Slope stability analysis of the drainage tunnel portal in Tanju Dam, Dompu District, West Nusa Tenggara

This paper presents design results of the tunnel portal slopes at the Tanju Dam, Dompu, West Nusa Tenggara. The objective of this research was to analyse the stability of the tunnel portal slopes using circular failure chart (CFC) method, limit equilibrium method (LEM), and finite element method (FEM). Input parameters were obtained from drill core evaluations and laboratory tests. By considering the rock mass rating (RMR) values of rock masses, which are categorized as class II, at the two slopes, adjustments for the cohesion and inner friction angle values are made. The inlet slope (IL) have cohesion values of 350 kPa and 40ºinner friction angle and the outlet slope (OL) have cohesion values of 400 kPa and 45ºinner friction angle. The CFC method shows that the IL and OL have safety factor (FS) values of 3.5 and 3.44, respectively. The LEM shows that the IL and OL have the FS values of 3.69 and 3.65, respectively. Meanwhile, the FEM shows that the IL and OL have FS values of 4.78 and 4.79, respectively. The stability analysis results indicate that designed slopes are stable.