| Summary: | Coupled thermal–hydraulic–mechanical (T–H–M) simulations were conducted by using COMSOL Multiphysics software to clarify the mechanical properties of stabilising expansive soils under freeze–thaw (FT) cycles using fibre–coal gangue composites. The optimal dosage of coal gangue and polypropylene fibres was determined through laboratory tests based on free and no-load expansion rates. Unconfined compressive strength (UCS) tests were conducted after 12 FT cycles to evaluate mechanical performance. A three-field coupled model was developed by using COMSOL to simulate the temperature, moisture, and stress fields in the stabilised soil. The results showed that the modified expansive soil (MES) exhibited a 20.1% improvement in UCS retention compared to plain expansive soil (PES) after 12 FT cycles. The volumetric strain in MES was reduced by 23.3% and stress concentration remained higher, particularly in the mid-depth region of the specimen. The stress–strain response followed a typical strain–hardening pattern, with MES displaying greater stiffness than PES. Model predictions demonstrated high consistency with experimental data (R2 > 0.95, MAPE = 5.7%), which validate the simulation framework. This study provides new insight into the FT behaviour of MESs and offers a robust modelling tool for optimising subgrade reinforcement designs in cold regions.
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