Research on the large deformation mechanism and control measures of a layered soft rock tunnel

Located on the Qinghai-Tibet Plateau northeast edge, the Muzhailing Tunnel has complex geological conditions: a large buried depth, high in situ stress, developed bedding, and weak-fractured rock mass. Strong deformation and serious damage occur during tunnel construction. Taking the 3# inclined shaft as an engineering example, a large-scale geomechanical model experiment for tunnel excavation and loading under different lateral pressure coefficients is performed to reveal the deformation evolution process and stress‒strain characteristics of layered soft rock tunnel surrounding rock. With increasing lateral pressure coefficient, tunnel cracks develop, the bedding slides and surrounding rocks deform. When the lateral pressure coefficient is greater than 1.5, the tunnel is severely deformed and damaged. A numerical model is established to analyse the surrounding rock deformation and stress during the whole process, and the surrounding rock deformation laws under different bedding inclinations and bedding spacings are studied. With increasing bedding inclination, the maximum displacement occurs in the bedding normal direction and develops from the top and bottom plates to both sides, in which the largest deformation occurs in the tunnel right shoulder for a bedding dip angle of 45 degrees. With increasing bedding spacing, the maximum displacement decreases and progresses from the bottom to top plate, and a sudden change in deformation occurs at the bedding interface. Finally, based on the bedded rock mass failure law, asymmetric coupling support measures with high-prestress constant-resistance anchor cables as the core are proposed and applied in the field. The monitoring results show a good large deformation control effect, which can provide a reference for similar projects.