Finite Element Method of Slope Stability Analysis: Case Study of Inlet Portal in the Diversion Tunnel of Bagong Dam, East Java, Indonesia
Ganny Indrajid (a,b*), Hendy Setiawan (a), Wahyu Wilopo

a) Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
b) Ministry of Public Works and Housing, Jakarta, Indonesia
* gannyindrajid[at]mail.ugm.ac.id

Abstract

Construction of the Bagong Dam^s diversion tunnel commenced in 2022. The tunnel, stretching 480 meters in length and measuring 4 meters in dimensions, is positioned directly beneath the dam. Due to the change in the trajectory of the inlet portal, a thorough analysis of the slope stability of the inlet portal is essential. This report presents a comprehensive slope stability analysis using the finite element method (FEM), both with and without earthquake loading. It also includes data for FEM analysis such as subsurface lithology, drill core analysis, observations of the geological strength index (GSI), and pertinent laboratory results. The study area encompasses two main lithologies: slightly weathered calcipulverite limestone and moderately weathered andesite breccia with interbedded claystone. GSI observations on core drill samples indicate that calcipulverite limestone is poor to fair quality (21 until 55), and andesite breccia with interbedded claystone is poor to good quality (21 until 75). The results of the FEM analysis showed that the Critical Strength Reduction Factor (SRF) of the slope decreased after adding earthquake loads. Under static and earthquake loads, the excavated slope at the tunnel inlet with a slope of 45 degree has a critical SRF value of 2,2. Based on the calculated SRF value, it can confidently affirm that the condition of the inlet portal slope is safe.

Keywords: slope stability, inlet portal, geological strength index, finite element method, strength reduction factor

Topic: Interdisciplinary Earth Science and Technology