The mechanical stability of a tunnel is subjected to the development of loosening zone. Convergence measurement is the most convenient and practical method to estimate support requirements. In 12 m span tunnels, in order to predict the final deformation and the development of loosening zone based on the measurement at the earlier stage of excavation, the Rock Behavior Classification in Tunneling (RBCT) had been proposed. It has five classes: Class I to V. Japan Atomic Energy Agency has excavated a 4.5 m diameter ventilation shaft and a 6.5 m diameter access shaft in Horonobe URL project since 2005. And, it is expected to build a practical guide to feed the results from the convergence measurement to the excavation work at deeper stages. Therefore, the authors confirmed the applicability of RBCT to the convergence curves in tunneling projects with smaller crosssectional dimensions. Based on the evaluation, they applied RBCT to the measurement in the shafts. Consequently, it was clarified that the final deformation ratio converged in the range of Class II and approximately 15 % wide loosening zone in excavated diameter developed in the case that the initial deformation ratio was observed in the range of Class III in the shaft excavation.
The mechanical stability of a tunnel is significantly subjected to the development of loosening zone around the tunnel wall. Terzaghi  had proposed a relationship between the rock condition and the support load caused by the loosening. Kastner  had studied the stress distribution around a tunnel and the support pressure by a theoretical model focusing on the development of the plastic zone around an opening. According to the earlier experiences of underground projects, the loosening of rock related to the stability of a tunnel could be defined as the rock behavior which consists of dilatancy caused by shear deformation along the joints of rock and the inelastic deformation of the rock itself. The magnitude and the width in the field can be estimated by investigating the change of the permeability and elastic wave velocity of rock between before and after excavations. The loosening should be controlled by the appropriate choice of an excavation method, the magnitude of support members installed, and the timing of the installation. In addition, the allowable limit of the loosening zone should be determined by the required performance as an underground structure and should be one of main reasons for the determination of the practical guide of convergence measurements in tunneling, which is used as the allowable limit of deformation by site engineers. Therefore, in the stage of both design and construction in tunneling, it is necessary to estimate the mechanical stability of a tunnel from the viewpoint of not only the stresses of support members but also the development of loosening zone. Tanimoto et al.  suggested that the loosening of rock could be divided into strain-softening behavior and plastic flow, and the allowable limit of loosening zone was the strain-softening behavior. And, by means of analyzing a number of convergence curves observed in 12 m span tunnels, Tanimoto et al. [4, 5] indicated that the excessive deformation was allowed in a number of tunnels in Japan, and built the relationships among the initial deformation rate, the final deformation, the width of loosening zone, and the allowable limit of convergence.