Single-stage and Multi-stage Triaxial Deformation Experiments of Granite: Insights into Brittle Failure with Confinement Evolution

Geothermal energy plays a critical role in the global transition toward low-carbon and sustainable energy systems. To support the development of deep geothermal reservoirs, it is essential to understand the mechanical behavior of crustal rocks under realistic in-situ stress conditions. In this study, we investigate the deformation and failure behavior of granite—a key lithology in many geothermal contexts—using advanced triaxial testing techniques tailored for high-resolution control and measurement. Experiments were conducted on cylindrical granite specimens (5 mm diameter, 124 mm length) using a triaxial rock mechanics test system. The samples were instrumented with axial and lateral extensometers to monitor strain evolution. Tests were conducted under both single-stage and multi-stage confinement test protocols with axial and circumferential strain control modes, to assess the strength and deformation behavior of the granite under varying confining pressures. While single-stage tests offer higher accuracy and simpler interpretation, multi-stage tests are more material- and time-efficient, especially when core availability is limited. However, multi-stage tests introduce complexities in interpreting mechanical response due to potential cumulative damage and changing confinement history, which can affect strength measurements and deformation behavior. Our study addresses these challenges by evaluating damage evolution and failure behavior against a consistent pre-peak strength criterion at each confinement stage, aiming to improve the interpretability and reliability of multi-stage test results. In a single-stage test, each specimen is subjected to a constant confinement (5, 20, 60, 100 MPa) and loaded axially until failure. In contrast, a multi-stage test applies successive confinement (4 stages, from 5 to 100 MPa) to the same specimen, with peak strength detected at each stage, allowing multiple data points from a single sample. Our findings show that increasing confinement during multi-stage testing results in peak strength values comparable to those observed in single-stage tests, suggesting minimal cumulative damage prior to failure at previous confinement stage. However, the results of circumferential strain show that we have deformed and may have damaged the specimens at each stage. When confinement is reduced in subsequent loading stages, peak stress is more variable and often lower than in equivalent single-stage tests. This behavior is particularly pronounced at higher confining pressures and highlights the sensitivity of post-dilatancy crack evolution to confinement history.