Challenges in Design and Execution of a Transportation Tunnel

Abstract

Many hydroelectric, transportation, rail and metro rail projects are under construction in the country. The underground structures for these projects especially tunnels and caverns are mostly constructed on/in rocks. The presence of joints, shear zones and shear seams significantly effect the rock behavior under loading and unloading conditions. The shear zones and heavy seepage conditions pose challenging conditions for engineers and shall be handled in systematic manner.

The paper presents the analysis of double lane tunnel of more than 9.5 m diameter in a poor & challenging geological conditions for a project, located in Northern India. The in-situ stress conditions are anisotropic in nature with major in-situ stress ratio of 0.55 and minor in-situ stress ratio of 0.45, thereby making the tunnel behavior complex. The three major joint sets along the tunnel alignment adds to the complexity of the problem. The stress-deformation analysis has been carried out using RS 2 software using Mohr–Coulomb model. The excavation has been simulated in two stages for the tunnel i.e., heading and benching. The analysis has been carried out, to decide upon the optimal support system for the tunnel. The wedge analysis is also carried out to find out the effect of intersecting joints along the tunnel alignment.

Based on the stress-deformation analysis and analyzing the stress behavior, deformation characteristics and plastic zones around the tunnel, the optimal support system is decided and implemented at site. The support measures in terms of shotcrete with wiremesh/SFRS, forepoling, lattice girders and grouting are adopted.