Four types of acoustic emission characteristics during granular stick-slip evolution

The shear behavior of granular materials plays an important role in understanding the occurrence of geological hazards. This study introduces the use of direct shear tests and acoustic emission (AE) technology on glass beads to investigate the distributions features of AE during the stick-slip processes. Results show that the shearing behavior of granular samples was shown as a series of similar, periodic stick-slip events. Some AE features — energy and Root Mean Square (RMS) — showed significant spatial clustering. Combined with the distribution of AE in the stick-slip event, the AE signal can be divided into four types: 1. low-energy and high-frequency AE, which represent particle friction; 2. high-energy and low-frequency AE, which represent structural failure, that is, the slip process; 3. low-RMS and low-frequency AE represent internal local failure; and 4. high-RMS and high-frequency AE caused by overall structure failure. The b-value representing the energy distribution of AEs is used to describe the changing of AE sources during shear process. In addition, the amount and energy of AE had a significant positive correlation with normal stress. The shear rate mainly affects the AE representing particle friction, and the faster the shear rate, the lower the incidence of these friction AEs. According to the time sequence of the occurrence of different types of AE, AE rate, b-value and local failure AE signal can be applied to the prediction or early warning of geological hazards.