Dynamic behavior of humid granular avalanches: Optical measurements to characterize the precursor activity

Abstract

Laboratory study of slope stability of granular media remains a challenge for modeling, understanding and predicting natural hazards, such as avalanches and landslides, which precursory signs are controlled by numerous physical parameters. The present work focuses on the impact of the humidity, in the range 40-90 %, on the stability of monodisperse dense packings of spherical beads. The beads are in a transparent box that is slowly and continuously tilted and allows simultaneous top and lateral optical measurements of global displacements of grains at the surface, defined as precursors. Humidity increases the cohesion between the grains. By performing successive avalanches that destabilize deeper granular layers, we assess the role of the exposure time to the high humidity rates in the diffusion process to reach the hygroscopic equilibrium inside the packing. We highlight an increase of the stability and first precursor angles, associated to a constant angle increment between two consecutive precursors, with a dependency with both the diameter (0.2, 0.5 and 0.75 mm) and the material (glass and polystyrene) of the grains.

Robust experimental study of avalanche precursory events based on reproducible cycles of grain packing destabilizations

“…Changes in the shadows surface between two consecutive images are used to detect grain displacements: an ImageJ [24] script, initially developed by Kiesgen de Richter et al [2], consists in subtracting two consecutive images, thresholding the result to remove the noise and then quantifying the surface S of the displaced grain shadows as a function of the inclination angle. Based on the ratio S /S 0 hereafter called the surface activity, the detection of a precursory event is like the approach used in previous studies [2,5,17,19] and corresponds to a surface activity close to unity.…”

“…A better understanding of the precursor activity is re-quired to predict the onset of the avalanche. The avalanche triggering and the eruption of precursors are controlled by several physical parameters, such as the packing dimensions (height [7], length [2] and width [8][9][10][11]), the density or volume compaction of the packing [2,7,[12][13][14], the tilting regime [4,15,16] and the humidity inside the packing [17]. Most of the previous experiments [1-3, 5, 7, 13, 17-19] dedicated to precursor identification have been performed by direct tilting experiments of a grain packing from the horizontal position to the avalanche.…”

Powder flow behavior governed by the surface properties of glass beads

“…In this case, the powder has been kept in a humid chamber longer to ensure absorption of enough humidity. With this purpose, the sufficient range of humid control time has been reported to be two days as minimum and one week as maximum storage time in a humid chamber for glass beads [46,47]. According to the literature the powder behavior has the same trend in both storage times.…”