The quality of rock mass has significant impacts on the stability of underground excavations. Rock mass is a matrix
consisting of rock material and discontinuities/fractures (e.g., faults, joints). Such these discontinuities are source of weakness (e.g.,large displacements and rotations may occur across them), consequently; they may lead to crucial instability problems. The objective of this paper is to assess the stability of underground tunnel which is excavated between two inclined planes of weakness (rock joints). The numerical modelling analysis has been conducted using RS2D software adopting elasto-plastic Mohr-Coulomb criterion. The rock joints are allowed to slip to investigate their influence on the stress distribution and deformation close to the excavated
tunnel. The stability of underground tunnel has been assessed by investigating; extent of plastic zones, distribution of the induced stress and displacements in the surrounding rock mass and along rock joints. The results reveal that, the normal stress along rock joints is sharply dropped where joints pass over the excavation; Slip occurs (e.g., indicated by inward displacement of rock on the underside of the plane of weakness) over the excavation due to reversal direction of the shear stress and the maximum shear displacements occur above and below the tunnel.