Study on mechanical properties and strength criterion of mudstone under loading and unloading considering pre-peak damage

In the highway construction of the southwestern Transverse Mountain area of China, mass mudstone engineering disasters have occurred, primarily attributed to engineering disturbances and water-rock interaction. Engineering disturbances commonly lead to varying degrees of pre-peak damage. To elucidate the evolutionary laws of strength in pre-peak damaged mudstone, we first defined the pre-peak damage variable ( D _a ) for mudstone, and through triaxial loading and unloading tests, obtained the mechanical characteristics of pre-peak damaged mudstone, analyzing its brittle properties from an energy perspective. Subsequently, through scanning electron microscopy tests, we analyzed the microstructural features to reveal the failure mechanism. Finally, the damage ratio strength theory (DR) was introduced to characterize the strength of the mudstone and validate the suitability of the DR. The results demonstrate that: (1) Mudstone with pre-peak damage exhibits a significant weakening effect due to water-rock interaction, with a maximum reduction in peak strength of approximately 28%. Compared to the loading stress path (LSP), the overall strength of the mudstone is lower under the unloading stress path (ULSP), and the deformation modulus decreases more significantly with D _a under the ULSP. (2) Both the D _a and confining pressure contribute to a decrease in the brittleness index of the mudstone. Under the ULSP, the mudstone is more prone to brittle failure. (3) The development of micro-cracks in pre-peak damaged mudstone makes it more susceptible to water infiltration, exacerbating the deteriorating effect of water-rock interaction, thus affecting its mechanical properties. (4) The DR can effectively characterize the strength of pre-peak damaged mudstone. The Damage Ratio (ν _D,c ) of mudstone under the LSP is in the range of 1.07∼1.50, and under the ULSP is in the range of 1.11∼1.52. The ν _D,c under the LSP is smaller than under the ULSP, decreases with the D _a , and exhibits plastic deformation, indicating that the DR can simultaneously characterize the strength and brittleness of the mudstone. The research results can provide guidance for the design parameters and disaster prevention of disturbed mudstone engineering.