Tunnel Stability Analysis Under Seismic Load Using Finite Element Method: A Case Study of Spillway Tunnel, Sidan Dam, Bali, Indonesia

The location of the Sidan Spillway Tunnel, as shown on the Earthquake Hazard Zone Map of Bali Island, is in a moderate hazard zone. The support system of the spillway tunnel of the Sidan Dam, Indonesia, was empirically designed based on rock mass classification systems. The objective of this research was to verify the performance of the proposed support system by a numerical method considering the uncertainty in using rock mass classification systems for designing a tunnel support system. In addition, being located in an active seismic region, the seismic load was not considered in the empirical tunnel support design. The proposed tunnel support design was numerically modeled in two dimensions using a finite element method and subjected to a seismic load obtained from probabilistic seismic hazard analysis. A numerical analysis without seismic load was also conducted for comparison. Phase2 software was used for the tunnel stability analysis. The input parameters in the numerical analysis were derived from field investigations (surface engineering geological mapping and evaluation of drilling cores) and laboratory tests. The spillway tunnel area consists of pyroclastic rocks, and the Generalized Hoek-Brown Failure Criterion was selected for the strength parameter of rocks. The suggested support designs are systematic rock bolts (20 mm diameter fully grouted), wired mesh, shotcrete, and steel ribs at specific locations. The numerical models determined that applying the proposed tunnel support system recommended based on the empirical method resulted in a roof strength factor of more than 1.5 under static load & 1.1 under seismic load and roof displacements lower than a 10 cm maximum displacement at every stage of construction. The results verify that the tunnel construction will be stable and fulfill the required safety standard, both under and without seismic loads.