Engineering geological investigations for designs of excavation method and support system of diversion tunnel at Bulango Ulu Dam, Indonesia

This paper presents engineering geological investigations for designs of excavation method and support system of a diversion tunnel at Bulango Ulu Dam, Indonesia. Evaluation of drill cores along the tunnel was carried out to determine the geological engineering conditions. Classifications of rock masses were carried out based on the Central Research Institute of Electric Power Industry (CRIEPI) and Rock Mass Rating (RMR) to analyze the excavation method and support system. The rock masses along the tunnel were classified into D, CL, and CM classes based on the CRIEPI and poor, fair, and good quality based on the RMR. The tunnel portals consisted of poor quality rock masses, while the main tunnel section consisted of good quality rock masses. The excavation method of full-face was proposed for the main tunnel section, while top heading and bench were proposed for the tunnel portals and main tunnel section having fair quality rock masses. The support system recommended for tunnel portals were 4-5 m long, 1-1.5 m spaced systematic bolts, 100-150 mm thick shotcrete, and 1.5 m spaced steel ribs. Meanwhile, the support system proposed for the main tunnel section was 3 m long, 2.5m spaced bolts, and 50mm thick shotcrete in the crown.