Perpustakaan Digital ITB

River diversion twin-tunnels are being built in Nanjung, West Java, Indonesia to reduce the water residence time of the Citarum River and to minimize flooding in the upstream area during the rainy season. This tunnel can increase the river debit from 570 m3/s to 643 m3/s. For the weak rock formation, the tunnel was excavated mechanically using a twin header, while a conventional drill and blast was performed to excavate the hard formation. The drill-and-blast excavation method was designed following the Swedish method, with some modifications considering the type of explosive used. It should be highlighted that the area of blasting is unfortunately close to a textile factory which is active in operation. Therefore, drill and blast must be designed to produce low vibration magnitude and to minimize impact on the factory. Having evaluated the literature as well as regulations related to the vibration threshold for the factory, all stakeholders of the Nanjung river diversion project including the factory personnel, tunneling contractor and consultant agreed to set the threshold value of peak particle velocity (PPV) at the factory at less than 3 mm/s. Consequently, the drill and blast design parameters including hole diameter, hole depth, drill pattern, and initiation pattern were carefully designed to prioritize producing low vibration magnitude. Packaged emulsion explosive had been selected in the early stage of the project as the main explosive charge. The initiation system used an electric detonator with long period delay. Before implementation, the proposed blast design was checked and simulated by FLAC to predict whether PPV was higher or lower than the threshold. The first blast design was then recommended for a top heading & benching, placing 84 blast holes with explosive charge concentration of 1.45 kg/m3. Vibration records at the factory showed that the maximum PPV was 2.95 mm/s. In terms of minimizing vibration magnitude, the blast design and the dynamic simulation can be considered successful. Having good results for the top heading blasts, the blast design was challenged to excavate a full face to speed up the project. The blast design of the full face was recommended to consist of 120 drill holes with explosive consumption of 1.32 kg/m3.