ABSTRACT: Providing efficient support system which prevents failures and deformations is one of the most important issues during tunnel construction especially in weak grounds. When conventional support systems such as rock bolts, shotcrete, wire mesh, steel frames and lattice girders cannot provide sufficient support for tunnels, using pre-reinforcement systems becomes necessary in addition to main support systems. Pre-reinforcement of weak ground is done before the excavation or ahead of advancing the tunnel. This will provide a safe and effective operation. Pipe roofing umbrella arch pre-reinforcement method is one of the conventional pre-reinforcement systems that can be implemented in tunnels, caverns and other infrastructures construction. Detailed 3D numerical simulations are useful tools to obtain a better understanding of the performance of a pre-reinforcement system. In this paper, sectional excavation of tunnel No.10 of the Ghazvin-Rasht Railroad is simulated with FLAC3D code by using pipe roofing pre-reinforcement method, side bolts and pipes, and initial support system. The results of numerical simulations are analyzed by using tunnel support interaction charts. Considering all technical parameters, pipes with 4 in diameter, 15 cm spacing and 5 degree installation angle is selected as the most appropriate pipe roofing method for this tunnel.
Tunneling in soft or weak ground may encounter many operational and safety problems. Under these conditions extra attention and field investigations will be required. When rock mass cannot be self-supporting, or when conventional support systems such as rock bolt, shotcrete, wire mesh, steel frame or lattice girder cannot provide sufficient support for a tunnel, using secondary support systems or pre-reinforcing become necessary. Furthermore, in urban areas shallow tunnels are often constructed adjacent to existing structures such as buildings, streets and railways (Funatsu et al., 2008). The possibility of tunnel induced displacements and subsidence that can damage the existing structures is very high. In order to minimize effects of tunneling on existing structures, engineers should pay special attention to ground displacements. Therefore, using efficient and proper support system is necessary with considerations of the project requirements and constraints such as amounts of settlements and displacements. Often, it is required to use secondary support systems or pre-reinforcing in addition to primary support system in order to ensure a safe working place during the tunnel excavation. It is obvious that if effective support system is not installed at proper time and place, tunnel will collapse and may cause extensive damage, resulting financial and human loss in the project.