## 3 Geological and Geotechnical Conditions of the Area
### Project Geology Details  
The NJHEP headrace tunnel is the deepest and the underground environment features superior geological settings when compared to global hydropower tunnel projects [18]. The site of the dam is located on the MBT itself, with the abut  
Figure 3: Neelum–Jhelum Hydroelectric Project located in the complex geological environment of the Sub-Himalayan region.  
of igneous and metamorphic rocks of the Punjal formation and the abutment on the left composed of the Murree formation (Figure 1). The headrace and tailrace tunnels of the project were excavated in the Murree formation, and the headrace tunnel crosses the Balakot-Bagh reverse fault or Jhelum-Balakot fault, locally called the MF, near the Jhelum crossing, which locally offsets the HKS [19]. The headrace tunnel inlet is at the northeastern limb, while the outlet of the tailrace is at the southwestern limb of the HKS. Along with MF, an unpredicted fault zone was experienced during tunnel excavation, named the Jhelum river fault zone (JRFZ). The size of the MF at the tunnel elevation is nearly 190 m and it is moderately to intensively fractured. Additionally, the widths of the JRFZ are 60 and 35 m in the left and right headrace tunnels, respectively. Moreover, this fault was responsible for the rapid water flow into the twin headrace tunnels from the Jhelum river and the occurrence of major collapse incidents below the river bed, owing to which the tunnels were temporarily plugged with concrete [11; 20].  
Prior to construction, small-scale local faults, along with fault plans, were identified; faults were also encountered during tunneling [21]. Large faults can usually be avoided when designing the projects, but small-scale joints, shear zones, and structural planes are significantly difficult to avoid during the underground construction of the projects [22]. From the investigations, repetitive series of shale and sandstone beds were expected during the feasibility study and detailed design stages of the project. However, during construction, interbedded sandstone and siltstone of the Murree formation, with occasional thin mudstone layers, appeared far more frequently than shale at all three construction sites. The interbedded formation commonly dips steeply with the tunnel axis perpendicular to the strike lines; moreover, the contact among beds is gradational. The fault structures affected this bidding structure, which resulted in a horizontal structure; thus, the steeply dipping structure was converted into a drag fold [23; 24; 25].  
In the project area, sandstone was further separated into SS1 and SS2 rock units. The first unit is comparatively stronger, contains fine to medium grain, and is well cemented. The fracture density is different in different beds, and the rock mass changes from massive to jointed. The second unit appeared to be steadily changing to form finer materials, including mudstone and siltstone. The sandstone unit (thickness of up to 30 m) is relatively massive and competent in high-stress areas. Silstone is intermixed with mudstone and shale, and its color changes from reddish to grayish brown. Shale and mudstone are the weakest rock beds in the project area with a reddish-brown color, and they are moderately weathered [26]. During tunnel excavation, geological mapping revealed that this weak rock bed had a widely developed sheared zone. The interspace between consecutive shear zones was recorded to be up to 10 m. The saturated \(\sigma_{\text{c}}\) (uniaxial compressive strength of intact rock) of mudstone, siltstone, SS2, and SS1 are 24, 65, 102, and 114, respectively [27].  
A number of tests were conducted on the rock specimens either in the field or the laboratory during the project design and construction stages. The test results were collected from literature owing to the project length (from the dam site to the powerhouse) and the interbedded geology, and Table 2 presents a detailed summary of the tests.