Beishan granite is a candidate host rock for the construction of China's high level radioactive waste (HLW) repository. Knowledge of the mechanical behaviors of the host rock is essential to engineering design of the radioactive waste disposal system in the rock. In this paper, uniaxial and triaxial compression tests on the Beishan granite were conducted using a servo-controlled compression system. The rock samples were taken from 560 to 575m below the ground surface in the Xinchang rock block of the Beishan area, China. Acoustic emission (AE) monitoring combined with stress-strain measurements were employed to investigate the deformation and failure behaviors of the rocks. Variations in waveform parameters were recorded continuously during rock fracturing. According to the characteristicAEparameters, a method of determining the crack initiation stress (σ:ci) and crack damage stress (σ:cd) was proposed. The experimental results indicate that the proposed AE method can be used effectively for the determination of crack initiation stress and crack damage stress thresholds in both unconfined and confined compression tests.
Deep geological disposal is internationally accepted as an appropriate and feasible way to deal with highlevel radioactive waste (HLW). A HLW repository can be constructed in a host rock at a depth of several hundred meters below the surface. During operation of a HLW repository in the rocks, rock cracking and damage near the excavation boundary may occur due to excavation-induced stress increase and sustained thermal loading. The damaged rocks with coalescent cracks or fractures can provide paths for groundwater flow and potentially radionuclide transport in and around the repository. In this complicated context, thermo-hydro-mechanical coupled processes govern the rock mass response. Among all the engineering properties of rocks, rock strength is one of the important factors because it has a direct impact on the layout of the repository. For instance, rocks with low damage strengths will require a wider distance between the disposal elements. Hence, a better understanding of strength characteristics of the host rock is essential for the design and performance assessment of a radioactive waste disposal system.
The rock failure process is associated with acoustic emission (AE). AE can be defined as the transient elastic wave generated by the rapid release of energy from a source within a material (Koerner et al. 1981) Due to high sensitivity to crack initiation, propagation and coalescence in rocks subjected to loading, the non-destructive and dynamic real-time monitoring method provides a powerful tool for investigating brittle rock failure and this technique has been widely used in rock mechanics studies and engineering applications (Mlakar et al. 1993; Chang & Lee 2004; Cai et al. 2007; Moradian et al. 2010). In general, AE event counts, AE count rates scaled with the stressstrain relationship, and source location of the events are frequently used by researchers to study the rock failure characteristics under different loading conditions. In addition, somewaveformparameters involved in the generation of an AE event also provide useful information to further investigate rock cracking mechanisms associated with macroscopic deformation and failure. For example, amplitude distributions of AE signals have been used to infer the relationship between variations of AE event magnitudes and failure processes of rocks (Sun et al. 1991). He et al. (2010) has correlated the frequency–amplitude of AE signals with the rockburst behaviors. A schematic illustration of these AE parameters is shown in Figure 1.