Research Highlight

Experimental insights into shear behavior and acoustic emission during granular shearing

Pubdate:2017-11-24    From: ANDF    Views:

Landslides are among the most hazardous of geological processes, causing thousands of casualties and damage annually. Landsliding phenomena generally exhibit a great diversity of natural gravitational granular-flows in terms of constituent materials (e.g., rocks, soils and mixtures of different materials), volumes involved (e.g., from several cubic meters to hundreds of cubic kilometers), displacement velocities (e.g., from a few centimeters per year to tens of meters per second), triggering factors (e.g., rainfall, earthquake, volcano, human activity and climate variability) and physical processes (e.g., fragmentation, segregation, erosion and deposition). Therefore, over the past several decades, efforts including experimental, theoretical, numerical and field investigations have been put forward to revealing the mechanisms regarding landsliding phenomena. For instance, the composed materials could be lubricated with the presence of the fluidizing medium (such as water, vapor, volcanic gases or suspension of fine particles) and thus landslide mobility would be elevated greatly due to the drastic reduction in shear resistance. Moreover, by examining the frictional properties of simulated shear zones at high slip rates, the ultralow steady-state friction has been reported in the literature owing to a number of physicochemical processes (such as melting, decomposition or dehydration). Although these investigations have effectively provided comprehensive information regarding the frictional strength characteristics of geological materials, it is still difficult to clearly know how the granular behaviors in shear affect the complex motions during landsliding. Because landslide materials interact with each other via different particle sizes, shapes, and gain frictional forces through the interactions of grains to resist movement. Fundamentally, what are the roles of particle features and mechanical conditions regarding the progressive failure processes and subsequent rapid landsliding phenomena? Because this question is significant in both predicting landslide disasters and promoting mitigation capabilities.

Importantly, insights from modern granular physics indicate that the evolutions of mesoscopic grain interactions play an important in controlling granular dynamics. Parts of the stored energy in the mass can be released through the generation of high-frequency components of elastic waves, termed acoustic emissions (AEs). The generated AEs deliver invaluable information concerning the physical processes and failure mechanisms in relation to granular deformations. However, the fundamental links between the grain-scale frictional behaviors in sheared granular materials and associated AE characteristics are still not clear. Therefore, my research work aimed at revealing the relationship between grain-to-grain frictional behaviors and the properties of AEs in sheared granular materials with particular emphasis on the influence of particle size and shear speed. In order to explore some fundamental variables in affecting granular shearing, cohesionless glass beads were used as analogy materials to remove particle shape as a variable and isolate the role of particle size. Two intelligent ring shear apparatuses were employed to meet the demands of a wide range of shear speeds (from 0.005 mm/s to 1.0 m/s in some cases) and large shear displacement (up to several meters). All dry granular assemblies with uniform particle size ranging from approximately 0.1 to 5.0 mm were sheared. For measurements of elastic waves, three high frequency AE transducers were installed near the shear plane, and AE signals were sampled at the rate of 1MHz together with the mechanical data during the shear tests.

The major achievement of research work is that we successfully observed the frictional instabilities and AE events for sheared granular materials, and one representative experimental result is shown in Figure 1. Based on the studies, several research findings have been published on the journal of Engineering Geology (2016, 210: 93-102) and Geophysical Research Letters (2017, 44(6): 2782-2791). and the addressed scientific concerns are presented as follows.

(1)    Do the frictional instabilities and AE events depend on the particle size and shear speed?

With decreasing particle size or increasing shear speed, the frictional behavior of granular materials transitions from unstable instability to stable sliding. In the unstable instability regimes, experimental results show that there is a strong correlation between frictional instabilities and AE events (Figure 1). The magnitudes of stress drops and the amplitudes of AE waveforms increase with particle size, but decrease with shear speed following power-law relations. The stress drops increase logarithmically with increasing recurrence interval of instability events but the frictional instability rate decreases with decreasing particle size. The mean number of AE events per second increase with shear speed. The primary frequency bands of recorded AEs are characterized in tens of kHz range for sheared granular materials. We concluded that the slip between particles is the only cause of observed frictional instabilities and recorded AEs.

(2)    Whether the AE events are precursors or resultant phenomena of mechanical failures?

We experimentally evidenced that ultrasonic amplitudes increase prior to the mechanical failure and local failures can transiently trigger the global failure. By analyzing the time-sequence between recorded AEs and mechanical failures, we found that the onsets of AE amplitudes precede the impending global mechanical failures (Figure 1D). This time sequence indicates that local failure within the grain mass occurs first, and results in the generation of AEs. Therefore, we concluded that such AEs can generate ultrasonic vibrations among the grains, which can trigger more failures along the local force chains, and finally lead to the impending global failure.

(3)    How the frictional instability affect the landslide motions?

We contributed a scientific knowledge of better understanding landslide dynamics. The frictional instability phenomenon shows that the energy needed for the landsliding debris to overcome the shear resistance will be less than that estimated from the stable sliding. In this case, the occurrence of frictional instability will have greater potential to elevate the mobility of displaced landslide materials.

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Figure 1. One selected our result for glass beads. (A): observed shear resistance; (B): monitored acoustic emission; (C): recorded vertical displacement; (D): event sequence between shear resistance and spectrum of AEs.