Mechanical characteristics of large-section tunnel in soft rock based on various rock conditions and excavation footages

Taking the V-class surrounding rock section at the entrance and exit about Qianqi Tunnel as an example, the deformation rules as well as mechanics about the tunnel in different rock conditions and excavation footages are compared by numerical calculation, and the suitable excavation footage of the relied engineering is also proposed. The results show that the displacements about surrounding rock are greatly influenced by the excavation footage, namely, the arch settlements, horizontal convergences as well as surface settlement of the tunnel increase significantly with the excavation footage. For the preferred rock condition, the maximum bending moments about the preliminary linings at middle step after excavation occurs in the vault, and its increase amplitude has little correlation with the excavation footage. For the poor surrounding rock, the maximum bending moment is transferred from the vault to the arch waist with the increase of excavation footage, and it is basically manifested as the inner tension of the lining. The relationship between excavation footage and maximum deformation as well as mechanical index of vault is also revealed, respectively. The excavation footage of shallow-buried tunnel sections in V-grade surrounding rock is suggested, that is, 1.8m for the preferred surrounding rock condition and 1.2m for the poor surrounding rock condition.