authors: | Nick Thompson Matthew R Bennett Nick Petford |
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Institution: | (1) Bournemouth University, Poole, UK |
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Abstract: | A large-scale avalanche of Earth material is modeled here as a granular flow using a distinct element numerical model PFC
2D. Such failures occur in a variety of geological settings and are known to occur frequently over geologic time-scales transporting
significant volumes of material basinward. Despite this, they remain poorly understood. The model used here begins with a
listric failure, typical of the flank collapse of a volcanic cone, and describes the movement of an assembly of several thousand
particles from failure to deposition. Within the model, each particle possesses its own material properties and interacts
with its immediate neighbors and/or the basal boundary during emplacement. The general mechanics of the particle assembly
are observed by monitoring the stresses, displacements, and velocities of distinct sections of the avalanche body. We monitor
the avalanches’ energy regime (e.g., gravitational influence, energy dissipation by friction, kinetic energy evolution, and
avalanche body strain). The addition of colored markers of varying geometry to the pre-failure avalanche was also used to
make qualitative observations on the internal deformation that occurs during avalanche emplacement. A general stretching and
thinning of the avalanche is observed. Monitoring of vertical and horizontal variations in stress, strain, porosity, and relative
particle stability indicate that the lower more proximal sections of the avalanche are subject to higher stresses. These stresses
are observed to be most significant during the initial phases of failure but decline thereafter; a situation likely to be
conducive to block fragmentation and in developing a basal shear layer in real-world events. The model also shows how an avalanche
which is initially influenced purely by gravity (potential energy) develops into a fully flowing assemblage as downslope momentum
is gained and kinetic energy increases. The horizontal transition where the failure meets the run-out surface is recognized
as a key area in emplacement evolution. The model has particular relevance to volcanic flank collapses and consequently the
implications of the model to these types of failure and the geological products that result are considered in detail although
the model is relevant to any form of large-scale rock or debris avalanche. |
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