Limit Equilibrium Method of Slope Stability Analysis: A Case Study of Outlet Tunnel Portal of Bagong Dam East Java Indonesia

Abstract Bagong Dam, located in Trenggalek Regency, has a diversion tunnel. The diversion tunnel is designed as a horseshoe-type tunnel with a diameter of 4 m and a length of 416 m. Although a general engineering geological investigation has been conducted at the dam site, slope stability has yet to be analyzed. This research seeks to categorize rock masses based on the Geological Strength Index (GSI) and evaluate the stability of the slope at the tunnel outlet using the Bishop Simplified and Morgenstern-Price Limit Equilibrium Methods (LEM), considering scenarios with and without seismic load. The results will determine the appropriate slope for the tunnel outlet portal. The data for the LEM analysis were gathered through surface geological mapping, subsurface core sampling, and laboratory testing of the rock samples. The geological engineering of the Bagong Dam tunnel outlet consists of andesite breccia (fair to good), interbedded sandstone clay (poor to good), and poor-quality limestone. The horizontal seismic load coefficient (k h ) at this location is 0.175. LEM analysis shows that the portal slope is stable after excavation according to the design, static load shows a safety factor (Fs) = 2.16 > 1.5 using the Bishop Simplified and also shows a safety factor (Fs) = 2.15 > 1.5 using Morgenstren-Price. Post-seismic load analysis also shows a safety factor (Fs) = 1.94 > 1.1 using the Bishop Simplified and also shows a safety factor (Fs) = 1.89 > 1.1 using Morgenstren-Price. Slope failure does not occur because the safety factor values can be achieved using the original slope design, both with and without seismic load.