Application of Rock Mass Rating for Excavation and Support Design: An Empirical Study of Diversion Tunnel A, Walimpong Dam, Indonesia

Abstract

Diversion tunnel is a tunnel designed to redirect river flow which allows the dam construction site to remain dry during the construction phase. The Walimpong Dam, which is planned to be constructed on the Walanae River in Soppeng Regency, South Sulawesi Province, Indonesia, will utilize two diversion tunnels during its construction: Tunnel A and Tunnel B. This paper focuses on Tunnel A, where no comprehensive rock mass classification has been conducted to date. The objective of this study is to evaluate the condition of the rock mass through the Rock Mass Rating (RMR) system and, based on the resulting classification, establish excavation sequence and support requirements that align with the geological characteristics encountered along the tunnel route. The RMR assessment for Tunnel A was conducted using rock samples obtained from four boreholes drilled in 2016. Core-logging data indicate that the tunnel alignment lies below the groundwater level and is underlain mainly by packstone–mudstone units that fall within the poor quality to fair quality range. The recommended excavation sequence for the tunnel is the benching and heading technique, while the suggested support system includes combinations of steel sets, shotcrete, and rock bolts. The findings produced in this work are intended to serve as a technical reference for diversion-tunnel development in settings with similar geological characteristics.