In order to evaluate the applicability of Acoustic Emission (AE) as an indicator of instability of active joints in dam structures, an extensive feasibility study was done on 40 joint samples. To this end, direct shear tests were conducted under different conditions (in various normal loads and displacement rates) and different joint characteristics (with various roughness and bonding percentages) and AE signals were acquired using attached sensors to the samples. In this study two methods were used to verify the effect of normal load as one of the most important parameters affecting shear behavior and consequently generated AE signals during direct shear test of rock joints. In the first method several samples were tested under normal stress of 0.5, 1 and 2MPa respectively. In the second method, normal stress was changed from 0.5 to 2MPa in the residual section of the same sample. The results showed that when normal load increases, the AE signals show a more significant peak in maximum shear stress point. Increasing and decreasing normal load during residual section make significant changes in corresponding AE signals.
Previous researches have addressed application of AE for monitoring rock joints both in laboratory (Moradian et al. 2008, Hong & Seokwon 2004, Li & Nordlund 1990) and in site (Sasao et al. 2003, Shiotani et al. 2001). Among several parameters affecting shear strength of rock joints, the magnitude of the normal stress has the most important role (Barton & Choubey 1977). Any change in this parameter makes a significant change in shear behavior and, consequently, in the generated AE signals (Li & Nordlund 1990, Filimonov 2005). In order to apply AE technique for stability monitoring of rock joints successfully, it is necessary to understand the effect of normal stress onAE activity in rock joints.