Engineering Geology Characteristics of the Bagong Dam Diversion Tunnel, Trenggalek District, East Java Province of Indonesia

Abstract Knowing the engineering geological characteristics of the tunnel area is very important to prevent collapse during construction and operational stages. The Bagong Dam diversion tunnel is 480 meters long with a trajectory under the main dam body. During the construction phase, new information was found regarding the lithology and geological structure, therefore it is necessary to update the engineering geological characteristics of the tunnel area. This paper aims to identify the engineering geological characteristics of the tunnel area, including morphological units, lithology, geological structure, X-Ray Diffraction (XRD) testing, and identification of rock mass quality using the Rock Mass Rating (RMR) method. Results indicated that the research area consists of steeply and gentle sloping hills. The tunnel site comprises two main lithologies: calcipulverite limestone and andesite breccia. XRD testing shows that the clay is dominated by smectite mineral at 15.66 A. In addition, alteration of limestone and breccia was also found in surface and subsurface observations based on XRD testing. The tunnel trajectory will pass through limestone with a fair RMR value (43≤RMR ≤57) and andesite breccia with a good RMR value (69≤RMR ≤81). According to results, further studies are needed to consider the most reliable methods during the excavation and installation of support systems of the tunnel.