Evaluation of Non-Destructive Stress Measurement Methods for Investigating Stress Memory in Rocks

Abstract

ABSTRACT Rock bursts and spalling are major concerns for the design of deep underground structures under high in-situ stress conditions. A full constrain of in-situ stress magnitude and spatial distribution is therefore essential for managing the occurrence and severity of such phenomena. In recent decades, non-destructive stress measurement methods have been developed as a reliable alternative to traditional methods. These methods are based on rock stress memory and rely on accurate measurements of strain and acoustic data during uniaxial cyclic loading of specimens. In this study, several non-destructive stress measurement methods based on strain measurement and acoustic emission were evaluated. Uniaxial cyclic loading and unloading tests were performed on cylindrical specimens of Hawkesbury sandstone, and the deformation was monitored using strain gauges, Linear Variable Differential Transformer (LVDTs), Digital Image Correlation (DIC) and high-speed acoustic emission (AE) system. The strength and capability of each method in determining the applied stresses were evaluated by analyzing the strain and acoustic data collected during uniaxial cyclic loading. Although all methods were found effective in determining the applied stresses, our results indicate that the DRA and SMM methods were more precise and consistent. INTRODUCTION The Kaiser effect is a phenomenon observed in rock specimens when a uniaxial cyclic loading is applied in a laboratory. Acoustic emission (AE) activity begins just after the applied stress exceeds the peak of previously applied maximum stress to the specimen (Lavrov, 2002). This memory of previously applied stresses is found not only as a change in AE activity but also in the strain rate of rock specimens under a constant loading rate, known as the stress memory (Yamamoto et al., 2009). Kuwahara et al. (1990) proposed a model for the inelastic deformation of rock specimens under axial loading of compression to explain the mechanism of the Kaiser effect. According to Kuwahara et al. (1990) there are many potential micro-cracks in a rock specimen, and the strength of the specimen depends on potential cracks and applied stress to the specimen. If the applied stress is increased to a certain magnitude, all the potential micro-cracks with strengths smaller than the applied stress should initiate or propagate (Hoek and Martin, 2014). When the specimen is subjected to stress again after unloading, new micro-fractures hardly occur at first, and the pre-existing crack produced in the previous loading steadily moves with an increase in applied stress until the applied stress reaches the peak of the previously applied stress. New micro-fractures begin to occur only after the applied stress has reached the previous stress.