Anisotropic Creep Characteristics of Sericite Phyllite for Long-Term Service Safety of Tunnel Structures

Foliated metamorphic rock exists widely in the surrounding rock of tunnel structures, underground utility caverns and mountain civil structures, and its laminated foliation structure induces obvious anisotropic time-dependent creep deformation, which seriously threatens the long-term service safety of underground building support systems. The strength and time-dependent deformation of foliated phyllite vary sharply with bedding dip angles, and long-term creep easily triggers large convergence, secondary-lining cracking and structural intrusion failure of tunnel structures. In this work, multi-stage uniaxial compression creep tests were carried out on sericite phyllite specimens with foliation dip angles of 0°, 30°, 45° and 90° using a programmable rock creep testing apparatus, aiming to investigate the anisotropic creep behaviors of phyllite surrounding rock for tunnel engineering. The anisotropic P-wave velocity characteristics of phyllite under different bedding angles were first analyzed to quantify the directional structural difference in the rock matrix. Based on the creep test curves, the full-stage deformation laws including instantaneous strain, decelerating creep, steady-state creep and accelerating creep were systematically summarized for specimens with different bedding orientations. Combined with macroscopic creep failure modes, phenomenological analysis was performed on the anisotropic creep damage behavior controlled by foliation weak planes. The results demonstrate that inclined bedding phyllite presents the most severe creep deformation and steady-state creep rate, and three typical creep failure modes (cross-bedding shear failure, tensile splitting failure, bedding slip shear failure) correspond to horizontal, vertical and inclined foliation specimens respectively. This study quantitatively reveals the time-dependent anisotropic mechanical properties of foliated soft rock, establishes a systematic experimental database and provides experimental basis for long-term stability prediction, and enriches the rheological research system for the durability evaluation of underground civil structures.