The creep deformation of surrounding rock masses is a commonly encountered geological hazard that presents significnat harm to deep buried soft rock tunnels. Creep deformation frequently occurs during the operation periods of tunnels. In this study, the long-term rheological properties of the surrounding rock in a Lanjiayan deep buried phyllite tunnel were investigated. A rheological model and the rheological parameters of the tunnel were established based on the results of a triaxial creep laboratory test. The theoretical curves obtained from the proposed rheological model corresponded with the test curves. The variation laws of the plastic zone and deformation in the secondary lining over time were also investigated. Furthermore, the cross sectional shape of the tunnel boundary and thickness of the deformation layer were optimized
Phyllite is a phyllitic-structured type of low grade metamorphic rock. Phyllite softens easily in water and has a uniaxial compressive strength that decreases significantly in a water-saturated state. The saturated uniaxial compressive strength of phyllite (<25 MPa) can be reduced to ½ ~ 1/3 of its original strength . According to the Standard for the Engineering Classification of Rock Masses  and the Code for the Investigation of Geotechnical Engineering , phyllite is a soft rock. Engineering practice shows that, after the excavation and supporting of a deep soft rock tunnel, the deformation of its surrounding rock undergoes increasing amounts of rheological processes over time. After a certain period of time, large amounts of deformation occur, which can lead to the failure of supporting structures. Thus, the rheological properties of the surrounding rock of tunnels have been studied frequently. D. Sterpi and G. Gioda  used plastic, viscoelastic and viscoplastic elastic and rheological models to simulate the three stages of the creep behavior of rocks subjected to the high amounts of geostress in a deep tunnel. D. Debernardi and G. Barla  proposed a new viscoplastic constitutive model to describe the mechanical properties of the surrounding rock stratum of a large extrusion tunnel, while considering both the elastoplastic and viscoplastic properties of the tunnel. A. Fahimifar  deduced the analytical solutions for circular tunnel deformation under hydrostatic pressure and predicted the displacement of a wall over time by assuming that the surrounding rock consisted of incompressible Burgers viscoelastic materials. P. Nomikos  studied the stress and displacement characteristics of the surrounding rock of a tunnel using an axisymmetric Burgers model and analyzed the effects of the viscoelastic parameters and rheological properties of the surrounding rock on those characteristics.