Rock Mass Characterization for Diversion Tunnels at Diamer Basha Dam, Pakistan – a design perspective

This research work presents a case study of rock mass characterization on diversion tunnels at Diamer Basha Dam, Pakistan. Geological and geotechnical studies were performed to characterize the rock mass along the axis of diversion tunnel. The core specimens retrieved were tested in laboratory and physical and mechanical properties of the rock mass were determined. Detailed discontinuity survey was carried out to determine the type and orientation of discontinuities with respect to direction of tunnel drive. The rock mass was classified using Rock Mass Rating (RMR), Q-System and Geological Strength Index (GSI). Based on rock mass classification results, the rock mass along the tunnel alignment was divided into three Geotechnical units, where the combined effects of the engineering-geological conditions, the initial stress situation and the ground water conditions were predicted to present consistent tunneling conditions. It was concluded from the rock mass behaviour that conventional drill and blast excavation method can be used, assuming smooth blasting by skilled workers. Ground support system was proposed for the diversion tunnels using Q-System support chart. In order to facilitate the easy classification of the rock mass at the project site, new empirical relationships were developed between Q-system, RMR and GSI by using original field data statistically. The relationship between RMR and GSI was found to have a greater degree of confidence with a higher correlation coefficient (R 2 = 0.84).