Tunnel support practice in small hydropower tunnels in the Himalayas through observational approach

With the high mountains, steep terrain and fast flowing river, the Himalayan region has a great opportunity and potential for hydropower development. In Nepal, many hydropower projects have been constructed and many are under construction among which majority of the project are small to medium scale project. For such project, medium size tunnel provides an economic and safe route for water convergence. Squeezing and collapse have been observed in most of the hydropower tunnels, which have resulted in support failure, as the tunnel passes through weak rock mass under high in-situ stress during construction. Due to insufficient tunnel closure measurements, field-based observation has been conducted to study the installed support at different tunnels in the Himalayas. The observational approach is used for analysis and recommendations have been made for designing a stable and safe support system for tunnels. Through an analysis of a case study from Nepal, comparison has been made on the design and practiced support systems, and description of supports used in various case histories from Nepal have been provided.