Study of Large Deformation Classification Criterion for Layered Soft Rock Tunnels under High Geostress

The asymmetric deformation failure characteristics and asymmetric mechanical behaviour of supporting structures in layered soft rock tunnels under high geostress, combined with the anisotropic mechanical properties of carbonaceous phyllite, were investigated. Based on the deformation data of 93 layered soft rock tunnels under high geostress, the relationships between roof settlement, horizontal convergence, largest deformation and geostress, rock compressive strength, and overburden depth were systemically analysed. The results show that the deformations are discretely distributed according to the variations in geostress, rock compressive strength, and overburden depth. Under certain parameters of geostress, rock compressive strength, or overburden depth, the deformations are distributed in both the high- and low-value zones. With increasing geostress and overburden depth, and with decreasing rock compressive strength, the deformations gradually approach the high-value zone. This means that the large deformations of layered soft rock tunnels under high geostress are determined by the combined action of high geostress, weak surrounding rock, and a weak bedding plane. Based on the power exponent variation law between the largest deformation and the strength-stress ratio, a new classification criterion for large deformations is proposed for layered soft rock tunnels under high geostress.