Mechanical behavior assessment of a deep geological repository in the Kinmen Granite using particle flow code

Mechanical stability of underground geological disposal openings for radioactive wastes is a concern because excavated surfaces experience elevated or lowered stress concentration, leading to failure post excavation. The PFC (Particle Flow Code) model, applied to the Kinmen Granite (Taiwan) in this study, reveals that the crack initiation and damage stresses are 0.4 UCS and 0.8 UCS, respectively. Simulated acoustic emission events increase near peak strength, accompanied by clustering of cracks. The input parameters for the PFC were numerically calibrated using laboratory experimental stress–strain curves. A key finding of this study is that the random bond-strength PFC model better characterises the stress-strain behaviour and brittle failure of the Kinmen Granite compared to uniform bond-strength PFC model. Triangular tensile failure zones arise near the spring line of the deposition tunnel post excavation. Thereafter, when the deposition hole is created below the tunnel, much of the contact force fails to reach its top due to the shelter effect from the wider tunnel. Low contact forces appear at the top quarter of the deposition hole, which will likely promote spalling (tensile failure) in this part and near the spring line of the overlying tunnel. This observation agrees with the Aspo Hard Rock field tests.