Multi-Rock Mass Classification Systems for the Assessment of Excavation Method and Support Design of Spillway Tunnel, Sidan Dam, Bali, Indonesia

Abstract

This paper presents the implementation of rock mass classification systems to determine the excavation method and the support design of the Spillway Tunnel, Sidan Dam, Bali, Indonesia. In order to mitigate the uncertainty associated with using single rock mass classification during designing tunnels, this study provides an evaluation by applying multiple rock mass classifications. Rock Mass Rating (RMR), QSystem, Geological Strength Index (GSI), Japan Society of Civil Engineering (JSCE), and Rock Mass Index (RMi) were among the rock mass classification systems used for comprehensive analysis. Rock mass parameters were determined based on engineering geological mapping and rock & soil core analysis. The rock mass quality of pyroclastic rock along the tunnel is generally poor to fair. Based on the GSI rating, digging and ripping excavation were recommended. The top heading with benching (1-3 m advance in top heading) was the proposed excavation method by RMR and JSCE. Each classification system advised the combination of reinforced shotcrete and rock bolt for the primary tunnel support with varying shotcrete thickness (30-400 mm), rock bolt length (2.6-6 m), and rock bolt spacing (1-2.5 m). However, field investigations during excavation and numerical stability analysis were recommended to eliminate risks and ensure safe construction.