Elasto-plastic Finite Element Analysis of Deformations in Deep Interacting Tunnels-A Comparative Study

The purpose of the present study is to carry out a parametric study and compare the deformations developed in twin interacting horse shoe tunnels and twin interacting circular tunnels with different W/D and Koconditions in the rock mass. The plain strain elastic and elasto-plastic finite element analysis with Hoek-Brown yield criterion have been performed. Three pillar width (W) to diameter (D) ratios (W/D) as 0.3, 0.6 and 1.2 have been introduced for each of three considered insitu stress ratio (Ko) as 0.5, 1.0 and 1.5 in the rock mass. The tunnels have been considered to be excavated simultaneously. Variation of deformations around tunnel boundary have been studied in the analysis. The results have also been compared with the results of that of excavation of a single tunnel. It has been concluded that for larger insitu stress ratio of Ko = 1.5 and smaller W/D ratio of 0.3, the yielded zone is highest. There is an overall horizontal movement of tunnel towards the pillar side in elastic analysis. When the material behavior is elasto-plastic, the trend in the percentage difference in displacements in interacting and single tunnels on abutment and pillar sides have been reversed. Ko = 1.5 shows the maximum variation in elastic analysis whereas in elasto-plastic analysis Ko = 0.5 shows the maximum variation. From a comparison of displacements in elasto-plastic and elastic analysis of interacting tunnels, it has been found that displacements are largest at springing level on pillar side than any other location on the tunnel boundary. Circular tunnels show comparatively higher values as compared to horse shoe tunnels.