Combining of Rock Mass Classifications for Preliminary Design of Excavation Method and Support System of Diversion Tunnel at Rongkong Dam, South Sulawesi, Indonesia

This paper presents rock mass classification for the empirical design of the excavation method and tunnel construction support system at Rongkong Dam, South Sulawesi, Indonesia. An evaluation of core drilling along the tunnel for analyzing the subsurface geological conditions. The Japan Society of Civil Engineers (JSCE) and Rock Mass Rating (RMR) were used to classify rock masses and suggest the reinforcement system and underground excavation. Based on the JSCE, the rock masses along the tunnel were categorized as CI and CII classes, by the RMR method, categorized as poor and fair quality. The RMR system recommends an excavation method for poor to fair rock mass quality: top heading and bench. Meanwhile, in the JSCE system, the excavation method is full-face with an auxiliary bench cut. The RMR method recommends the following support systems for tunnel portals because the stand-up time value of the rock mass was relatively low: rock bolts with a diameter of 20 mm, length of 4-5 m, spacing of 1.0-1.5 m, and shotcrete thickness of 100-150 mm on the crown and 100 mm on the walls, as well as steel sets with a length 1.5 m. According to the JSCE system, the most recommended reinforcement system for the conditions in the research region comprises rock bolts measuring 4 m long and 1.2 m in spacing along with steel beam H-150 spaced 1.2 m apart. Furthermore, 15 cm thick shotcrete and lining with thick 40 cm on walls and 45 cm on invert.