An experimental system capable of bidirectional fluid injection under true triaxial compression conditions for the evaluation of hydro-mechanical properties of natural barriers

Rock masses characterized by low permeability (e.g. crystalline rocks) have been considered as natural barriers in the deep geological disposal of high-level radioactive waste. Therefore, groundwater flows within the natural barriers are typically governed by the spatial distribution of fractures within the rock mass. This study focuses on introducing an experimental system designed to evaluate hydro-mechanical properties in the natural barriers. This system allows hydro-mechanical experiments on fracture specimens by employing fluid injection under true triaxial compression conditions. The principal stress state of the underground rock mass can be induced through true triaxial compression conditions, providing the means for its application in the analysis of site-specific properties associated with rock fractures. In particular, the main feature of the experimental system is its capability for bidirectional fluid injection, allowing for hydro-mechanical experiments to be conducted on both individual and intersecting fractures. Detailed specifications have been designed considering the principal stress and groundwater conditions surrounding the KURT (KAERI Underground Research Tunnel), an underground research laboratory in Korea. The preliminary experiments utilizing the constructed experimental system confirmed that true triaxial compression conditions could be implemented while sustaining the loading speed and target stress at a reasonable level. Moreover, it was demonstrated that stable bidirectional fluid injection conditions could be achieved. The examination of applicability was also carried out by employing the experimental system to derive fracture apertures, which represent the hydro-mechanical properties of rock mass. In future investigations, we plan to conduct various hydro-mechanical experiments, employing fracture specimens sampled from the KURT site. This effort is expected to contribute to developing a Korean hydro-mechanical coupled model by evaluating the fracture properties in the KURT site.

Keywords: experimental system, bidirectional fluid injection, true triaxial compression, hydro-mechanical properties, natural barriers

Acknowledgements: This work was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and the National Research Foundation of Korea (NRF) grant funded by the Korea government(Ministry of Science and ICT, MSIT) (No.2021M2E1A1085200)