## 9 Discussion  
Hydroelectric projects have been developed in the Himalayan mountain range in the northern part of the country with plans for future developments in order to satisfy the increasing demand for energy in Pakistan. However, during the underground excavation in the Himalayan region, several technical challenges pertaining to the design and construction of these hydroelectric projects are encountered, including high overburden, regional tectonics, insufficient exploration during feasibility studies, complex geology, virgin and excavation-induced stresses, and the presence of groundwater. These adverse conditions produce risks not only during construction in terms of huge rock mass failure, rock bursts, squeezing, and flooding, but also during the operational phase of the project, as experienced during the construction and operation of the NJJHEP. The major focus of this research is deep underground tunneling, which is not so common. There are few examples of such deep civil engineering tunnels in recent literature. The Jinping II hydropower project is one of those referred in this manuscript that have faced the same challenges, like NJJHEP. The successful completion of the complex NJJHEP in the challenging and unique in-situ environment of the Himalayas is a great experience for the study of a significantly deep civil tunneling process. Rock burst was the main problem during tunneling in this part of Himalayas, which has great implications on human life and property. In the future, many tunneling projects are planned in this area, due to its great hydropower potential. This study provides a valuable reference, not only for research in relevant fields, but also for the planning and construction of deep underground excavations worldwide, and specifically, in the Himalayas. Figure 13: Filling of the headrace tunnel and subsequent infiltration into the adits, drainage gallery, and powerhouse. The project-related features performed a critical role in the successful completion of the significantly long excavation work of the NJJHEP under the difficult geological conditions. New methods and models have been applied and developed based on the experience of the project, which have significantly contributed to the research on rock mechanics and are reviewed in this paper. The project layout indicates that the tunnels experienced a number of challenges in their route. These challenges include the complex geology of the Himalayas with unique sedimentary settings at significant depths (up to 2000 m overburden), abnormal stress conditions (K0 > 3.0), the presence of fault zones, and river crossing under low overburden. The intact rock and rock mass properties are different for the same type of rock along the tunnel route and under different overburdens, which was determined based on the results that are summarized from the test campaign conducted during the project. A combination of the single and twin headrace tunnels was completed with both conventional and mechanized excavation methods. The actual problems encountered during construction included the prediction of actual rock mass behavior ahead of the excavation, slow progress from conventional excavation, rock burst occurrences during TBM-based excavation, water inrush problem in fault zones, and water leakage problems during the construction stage of the project. During the project construction period, the construction strategy was revised at different times for its timely completion. The primary modification was the decision to use TBM instead of the conventional approach in a specific section of the headrace tunnel. Although modifications were adopted for accelerating the tunnel construction, the challenges faced in the Jhelum river crossing, presence of fault zones, occurrence of natural disasters, and changes in policies were the major factors responsible for project delay [48]. Although the TBM-based tunneling progressed faster than the conventional excavation process, this progress was lower than expectations, owing to high stresses and rock burst events.