Brittle Creep and Associated Acoustic Emissions in Granite Under Unconfined Compression

Abstract

ABSTRACT Time-dependent processes in rock govern its long-term behavior hence understanding of these processes is of great importance especially for long-lasting surface and underground structures. Results from conventional creep experiments have shown that three stages of creep (trimodal creep curve) exist for brittle materials. The first stage is the primary stage where the strain rate is inversely proportional to time. In the secondary stage, partially reversible strain occurs in which the strain rate is small and constant. The final stage is the tertiary stage where accelerated strain rate occurs, causing the failure of the rock. It is hypothesized that Acoustic emission (AE) can be used as a proxy to identify the transitions from one stage to another in creep; however, the different focal mechanisms of the fractures produced in the three stages of creep in brittle rocks have not been discussed in detail. The study presented in this paper aims at investigating how AE signatures can monitor the brittle deformations during the three stages of creep and its correlation with the mechanical observations, so that these AE signatures can be used to gain important information in the field. Creep experiments were conducted on double-flawed prismatic Barre granite specimen in the laboratory under uniaxial compression to investigate the temporal and spatial evolution of the AE events during the primary, secondary and tertiary stages of creep. Results illustrated that brittle creep in granite follows the Omori's law and inverse Omori's law in the primary and tertiary creep regimes, respectively. Non-double-couple (NDC) sources were the dominant failure mechanisms in all stages of creep. These results can be of great importance as continuous and non-destructive monitoring of structures in rock can serve as precursory indications of instability of the rock engineering structures as well as in the analysis of earthquake aftershocks and recurrences. INTRODUCTION Fracturing in rocks can happen over time; hence, it is important to understand the microcracking mechanisms of time-dependent deformation in rocks. The analysis of time-dependent effects is significant in multiple areas, including the study of seismic hazards like earthquake recurrence and aftershocks (Scholz in 1968). It is also crucial in evaluating the long-term stability of rock engineering structures, such as tunnels, mining rooms and pillars, rock slopes, and nuclear waste repositories (Diederichs and Kaiser 1999, Nara et al. 2010, Paraskevopoulou et al. 2018, Zhang et al. 2020, and Zhao et al. 2022). Moreover, time-dependent effects also have a significant impact on the exploration and utilization of resources such as oil and gas reservoirs and enhanced geothermal systems (EGS) (Demarest 1976; Cornet 2007).