Simplified Thermo–Hydro–Mechanical Coupling Modes for High-Temperature Tunnels in Naturally Fractured Rock Masses: A Combined DFN and ECM Approach

This study investigated the thermo–hydro–mechanical (THM) coupling behaviors in naturally fractured rock tunnels through an innovative integration of the discrete fracture network (DFN) model and the equivalent continuous medium (ECM) model. The DFN model was generated using the Monte Carlo simulation approach based on the discrete-element method (DEM). Calibration of the DFN model was performed using ultrasonic pulse velocity tests, ensuring alignment with the structure of very blocky rock mass as depicted in the Geological Strength Index chart. The fully THM-coupled ECM model, formulated based on the FEM, incorporates equivalent mechanical, hydraulic, and thermal properties from the DFN model. The existence of representative elementary volumes within the DFN model was evaluated and confirmed. The study explored the THM intercoupling effects in high-temperature fractured rock tunnels under both elastic and elastic–plastic conditions. It was observed that, in the elastic stress state, only the one-way influence of the H field on the M and T fields is significant. Conversely, under elastic–plastic stress conditions, the one-way impact of the M field on the T field can be neglected. Based on this analysis, the study innovatively proposes simplified THM coupling modes for high-temperature tunnels.