Design and optimization of single roadway tunnel at tokha-chhahare section

Abstract

This project presents the design and optimization of a 4.01 km single roadway tunnel along the Tokha-Chhahare section in Nepal, enhancing connectivity between Kathmandu and Nuwakot. The existing route, with steep gradients, landslide-prone terrain, and hazardous bends, poses risks to travelers and trade. The tunnel aims to reduce travel distance by over 60%, mitigate disruptions, and improve safety. The study integrates empirical methods and numerical modeling to assess tunnel alignment, geological stability, and support systems. Rock mass classification such as Q-system and RMR guided the design, while finite element analysis using Phase2 software optimized supports by evaluating stress distribution, displacement, and rock behavior. A horseshoe-shaped cross-section was selected for better stress distribution, with support systems including shotcrete and rock bolts. Numerical validation showed reduced displacement and stress concentrations. The optimized support system shotcrete and rock bolts reduced tunnel wall displacement by 48% and improved stress redistribution. Rock mass classification guided zone-specific support, validated by finite element analysis showing 15�25% principal stress reduction. Furthermore, the portals were identified to be at a risk of toppling failure which was identified from kinematic analysis.