## 8 First Filling of Headrace Tunnel  
After project completion, the successful re-saturation of the headrace tunnel was crucial for design verification. Tunnel stability and water leakage were critical, because of the complex geological environment in this super-long tunnel. The structural reliability of tunnel lining is significantly reliant on several parameters, including rock types, strength of the rock mass, structural features, tectonic setup, and groundwater levels [76]. According to [PERSON], in the case of a shotcrete-lined tunnel, the maximum limit for leakage per linear meter of tunnel is 0.31 L/min.; however, for an economic solution, a limit ranging from 1 to 1.5 L/min. is acceptable [77].  
The tailrace tunnel filling of NJHEP was started first (13 January 2018), followed by the filling of the headrace tunnel (6 March 2018). A slow filling process was implemented with six controlled stages to avoid cracks in the liner and maintain tunnel safety. The tunnel filling rate was 1.5 m/day with intermittent pauses of 24 h between each stage [78]. Figure 12 shows the headrace tunnel water-filling stages and corresponding pauses, along with water loss due to leakage.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Values of \(\alpha_{c}\)/\(\alpha_{1}\)**} \\ \cline{2-4}
**RMR Version** & 4–5 & 3–4 & 2–3 \\ \cline{2-4}  & \multicolumn{3}{c}{F\({}_{\text{stress}}\)} \\ \hline RMR89 & –5 & \(-\)10 & \(-\)15 \\ RMR14 & –22.326 & \(-\)27.169 & \(-\)32.012 \\ \hline \hline \end{tabular}
\end{table}
Table 7: F\({}_{\text{stress}}\) based on strength–stress ratio for the two versions of rock mass rating.  
Figure 11: Procedure for determining F\({}_{\text{stress}}\) in RMR\({}_{14}\), while considering the five cases listed in Table 6 (modified after [32,72]).  
The major aims of the headrace tunnel filling were as follows: (1) to apply less drastic forces on the concrete liner; (2) fill with increments in pressure; (3) allow time to monitor for leakages and hydro-jacking; (4) provide time to monitor, quantify, and stabilize the water loss; and, (5) minimize hydraulic gradients during the initial saturation of the rock mass [28].  
This stepwise filling operation was mandatory in avoiding unnecessary pressure buildup on the tunnel lining. The inherent geology was characterized by open joints and leakages at places, although these were dried later after excavation. During filling, back loading on the liner, as well as strength reduction of the rock mass, were expected.  
Figure 13 shows the seepage response in the powerhouse and auxiliary structures during the first filling of the headrace tunnel. The leakage responses were high in adits A4 and A4a when compared to other leakage observations. The adit A4 runs parallel to the headrace tunnel. The maximum leakage was due to the small pillar width (approximately 50 m), which was an expected response. There was a sudden increase in leakages in adit A4a, which was caused by post-water up-grouting in adit A4, as shown in Figure 13. This is reflected in the results, as the leakage rate in adit A4 decreased, owing to grouting, and the leakage increased in adit A4a in parallel.  
During the actual tunnel filling operation, the consolidated grouting work and drainage system worked effectively, with no deformation in the rock mass around the caverns. No significant changes in the extensometers, load cells, and convergence points/targets were noted during the filling operation. However, observations at certain piezometers revealed that water flow occurred in the sandstone beds and the overall leakage was within the design/permissible limit. During the tunnel filling and re-saturation operation, overall, the geomechanical instruments did not show any significant change.  
Figure 12: Filling stages and water loss during headrace tunnel filling.