Rock mass classifications for empirical support system design of the Cidanau Dam diversion tunnel, Banten, Indonesia

Abstract

The Cidanau Dam, located at Banten Province, Indonesia, has been designed to have a diversion tunnel on the dam’s left abutment to divert the Cidanau river water during the earth-fill dam construction. The diversion tunnel is proposed to have a horse-shoe form with 413 m in length and 5 m in inner diameter. This research aimed to determine the support systems design of the Cidanau Dam’s diversion tunnel based on rock mass characterization and classification obtained by a surface engineering geological investigation, drill core assessment, and laboratory testing. A surface engineering geological investigation and a drill core assessment were conducted to determine the rock mass condition on the diversion tunnel alignment. The intact rock’s physical and mechanical properties of the drill core specimens were obtained by laboratory testing. The tunnel support system was proposed by an empirical approach based on the rock mass rating (RMR) and Q-system. Based on the results of the engineering geology investigation, the research area was divided into two rock units, namely the slightly weathered – fresh tuff breccia unit, which is the older unit, and the decomposed – slightly weathered lapilli tuff unit. As the main focus of this research, the diversion tunnel is located at lapilli tuff with poor rock mass quality (inlet and outlet section) and tuff breccia with fair rock mass quality (middle section). Three segments of ground support systems were proposed based on the rock mass quality along the diversion tunnel.