Control technologies for the deformation of a tunnel excavated in steeply inclined layered phyllite under high geo-stress

This study aims to control the deformation of a large section of tunnel excavated in steeply inclined layered phyllite under high geo-stress. To this end, anisotropic mechanical properties, which are factors influencing the deformation and stress characteristics of the phyllitic structures, were studied by combining rock tests with field tests based on Yangjiaping Tunnel on the Chengdu-Lanzhou Railway, China. Moreover, measures to control the surrounding rock deformation are proposed. The main conclusions are as follows: mechanical properties of chlorite-phyllite present significant anisotropy and its strength and failure modes are related to the azimuth of the applied load. There are significant differences in the failure modes of the surrounding rock at different positions in the tunnel excavated in steeply inclined layered strata. The rock surrounding the sidewalls is dominated by tension and the bending failure of beddings, whilst the vault and inverted arch are characterised by shear failure. Furthermore, the surrounding rock deformation of the tunnel excavated in steeply inclined layered phyllite under high geo-stress is affected by factors such as tectonic stress, strata occurrence, a soft interlayer and construction disturbance. The surrounding rock deformation and stress of the support structures are distributed non-uniformly. Large displacements and deformations are found on sidewalls, where targeted control measures should be taken. The measures that can be taken include optimising the cross-sectional profile, strengthening support rigidity, simplifying the excavation steps, enhancing advanced pre-support measures, adjusting the angle at which anchor bolts are applied and using mechanised construction. They can timeously provide support to the surrounding rock and limit the development of plastic zones, which not only controls tunnel deformation, but also improves the progress of construction by around 50%.