Rock mass quality in the study area of Cijurey dam diversion tunnel, Bogor Regency, West Java Province, Indonesia

Abstract This study investigated the rock mass quality in the diversion tunnel of the Cijurey Dam through a detailed case study. Detailed examinations of the geological conditions were carried out on the surface and subsurface of the designated area, including residual soil, sandstone, carbonate sandstone, and carbonate mudstone layer. The method of this study is based on the classification of the Rock Mass Rating (RMR) and Geological Strength Index (GSI). RMR was the basis for determining the strength of rock masses in the form of shear strength of discontinuity surfaces, which depends on several factors such as distance, orientation, continuity, surface characteristics, separation, thickness, and nature of the filling material, while GSI was related to the level of fractures and surface conditions of discontinuities (9). The study area consists of three types of rock mass classes, namely poor to fair-quality sandstone with an RMR range between (41-60), poor to fair-quality carbonate mudstone with an RMR range between (37-63), and fair to good-quality carbonate sandstone with an RMR range between (60-63). Equations were derived to express this correlation GSI = 1.9845RMR-41.039 (R2=0.939), which had similarities with the linear line of the empirical equation of Wijaya et al. (27). Further analysis was needed, including the evaluation of the slope stability of the portal, the determination of the excavation methodology, and the formulation of the tunnel support system based on the rock mass characterization results obtained from this study.