Pre-nailing support for shallow soft-ground tunneling

• Governing equation of nail behavior due to tunneling is driven.
• Design factors and their effects of pre-nailing are investigated.
• Field application and its 3D numerical analysis of pre-nailing for validation.

In tunneling, supports are usually installed after excavation. Thus, the risk of tunnel collapse is highest immediately after excavation at the tunnel face. This is one of the main problems faced when constructing shallow tunnels in soft ground. In this paper, a pre-support tunneling method using a pre-nailing technique is proposed to improve face stability and reduce tunnel deformation. By driving nails into the ground before excavation, it is possible to achieve mechanical advantages such as restricting the deformation of the ground and preserving the arching of the ground. A theoretical governing equation for the pre-installed nails was formulated, and the mechanism whereby the deformation is reduced was investigated. The mechanical advantages of the method were validated in terms of the ground reaction behavior by carrying out small-scale model tests. A parametric study of the main design factors showed that the distance between the nails is the most important factor. Finally, a case study whereby the method was applied in the field was conducted; the pre-nailing technique was found to be particularly useful for reducing surface settlement and is appropriate for shallow, soft-ground tunneling.