The Hachinoshiri Tunnel on the Chubu Odan Expressway is a two-lane tunnel with a total length of 2469 m. In the center of the tunnel there is weak highly-weathered mudstone with a competence factor less than unity (1.0). In light of these circumstances, the authors used the competence factor to estimate the earth pressure acting on the shotcrete, calculated the necessary load-bearing capacity of supports, and used it as a reference in designing a high-strength tunnel. As the tunnel construction method, the authors adopted early closure through full-face section mechanical excavation using a stable spherical face and a 330kW boom-type roadheader.
As a result, mechanical stability of the tunnel was achieved to enable steady tunneling, the effectiveness of this method was verified, and the mechanical behavior characteristics of early closure tunneling were clarified. Moreover, new technological issues came into focus.
