Instability mechanism of layered surrounding rock tunnels affected by layer thickness under dynamic and static loads

In the context of the San Gavain hydroelectric power station diversion tunnel project in Peru, this paper studies the surrounding rock instability mechanism using a combination of numerical simulation methods and model test and presents the enclosing rock instability criterion based on the catastrophe model. The study demonstrates that the distribution of stress and displacement during tunnel excavation is more affected by the presence of laminated joints; the thinner the layer thickness, the more prominent the influence of laminations, and the higher the deformation of the surrounding rock. With the increase in layer thickness, the resultant displacement shows the law of gradual decrease; when the layer thickness is greater than 16 cm, the rate of decrease gradually slows down, and the maximum displacement is 0.201 mm. The failure mode of the layered perimeter rock tunnel with a 45° inclination angle primarily shows up as tensile deformation at the left arch shoulder and shear slip deformation at the right arch foot under the combined action of blasting disturbance load and excavation. Based on the instability analysis of the fold catastrophe model and cups catastrophe model, the influence of bending deformation between layers of surrounding rock is generally greater than shear slip deformation. The instability criterion Δ＜0 exists in all working conditions, bending instability damage may occur in all cases. When the layer thickness is greater than 24 cm, the shear slip instability criterion is greater than zero for 0–2 ms, and the system is always in a stable state, and when the layer thickness is less than 24 cm, the system may be destabilized at 1.5 ms.