Calcareous mudstone, a type of red-bed soft rock, is prevalent in the surrounding rock of the Central Yunnan Water Diversion Project (CYWDP) in Yunnan Province, China, significantly impacting both construction and operation. The mechanical properties of calcareous mudstone vary with depth. This study investigates its mechanical properties, permeability characteristics, energy evolution, and macro- and micro-failure characteristics during deformation using triaxial compression tests under different confining pressures. Results reveal distinct stage characteristics in the stress–strain behavior, permeability, and energy evolution of calcareous mudstone. Crack propagation, permeability evolution, and energy dissipation are closely linked, elucidating the deformation and failure process, with fluid pressure playing a crucial role. The confining pressure σ3 increased from 2 MPa to 4 MPa and 6 MPa, while the peak stress σc (Pw = 1 MPa) of the calcareous mudstone increased by 84.49% and 24.89%, respectively. Conversely, the permeability at σc decreased from 11.25 × 10−17 m2 to 8.99 × 10−17 m2 and 5.72 × 10−17 m2, while the dissipative energy at σc increased from 12.39 kJ/m3 to 21.14 kJ/m3 and 42.51 kJ/m3. In comparison to those without fluid pressure (Pw = 0), the value of σc at Pw = 1 MPa was reduced by 36.61%, 23.23%, and 20.67% when σ3 was 2, 4, and 6 MPa, respectively. Increasing confining pressure augments characteristic stresses, deformation and failure energy, and ductility, while reducing permeability, crack propagation, and width. These findings enhance our understanding of calcareous mudstone properties at varying depths in tunnel construction scenarios.
