## 4 Stability Analysis of the Dam  
This study analyzes the stability of the Tiga Dam in Kano State, Nigeria, under drawdown conditions using Plaxis 2D, a finite element method software. A rapid reduction in reservoir levels can lead to dam instability due to the persistence of high pore water pressures within the dam structure. A fully coupled flow-deformation analysis is employed to accurately model this scenario, wherein time-dependent pore pressure is integrated with deformation development to inform the stability analysis. The dam's stability has been evaluated under the following specific conditions: full (high) reservoir level; rapid drawdown (RDD) with a 5-day duration; slow drawdown over a 50-day duration; and low water level of the dam. The dam under consideration exhibits a height of 40 m with a base width tapering to 20 m at the crest. Under normal operating conditions, the water level behind the dam reaches 38 m. The analysis examines a scenario where this water level experiences a significant drop of 33 m. The normal phreatic level on the dam's downstream side is 8 m below the ground surface. Table 5 shows the material properties of the dam. Figure 11(a)-d presents a comprehensive stability analysis of the Tiga Dam in Kano State, Nigeria, using Plaxis 2D version 2024. This advanced finite element software is widely used in geotechnical engineering for its ability to model complex soil-structure interactions under various loading conditions. This study examines the dam's stability under four critical scenarios: high reservoir level, rapid drawdown, slow drawdown, and low reservoir level, each characterized by its Factor of Safety (FOS). Figure 12(a) depicts the dam's stability during high reservoir conditions, where the impoundment is at its  
Figure 11: Shear box test results on twelve soil samples from the downstream area of the Tiga Dam in Kano State, Nigeria. maximum design level. The analysis yields an FOS of 1.322, indicating that the resisting forces are about 32% greater than the driving forces. According to the US Army Corps of Engineers (2018), a FOS above 1.5 is typically recommended for steady-state seepage conditions. While the dam's FOS (1.322) is lower than this guideline, it still exceeds the minimum requirement of 1.1 set by the International Commission on Large Dams (ICOLD) for unusual loading conditions (2018). Figure 12b,c illustrate the dam's performance during drawdown scenarios--rapid and slow--which are critical phases in dam operation. Rapid drawdown (Figure 12b) occurs when the reservoir level is lowered quickly, often for emergencies or flood control. This sudden change can lead to high pore water pressures in the dam's upstream face, reducing effective stress and stability. The analysis shows an FOS of 1.006 for this condition, barely above the failure threshold of 1.0. This low value aligns with findings by [PERSON] et al., who reported FOS values between 1.0 and 1.1 for several ear  
Figure 12: _Cont._ \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Parameters** & **Subsoil** & **Fill** & **Core** \\ \hline Model & Mohr–Coulomb & Mohr–Coulomb & Mohr–Coulomb \\ \hline Drainage type & Drained & Drained & Undrained (B) \\ \hline \(\gamma_{\text{unstart}}\)\(\gamma_{\text{sat}}\) (\(\text{kN}/\text{m}^{3}\)) & 16.58, 19 & 14.84, 18.5 & 16.54, 18.44 \\ \hline Young’s modulus (\(\text{kN}/\text{m}^{2}\)), Poisson’s ratio & 40 \(\times\) 10\({}^{3}\), 0.31 & 15 \(\times\) 10\({}^{3}\), 0.34 & 1.4 \(\times\) 10\({}^{3}\), 0.36 \\ \hline Cohesion (\(\text{kN}/\text{m}^{2}\)), Undrained shear strength (\(\text{kN}/\text{m}^{2}\)) & 1.5, - & 8.88, - & -, 6 \\ \hline Friction angle (\({}^{\circ}\)), Dilatancy angle (\({}^{\circ}\)) & 34, 4 & 12.7, 1.5 & -, - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Material properties. Figure 12: Cont. Figure 12: (**a**) High Reservoir of the Tiga Dam in Kano State, Nigeria. (**b**) Rapid Drawdown of the Tiga Dam in Kano State, Nigeria. (**c**) Slow Drawdown of the Tiga Dam in Kano State, Nigeria.