To elucidate coal permeability evolution and its dominant mechanisms under continuous thermo–mechanical–gas coupling, continuous methane flow tests were conducted on coal specimens from the same sampling area with consistent bedding orientations along a fixed loading path, covering temperatures of 30–70 °C, confining pressures of 4–12 MPa, and gas pressures of 1–2.5 MPa. A thermo–mechanical–gas coupled permeability model accounting for differences in baseline permeability was also developed. Increasing the confining pressure from 4 to 12 MPa reduced the mean permeability by 57.40%, indicating that effective-stress-induced fracture compaction was the primary control on coal permeability evolution. Increasing temperature generally reduced permeability, while the reduction from 30 to 70 °C decreased from 38.21% at 4 MPa to 19.63% at 12 MPa, indicating that the temperature response varied with stress conditions. Over the gas-pressure range of 1–2.5 MPa, the overall variation in mean permeability was approximately 2%, while the magnitude and direction of the gas-pressure effect varied with temperature and confining pressure. The model incorporates effective stress, temperature, a Langmuir-type adsorption state, an effective stress–temperature interaction term, and a baseline-permeability correction to represent the principal coupling effects and differences in response magnitude among coal specimens. The overall fitting coefficient of determination was 0.9198. Leave-one-specimen-out cross-validation (LOSO-CV) yielded a coefficient of determination of 0.8829 and a mean absolute percentage error (MAPE) of 10.50%. Given the baseline permeability k0 of a target coal specimen, the model can predict permeability under different combinations of temperature, confining pressure, and gas pressure within the investigated parameter range, providing a modeling basis for coal permeability prediction under multifactor coupled conditions.

Rainfall-induced seepage instability is a major concern for internal waste dump slopes in cold-region open-pit coal mines, where slope performance is influenced by groundwater conditions, moisture state, and seasonal temperature variations. This study investigates the internal waste dump slope of the Chaoyang open-pit coal mine and evaluates its seepage and stability responses under prescribed moisture and temperature states before and after rainfall, together with the effectiveness of drainage control. Soil specimens with moisture contents of 14%, 17.6% (natural), 23%, and 26% were tested at ambient temperature, −5 °C, and −15 °C by uniaxial compression and direct shear tests. The mechanical parameters measured under the prescribed moisture and temperature states were assigned to a GTS NX seepage–stability model. Twenty-four parametric cases, comprising four moisture contents, three temperature states, and pre- and post-rainfall conditions, were evaluated using the strength-reduction method, and an HDPE perforated drainage scheme was subsequently assessed. Under the ambient-temperature parameter state, increasing specimen moisture content from 14% to 26% reduced the pre-rainfall factor of safety from 1.41 to 1.18 and the post-rainfall value from 1.38 to 1.17. Parameter sets obtained from low-temperature-conditioned specimens produced higher calculated factors of safety; however, these cases represent prescribed mechanical states rather than the actual winter condition of the full-scale slope. Under the idealized drainage boundary, the pre- and post-rainfall factors of safety increased from 1.22 and 1.18 to 1.40 and 1.39, respectively. The results demonstrate the relative effects of laboratory-derived mechanical parameters, rainfall-induced seepage, and idealized drainage under the prescribed scenarios.