Tunnel stability modeling using finite element method: A case study of the inlet of diversion tunnel, Budong-Budong Dam, West Sulawesi, Indonesia

Abstract This article presents the results of the tunnel stability from the empirical design at the diversion tunnel of Budong-Budong Dam, West Sulawesi, Indonesia. The purpose of this study is to assess the effectiveness of the suggested support system using a numerical approach, taking into account the uncertainties in the design process and the use of face mapping during excavation. The tunnel support design is simulated using a two-dimensional finite element method and exposed to a seismic load derived from probabilistic seismic hazard analysis. Additionally, a numerical analysis is conducted with and without seismic load to serve as a comparison. Tunnel stability is analyzed using Phase-2 software. The input parameters used in the numerical analysis come from field investigations, which include surface engineering geological mapping, tunnel face mapping, core drilling evaluation, and laboratory tests. According to geological face mapping and drilling core data up to 25 meters deep, the diversion tunnel area consists of pyroclastic-epiclastic rocks. The Generalized Hoek-Brown Failure Criterion is selected to model rock strength. The horizontal seismic coefficient calculated for the pseudo-static tunnel stability analysis is 0.235. The recommended support designs include systematic rock bolts, steel ribs with wired mesh, and shotcrete installed at specific locations. Numerical modeling determines that implementing the suggested tunnel support system, as advised by empirical methods, results in a strength factor greater than 1.5 under static load and 1.1 under seismic load. The results confirm that the support system proposed by the empirical design methods provides a safe tunnel design under both static and seismic loads.