Transversely isotropic creep behavior of phyllite and its influence on the long-term safety of the secondary lining of tunnels

Phyllite, which is a low-grade metamorphic rock with well-developed foliations, is encountered frequently during the construction of tunnels in western China. First, uniaxial compressive creep tests were performed to study the time-dependent behavior of phyllite specimens from Zhegu mountain tunnel. Then, a numerical approach was put forward based on the particle discrete element method to describe the typical creep behavior of phyllite, including its decay, steady, accelerated creep stages, and its transverse isotropy. Finally, the numerical model was adopted to investigate the failure process of secondary tunnel lining in the phyllite stratum. The following results were obtained: Ai et al. (2014) (1) The creep behavior of phyllite was affected significantly by the weak plane-loading angles and water content. Its creep strength decreased as the water content increased, and the maximum and minimum values occurred at θ = 90° and 30°, respectively; Carrillo et al. (2016) (2) The fractal dimension of fracture surface for θ = 0° was the smallest due to the shear and tensile failure along foliations. For each angle, fractal dimension increased with the increase of water content; Chen et al. (2018) (3) Micro-cracks were initiated at the initial loading stage and propagated during the entire loading process at θ = 0°, while they appeared at a higher stress level after specimens had accumulated sufficient energy at θ = 30° or 90°; Chen et al. (2019) (4) The evolution of micro-cracks was affected jointly by the inclination angle of foliations and the geo-stress field, and the cluster of cracks shows evidently asymmetric features. The results revealed the transversely isotropic creep behavior of phyllite and proposed a method which can aid more accurate predictions for design and construction with respect to geotechnical engineering structures related to layered rock mass.