In the paper, the authors propose a new quantitative assessment method of rock condition based on observational classification obtained at the tunnel face, which will enable to know either strength or deformability of rock mass, further more appropriate amount of rock bolts, thickness of shotcrete and size and pitch of steel ribs just after the blasting procedure. The study is confirmed by the database of up to the 6,101 sections of tunnels constructed by Japan Highway Public Corporation.
NATM is generally considered a tunnel construction method that utilizes field measurement results as a judgment criterion, and a vast amount of measurement data has been accumulated over the years. In the field of tunneling, expectations are growing for even greater effort not only to ensure construction safety by making utmost use of accumulated measurement data, but also to build tunnels in a rational and economical way. Since 1997, Japan Highway Public Corporation (JH) has been working to diversify its standard tunnel support patterns applicable to NATM tunnels by developing new patterns or modifying existing ones on the basis of performance data for the tunnel support patterns by JH for NATM applications. Table 1 shows the standard support patterns by JH. The objective of this diversification is to select a support pattern best-suited to the geological conditions, instead of applying a standard pattern more or less uniformly. Flexible selection of tunnel support patterns requires modifying, if necessary, the support pattern chosen at the preliminary study stage according to field measurements. JH introduced new framework for the condition of rock mass on tunnel faces in 1996, and compared the conventional support determination method based on the conventional face observation items and a new rock mass classification based on new face observation items, hereafter referred to as the JH Method ¹ 2). Figure 1 shows the tunnel face observation data sheets used in JH method. Preserved records of observation data obtained in this paper, the authors proposed the degrees of contribution of different observation items to tunnel support determination are assessed as weights in order to evaluate observation results on tunnel face quantitatively for rational tunnel construction. Results thus obtained are then applied to past project data to verify the validity of the new evaluation system for the condition of rock mass on tunnel face during excavation.
It is quite possible that the degrees of contribution of different evaluation categories of each observation item vary depending on rock types. Having analyzed field observation data accumulated within ill and examined the relationship between support patterns and Convergence for different rock types, Yagi et al. report that rocks can be classified into a number of groups according to deformation behavior". For the purposes of this study, the authors divided general rock groups into four classes according to compressive strength in the fresh state and the modes of subsequent weathering and deterioration, and investigated the degrees of contribution of observation items in each class.