## 2 Factors Affecting the Deformation of Soft Rock Tunnels under High In Situ Stresses  
The engineering data from some typical largely-deformed soft rock tunnels in China are summarized in Table 1, including the buried depth, formation lithology, in-situ stress, deformation magnitude, and characteristics of deformation and failure. Surrounding rock masses that are found with large deformations are mostly soft rock with a highly developed bedding texture, such as shale, slate, and phyllite (Table 1). The bedding plane profoundly affects the strength and deformation characteristics of tunnel-surrounding rock mass and is considered the key factor determining the mechanical behavior and deformation-failure characteristics of anisotropic rock mass. In general, the compressional deformation of the surrounding rock mass of tunnels in laminated soft rock occurs mainly at the two sides along the normal direction of the weakest bedding plane. Moreover, the smaller the bedding spacing is, the worse the integrity and self-stabilizing ability of the surrounding rock mass are, which leads to increased proneness to local large deformation and thus a relatively large compressional load on the support structure and highly-deformed lining and surrounding rock mass in local areas. The factors affecting the large deformation of the surrounding rock mass can be summarized as follows:  
Based on lithology, the surrounding rock mass can be grouped into two types, namely, plastic and brittle rock. The former consists of soft rock and is typically characterized by low mechanical strength, softening and swelling on wetting, and low slaking resistance, which are all unfavorable for the load-bearing and stability of the tunnel's surrounding rock mass. On the contrary, the latter is of hard rock and generally has a high load-bearing capacity, low softening and swelling tendencies on wetting, and high slaking resistance, which are favorable for the load-bearing and stability of the tunnel's surrounding rock mass. The lithology combination can be generally summarized into four cases, namely, the consistent lithology, hard interbeds (in much softer surrounding rock mass), soft interbeds (in much harder surrounding rock mass), and soft-hard alternating lithology. Different lithology combinations are associated with varied deformation strengths. A soft interbed in the surrounding rock mass is equivalent to a weak part in the surrounding rock mass. In most cases, deformation-failure occurs first in the soft interbed during tunnel excavation. If a hard interbed exists in the surrounding rock mass, it will often suppress the rock deformation, due to its more rigid mechanical properties than those of the surrounding rock mass. Therefore, the deformation magnitude is often relatively small at the hard interbed. The weathering effect is a common geological phenomenon in nature, which can weaken the cementation between rock particles, form damage cracks, reduce surface roughness, and worsen the physical and mechanical properties of rock. Rapid weathering of soft rock in tunnels will reduce the stability of the surrounding rock mass and affect the safety of the tunnel construction. After the tunnel excavation, the weak layered surrounding rock mass will form internal cracks due to the unloading effect; then, an effective seepage channels are formed, groundwater seeps through these channels, reducing the effective stresses and thus the shear strength along discontinuities and therefore the strength of the rock mass. Therefore, the presence of underground water makes the rock mass more prone to large deformation. The varied underground water seepage pathways or inconsistent thickness of the broken rock zone in the surrounding rock mass after excavation may result in local large deformation of the surrounding rock mass.