acoustic probing of elasticity in the bulk of tilted granular layers-precursors of avalanches
Résumé
Earthquakes, avalanches and landslides have a shearing nature. The related mechanisms that take
place are strongly complex due to heterogeneities of geometry as well as internal and applied
stresses. Analogue experiments in laboratories are often designed with granular media to mimic the
mechanical behaviors of the crust at a much larger scale. For tri-dimensional granular materials, the
use of acoustic waves is a way to probe modifications in the bulk where the shearing process develops.
Here, we conduct experiments with monodisperse glass beads of 2 mm in diameter where humidity
and temperature are controled. We experimentally study by both linear and nonlinear acoustic
methods the destabilization of tilted granular layers where precursors of avalanches occur pseudo-
periodically. The quasi-periodicity of the avalanche precursors as a function of the tilt angle still
remains poorly understood. These precursors can be observed on the free surface of the layer as
collective motions of grains which increase exponentially in intensity until avalanche. A few important
parameters influence the precursor dynamics: the surface features of grains and the moisture.
Both of them are related to the cohesion between grains in the medium. The acoustic measurements
for probing the bulk elasticity of the granular layer are conducted at frequencies of a few kHz with two
piezoelectric transducers embedded in the sidewalls and thus working non-invasively.
A linear acoustic method allows to probe elastic relaxations to, which follows each precursor event.
In transmitting single gaussian pulses through the medium, it is also possible to observe an elasticity
softening effect. The method allows the measuring of the acoustic transfer function of the granular
layer and the acoustic wave phase velocity. When working in the frequency region where the
acoustic transfer function is significantly modified during the tilt, we show important variations of
the elasticity of the medium. This frequency region is in the low frequencies (under ~ 10 kHz), that
is for wave lengths much larger than the grain size. The granular assembly is thus seen as an effective
medium. The measurement of the phase velocity allows to estimate the Young moduli which is
found to slowly decrease during the tilting process. For each precursor softening occurs leading to a
loss of rigidity of the medium, and then hardening follows. This work may bring an interesting train
of thought on catastrophic events in geophysical processes.