A new transversely isotropic nonlinear creep model for layered phyllite and its application

Phyllite, which is a low-grade metamorphic rock with well-developed foliation planes, is encountered frequently during tunnel construction in western China. Its creep behavior is affected significantly by the foliation planes and has a crucial influence on the long-term safety of tunnel structures. Uniaxial compressive creep testing was conducted to analyze the time-dependent features of phyllite obtained from the Zhegu mountain tunnel on the Wenma expressway, China. A new creep model that connects a Maxwell body, a Kelvin body, and a nonlinear visco-plastic body was proposed to describe both the full creep process (including the transient, steady, and accelerated creep stages) and the transversely isotropic characteristics of phyllite. The creep model was also applied to investigate the long-term safety of a cracked tunnel lining in phyllite bedrock. The results showed that the creep strength and corresponding axial strain of phyllite exhibited maximum and minimum values at theta (the angle between the loading direction and the weak planes) = 90 degrees and 30 degrees, respectively. Good agreement was found between the calculated and experimental creep curves, indicating that the creep model replicates the physical creep process of phyllite well. The safety of the cracked lining was affected mainly by the damage degree of cracks and the creep behavior of the surrounding rock. Uncracked sections, because of their greater stiffness, were more sensitive to creep load than cracked ones. The inclination angle of foliation planes influenced the location of unsafe sections (those with a safety factor less than one), and this effect was weakened as the number of pre-existing cracks increased.