Analisis Forensik di Jalur Gumitir dengan Pendekatan Limit Equilibrum Method (LEM)

Abstract

The Gumitir Road Section at KM 233+550, a segment of the Jember-Banyuwangi national road under the authority of BBPJN East Java-Bali, has experienced structural failure in the form of slope collapse and road subsidence. This study aims to reconstruct the failure mechanism through a quantitative descriptive numerical approach based on slope failure forensic analysis, focusing on the influence of Ground Water Level (GWL) fluctuation on changes in the Safety Factor (SF) value. The analysis was conducted using GeoStudio SLOPE/W software version 2025.2.1 with the Spencer Limit Equilibrium Method (LEM) across nine GWL depth scenarios, subsequently validated comparatively using a Coupled Finite Element Method hydrogeological approach (Coupled FEM SEEP/W-SLOPE/W). The results indicate that the rise in groundwater level has a significant and non-linear effect on the decline of SF, with values decreasing from 1.482 at a GWL of -18 m to 0.371 at a GWL of -2 m. Referring to the critical threshold of SF 1.20 in accordance with SNI 13-7124- 2005, the slope was found to no longer meet the safety requirement within a GWL range of -15.8 m (LEM Spencer) to -16.5 m (Coupled FEM SEEP/W). Comparison between the two methods showed a consistent trend pattern, although Coupled FEM SEEP/W systematically produced more conservative SF values due to its capability to explicitly capture pore water pressure distribution and seepage forces, making it more representative for complex hydrogeological conditions on similar national road slopes.