Mechanical properties and failure mechanisms of coarse carbonaceous mudstone particles as highway embankment filler under coupled temperature-moisture conditions

Carbonaceous mudstone is a potential embankment filler in mountainous regions with limited high-quality materials; however, its engineering performance in highway embankments under complex environmental conditions remains poorly understood. This study aimed to investigate the mechanical properties and failure mechanisms of carbonaceous mudstone filler under different temperature-moisture coupled conditions. Triaxial shear tests were conducted under four temperature-moisture coupled conditions: dry-heat to dry-cold (DHDC), wet-cold to wet-heat (WCWH), dry-cold to wet-heat (DCWH), and dry-heat to wet-cold (DHWC). The effects of these conditions on the strength characteristics, relative breakage ratio, failure mode, and microscopic morphology were examined. A segmented prediction model based on the Duncan-Chang model was applied to validate the experimental results under the DHWC condition. The failure mechanisms under different conditions were also analyzed. The results indicate that the degradation of carbonaceous mudstone increases in the following order: DHDC, WCWH, DCWH, and DHWC. Under the DHDC condition, the stress-strain curves exhibit strain-softening behavior, while other conditions show strain-hardening behavior, with peak deviatoric stress occurring at 2% and 4% axial strains, respectively. The shear strength decreases by up to 40% under the DHWC condition but remains nearly unchanged under the DHDC condition, showing a positive correlation with particle breakage. As the number of cycles increases, the failure surfaces gradually move downward. Higher confining pressure shifts failure mode from shear failure to shear slip or localized compression, and eventually to overall compression or expansion failure. The modified Duncan-Chang model accurately predicts the experimental results. These findings provide important guidance for the application of carbonaceous mudstone filler in highway embankment construction in humid mountainous regions.