Optimization of the distance between twin tunnels by stress analysis

In this research, the stress characterization around twin-tunnels passing through a hard rock mass mostly consisting of Charnockitic Gneiss, Biotite Gneiss and Garnet Granulitic Gneiss in Kaluganga Development Project was analyzed by Boundary Element Method numerical solutions. The distribution and magnitude of major and minor principal stress contours, mean stress, differential stress, total displacement, maximum shear strain, and strength factor contours around the tunnels were simulated using the “Examine 2D” software. Examine 2D is a plane strain boundary element programme for calculation of stresses and displacements around underground and surface excavations in rock. Modeling results show that the countour values of the strength factor around the tunnel is greater than 1 when the distance between tunnels is 4.2 m which is the actual designed distance between the twin tunnels. When the distance is less than 4.2 m, model results shows that the strength factor reaches 1 in 1.88 m, and the excavation becomes unstable