Numerical Modeling of Tunnel Support System in Himalayan Rock Masses for Seismic Resilience

This study uses numerical modeling in combination with rock mass classification approaches to develop tunnel support systems that are safe from earthquakes, ultimately contributing to the seismic resilience of the important infrastructure. A case study of tunnel in Himalayan region of Nepal has been considered for seismic assessment of tunnel supports in different geological conditions. For better quality rock masses (GSI>30), strain-softening model was used, and for weaker rock masses (GSI<30), elastic-perfectly-plastic model was used. To ensure safety in the case of extremely poor rock mass, shotcrete thickness had to be increased by 100 mm over that recommended by empirical methods. In both models, shotcrete in the sections with high overburden showed a significant decrease in FOS. Even though this type of unusual finding has also been reported in the Melamchi water supply tunnel after 2015 Gorkha Earthquake, no further studies to investigate the influencing factors have been done. While this condition occurred in the model used in this study because of the application of higher in-situ stress for sections with a larger depth of overburden, the actual reason for the occurrence of this situation as a result of an earthquake remains unexplained. Future research should take into account the effects of discontinuities, epicentral distance, aftershocks, and the incorporation of actual stress measurements from the field for numerical modeling.