Determination of the Tunnel Excavation Methods, Support Systems, and Stand-Up Time Based on Rock Mass Rating 1989 (RMR89) of the Spillway Tunnel, Bulango Ulu Dam, Gorontalo Province, Indonesia

The spillway tunnel of the Bulango Ulu dam was initially designed based on drill core evaluation. When the tunnel construction stage advances, surface rock outcrops are available for more detailed characterization of rock masses, particularly discontinuity orientation. This study's objectives are to forecast the tunnel stand-up time using the Rock Mass Rating (Bieniawski, 1989), or RMR89, and to offer suggestions for the tunnel excavation method and support system. To validate the RMR89 parameters found by drill core evaluation, surface engineering geological mapping was carried out. The results show the spillway tunnel consists of diorite and andesite which have very poor to fair rock masses. The excavation methods for the very poor and fair rock masses are multiple drifts and top heading and bench, respectively. For the support systems of the tunnel, bolt, shotcrete, and steel ribs are needed. The stand-up times for the very poor and fair rock masses are less than 30 min and from 1 day to 6 days, respectively. This paper demonstrates the significance of geological mapping for surface engineering in tunnel design.