The construction of tunnels in hilly terrains poses significant challenges, particularly in ensuring the stability of the portal slopes. This study focuses on proposing an optimum design for the portal slopes of a twin traffic tunnel in a part of the National Highway (NH-13) near Hospet town, Karnataka, India. The geotechnical investigation revealed that the tunnel primarily passes through litho-units of the Sandur schist belt, viz. metabasic, phyllite and banded iron formation. The phyllite rock exposed at the tunnel portal faces exhibits close-spaced joints and bedding joints as major weak structural features. The stability of the slopes was analysed using two-dimensional finite element modelling (FEM), which allows for a detailed analysis of stress and strain distribution within the rock mass. Various slope heights ranging from 12 to 26 m were considered with different permutations and combinations of stripping ratios (1:2 and 1:3). The FEM analysis indicated that slopes with a 1:3 ratio and 20 m height showed a factor of safety less than unity, requiring support measures such as wire meshing, shotcreting, and bolting. Lower heights, such as an 18 m bench provided a safety factor above unity. The study also proposed designs for the portal slopes based on field investigation, geo-mechanical characterisation, and numerical simulation. These designs included support measures such as rock bolting, shotcreting, and wire meshing. Overall, the study highlights the importance of employing advanced numerical methods like FEM for designing stable and safe portal slopes in tunnel construction projects.