ABSTRACT
The technological innovation in underground structures instigates complex, economical, and efficient structures like powerhouse caverns, stations, tunnels, subways, metros, and many more. The complexity in these structures increases with the addition of intersections. These intersections can be horizontal, vertical, or inclined, subjected to stress concentrations, requiring special design considerations. The requirement of an extra support system is dependent on the rock mass properties, cavities dimensions, intersection angles, overburden, and in-situ stress conditions. The present study aims at assessing the effect of variable depth and in-situ stresses on various angles of tunnel intersections under similar rock mass conditions. Thirty-nine cases are analyzed to evaluate the variation in stress and deformation due to changes in the three input parameters: depth, in-situ stress, and intersection angles. FLAC3D, a finite-difference method-based numerical software, is used for the analysis. Thirty-six cases were evaluated by varying the in-situ stress ratio from 0.5 to 3, overburden from 100 m to 1500 m at intersection angles of 90°, 60°, and 30°. The results show that the 30° intersection is most critical. An increase in the in-situ stress and overburden results in a considerable increase in deformation. The values of the strength-stress ratio show the potential squeezing condition primarily for 30° intersection and high overburden.An analysis of vertical intersection to account for stress conditions at shaft intersections was performed. The results show that edges of vertical and horizontal junctions demand longer inclined bolts and support. The addition of vertical intersection with the multiple in-plane intersections proves to be most critical. The effect of parallel tunnels and their intersection showed a minimum of 2.5 times the diameter between the tunnels as a clear distance is required to avoid overlapping the plastic zone. The application of three-dimensional numerical analysis is assessed for two intersections of cross passage and adit for the T-48 tunnel project in the Himalayas. The results showed the intersections to be stable with extra support. However, the comparison of results with monitoring data did not provide sufficient information due to errors in the positioning of instruments. Still, overall the analysis showed that the rock mass parameters considered are conservative for the tunnel design. However, the necessity for a 3-D analysis for stability assessment and an extra support system at intersections is required.
