<p>Layered rocks, commonly encountered in underground engineering, often contribute to weak and fractured geological environments. Sericite phyllite, a typical thin-layered rock with a well-developed foliation, exhibits significant anisotropic mechanical properties. The aim of this study is to investigate the anisotropic mechanical behavior of sericite phyllite. Triaxial compression tests were conducted on specimens with foliation angles of 0°, 45°, and 90°, under dry and saturated conditions, and different confining pressures. The results show that the strength curves and strain curves of sericite phyllite exhibit a U-shaped distribution across foliation angles, while the strain curves under saturated conditions with confining pressure follow an A-shaped pattern. Specimens with a 45° foliation angle may fail due to frictional sliding between layers, and the disconnection between layers leads to a sudden decrease in modulus, while specimens with a 90° foliation angle fail due to vertical foliation buckling. A foliation-related modulus (FRM) is proposed to address the sudden changes in modulus for inclined foliation and the decelerating modulus growth under increasing confining pressure. An enhanced piecewise strength criterion (EPSC) based on Jaeger's Plane of Weakness model and the Mohr–Coulomb criterion is introduced, allowing for piecewise fitting across different angle ranges to improve accuracy. These findings enhanced the design and risk assessment of underground engineering by providing quantitative predictions of strength reduction and deformation patterns in layered rocks, particularly valuable for tunnel boring and cavern excavation where varying foliation angles critically affect stability and support requirements.</p>