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  Tunnel Designed with Early Ring Closure in Squeezing Ground

  The Hachinoshiri tunnel is a two-lane road tunnel the total length of which is 2469m. The inner section of the tunnel is designed to be 71m, for which the tunnel needs to be excavated with a cross section of 115m. Highly weathered squeezing mudstone, the unconfined compression strength, q, of which measures not greater than 0.25N/mm, is present over approximately 570m in the major direction of the tunnel at the middle of the tunnel. The tunnel was surveyed to have the maximum overburden of 180m, and the competence factor became not greater than 0.1 along the tunnel alignment, which provoked concern about occurrence of excessive deflection or failure of tunnel support, and excessive ground deformation while it is being excavated. Hence, the tunnel was designed to employ early ring closure based on the load-carrying capacity required for tunnel support that was estimated from the thick cylinder theory, and the exerted earth pressure, Po, which can be determined from the competence factor, c. The tunnel construction adopted full-face excavation accompanied by early ring closure using a 330 kW boom header. Injection type long forepiling and curved tunnel face were also employed to stabilize the tunnel face. The employment of full-face excavation and early ring closure combined with curved tunnel face ensured the mechanical stabilization of a SEM (Sequential Excavation Method) tunnel in such poor ground where natural arching is unlikely achievable, so as to prove the validity of the proposed methods. Accordingly, the tunnel construction techniques on early ring closure have been reasonably established from insights into mechanical parameters such as competence factor, distance to early ring closure, tunnel shape where full-face excavation and early ring closure are adopted.early ring closure, full-face tunnel excavation, squeezing ground, earth pressure

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