## 1 Introduction  
In order to overcome the electricity crisis in Pakistan, the northern part of the country has been considered a suitable location for implementing hydroelectric power projects in the deep dissected valleys, with the aim of achieving large amounts of storage and high water heads for power generation. Underground excavation is a key aspect of almost every hydropower project in this area of the country, owing to the presence of valleys and mountains. However, constructing these hydroelectric power projects in the area is challenging, both technically and financially. From a technical perspective, the Himalaya is a young mountain range; it is technically active, and the rocks are reasonably incompetent and affected by a number of faults, folds, and thrusts of varying extents. Moreover, several locations in this area also feature with subsurface water. Geological factors, such as the properties of the rock mass, virgin stress settings, and inflow of groundwater through discontinuities, affect the stability of underground excavations and they are called ground composition parameters [1; 2; 3]. In the Himalayas, ground composition parameter-related problems, such as rock bursting, squeezing, and groundwater inflow, are encountered in nearly all deep excavation projects [4]. Here, in situ stresses are comparatively unbalanced, as they are geologically age dependent, to a large extent. As it is the youngest mountain chain worldwide, it is relatively more challenging to perform tunneling in this area using tunnel boring machines (TBMs) [5].  
Along with ground composition, project-related features, which include the shape and size of the tunnel and the method and rate of excavation, are also important in the successful completion of projects [3]. The underground excavation method (mechanical or drilling and blasting) is responsible for the stress-strain behavior at the excavation face, and this behavior depends on the cross-section of the excavation area, along with the quality of the rock mass [6]. Ground composition parameters are responsible for the risk that is associated with tunneling; therefore, they directly affect excavation-related features of the project [7]. Although mechanical excavation is advantageous in low-stress environments, it is destructive in high-stress environments, as compared to the conventional drill and blast (D&B) method [8; 9].  
The Neelum-Jhelum Hydroelectric Project (NJHEP) is a major underground project, constructed in the complex geological environment of the Himalayas, in Pakistan. Almost a decade was required to complete the construction of this project, which can produce 969 MW of electricity. The initial design for underground excavation considered the use of the conventional method; however, during construction, for a significant length of the tunnel excavation, the TBM-based method was used for reducing project construction time. This design altered the project-related features that are associated with challenging ground composition. Underground construction was delayed at different periods, owing to extreme ground compositions, which presented significant challenges during tunneling. The changes in design, quality, and scope of the work resulted in a considerable increase in the construction time and cost [10]. During and after the completion of this project, significant research efforts were devoted toward evaluating the critical role of rock mechanics in the design and construction of underground structures. This study focuses on reviewing the effect of ground compositions and project-related features on tunnel stability that is based on previous research associated with this project.