ABSTRACT
Z. Moradian Department of Civil and Environmental Engineering and Earth Resources Lab (ERL), Massachusetts Institute of Technology (MIT), Cambridge, MA, USA
Abstract: This chapter focuses on using acoustic emission (AE) technique for investigating shear mechanisms of rock discontinuities from the initial movement up to the residual state. Direct shear tests were done on rock joints and AE technique along with image analysis and 3D topography of the surfaces, scanned by a laser profilometer, were used to obtain insights into several stages in the shear failure process of rock discontinuities. These stages are: I: pre-peak linear period, II: pre-peak non-linear period, III: post-peak period and IV: residual shear strength period. The rate and cumulative graphs of the AE parameters such as number of hits, and energy were correlated to the shear stress-shear displacement of the tested rock joints. These correlations revealed that AE has a high competency as a precursor prior to shear failure of discontinuities and therefore it can be used as an indicator of instability in slopes and structures suffering from sliding along discontinuities. These observations can also open a door for a better understanding of the mechanisms of faulting and finally for earthquake prediction. Locations of the AE event sources were determined from propagation velocity of acoustic waves and the traveling time from the event source to the AE sensor. These sources correspond to damage zones that are caused by active asperities in shearing process, mostly asperities facing the shear direction. The distribution of the source locations and their associated AE energy can provide an estimation of contact areas between upper and lower surfaces of the rock joint. It also helps to detect location, size, and failure intensity of the damaged zones during each stage in the shearing process. Presence of the AE events with low energy before shear stress peak revealed that slipping/shearing process may start from zones with less frictional resistance and then it will be controlled by rough asperities with higher frictional resistance.
