The support measures determined by two empirical rock mass classification methods, Rock Mass Rating (RMR) and Tunnelling Quality Index (Q), for an access tunnel driven through jointed sedimentary rocks were compared with the actual support installed and the results of a numerical simulation undertaken using UDEC software package. The study indicated that the support measures recommended by the RMR method are in general agreement with the support installed, albeit some differences in the bolt spacing and shotcrete thickness used. In contrast, the Q recommended by the RMR method are in general agreement with the support installed, albeit some difference in the bolt spacing and shotcrete and fibre/mesh reinforcement, fall well short of the extent of shotcrete and mesh installed in the tunnel. The support performance monitoring and the results of numerical simulation showed that the installed support measures were required for the rock mass conditions present in the tunnel.
This paper presents an evaluation of the support derived using two empirical methods, Rock Mass Rating (RMR) and Tunnelling Quality Index (Q), for an access tunnel to an underground power station driven through jointed sedimentary rocks. The RMR and Q methods used in this study were developed by Bieniawski (1973) and Barton et al. (1974), respectively, and were subsequently revised to enhance the reliability of their support predictions. Their current versions are RMR89 (Bieniawski, 1989) and Q94 (Barton & Grimstad, 1994). Despite the revisions, these methods have limitations some of which are discussed by Palmstrom & Broch (2006), Pells & Bertuzzi (2008) and Ranasooriya & Nikraz (2007), 2008).
A practical approach to identify the limitations of the empirical design methods and suggest improvement, where necessary and possible, is to compare their support predictions with those derived by other applicable methods and also with the performance of the support installed. The paper compares the support derived by the two methods with those installed in the tunnel, and evaluated their adequacy by two dimensional numerical simulation of the rock mass behaviour around the tunnel.
The case tunnel considered is part of the Lam Ta Khong pumped storage project situated some 200 km northeast of Bangkok, Thailand. The major components of the project include an underground power station and several kilometers of tunnels and shafts. The D-shaped 6.8 m wide and 1390 m ling case tunnel driven by drill and blast methods is the main access route to the underground power station. The tunnel overburden varies from 15 m at the entrance portal to about 350 m at the powerhouse end. The general tunnel alignment is 107°.
Throughout the project, standard support systems comprising rock bolt, shotcrete, wire mesh and steel sets were used and their performance was monitored. The geological conditions and construction details of the project, as well as the results of convergence and support performance monitoring, were reported by Jinye (1993), Sirikaew (1993), Praphal (1993), Tran (1994), Sriwisead (1996), Nitaramon (1997), Gurung and Iwao (1998) and Phienwej (1999).
