Triaxial creep behaviors of sericite phyllite using data optimization method subjected to loading–unloading cycles

In underground engineering construction and maintenance, layered rocks frequently exhibit significant creep effects that lead to localized fractures or long-term instability of surrounding rock masses. This study investigates the anisotropic creep behavior of sericite phyllite, a typical thin-layered rock, through cyclic loading–unloading creep experiments. A data optimization method based on plastic inheritance was proposed, enabling creep curves to be equivalently transformed into single loading–unloading creep curves at multiple stress levels, thereby effectively eliminating the influence of stress history. The experimental results reveal that the long-term strength of sericite phyllite follows a “U”-shaped distribution pattern with respect to foliation orientation. Under dry conditions, the long-term strength exceeds 90% of triaxial strength, whereas under saturated conditions, it decreases to 40%–60%. The maximum creep strain of dry rocks does not exceed 0.07%, while that of saturated rocks can exceed 1%, reaching over 1.5% when frictional sliding occurs. Analysis of creep strain, creep rate, and plastic strain characteristics helped identify and quantify two distinct deformation mechanisms under saturated and confined pressure conditions: Frictional sliding in 0° and 45° foliated rocks, and localized fracturing in 90° foliation rocks. These findings provide a theoretical foundation for creep risk assessment and support design in tunnel engineering. • A plastic-inheritance-based data optimization method for cyclic creep is proposed. • Creep behavior, rate, and long-term strength of sericite phyllite are analyzed. • Conditions for frictional sliding and localized fracturing are quantitatively defined.