Empirical design for the excavation method and support system at tunnel no 2, Bintang Bano irrigation area, West Nusa Tenggara

Bintang Bano Irrigation Area aims to optimize water consumption downstream of the Bintang Bano Dam and distribute it to irrigate agricultural land in Rempe, Seteluk, Seloto, and Senayan villages. Bintang Bano Irrigation Area uses tunnel construction as its network. This research aims to provide recommendations for the tunnel excavation method and support system and predict the tunnel stand-up time based on an empirical method from Rock Mass Rating (RMR89). The research method includes analysis of surface geological mapping and analysis of the core drill along the tunnel to determine the classification of rock masses. Surface engineering geological mapping was conducted to validate the Rock Mass Rating parameters determined by drill core evaluation. The results show the tunnel consists of andesite having poor to good rock masses. The excavation methods for poor to fair rock masses are heading and bench; good rock mass is full face and uses complete support. The tunnel requires bolts, shotcrete, and steel ribs for the support systems. Stand-up time predictions for poor rock mass are less than one day, and fair to good rock is over a month. Although the empirical approach is functional, numerical analysis is needed to verify the empirical design and effect of geological structures for tunnel stability, including tunnel deformation during construction.