## 4 Stability Analysis of the Dam
(**d**) Low Level of the Tiga Dam in Kano State, Nigeria. Figure 13: _Cont._  
Similarly, slow drawdown (Figure 12b) presents significant stability challenges, with an FOS of 1.002. Although less sudden than rapid drawdown, this scenario still induces unfavorable pore pressure conditions as the phreatic line adjusts to the changing reservoir level. The remarkably low FOS (1.002) underscores the vulnerability of the Tiga Dam during any drawdown event, echoing concerns raised by [PERSON] in a study of 20 earthen dams, where 30% had FOS values below 1.1 during drawdown [52]. Lastly, Figure 13d shows the dam's stability at low reservoir levels, yielding an FOS of 1.147. While not as critical as drawdown scenarios, this condition still concerns stability. The FOS exceeds 1.1, meeting ICOLD's [50] minimum for unusual situations, but falls short of the 1.3 recommended by the Federal Energy Regulatory Commission for long-term steady-state conditions [53]. [PERSON] et al. used SLOPE/W, a limit equilibrium software, to analyze an earthen dam in Indonesia, obtaining FOS values of 1.5 (high reservoir), 1.2 (rapid drawdown), and 1.3 (low level) [54]. Furthermore, [PERSON] and [PERSON] employed a coupled hydro-mechanical analysis in ABAQUS to study dam stability, focusing on transient seepage effects [55]. For a clay core dam similar to Tiga, they reported FOS reductions of up to 25% during rapid drawdown compared with steady-state conditions. Our results corroborate this, showing a 24% decrease from the high reservoir (1.322) to rapid drawdown (1.006), validating the significance of transient effects. The novelty of this study lies in its application of the latest Plaxis 2D version (2024) to analyze an existing operational dam. Moreover, this research uniquely compares four reservoir states within the same dam, offering a holistic view of its stability profile. Most studies focus on fewer scenarios; [PERSON] et al. examined only high reservoir and rapid drawdown conditions using PLAXIS, missing insights from slow drawdown and low-level states [55]. Our analysis of slow drawdown (FOS = 1.002) is also particularly novel. This condition, often overlooked in favor of rapid drawdown, is shown to be equally critical. The minimal difference in FOS between rapid (1.006) and slow (1.002) drawdown challenges the assumption that slower reservoir changes are inherently safer. This finding  
Figure 13: (**a**) Reduction factor versus total displacement at the crest of the dam. (**b**) Reduction factor versus total displacement at the toe of the dam. aligns with theoretical work by [PERSON] and [PERSON] on delayed pore pressure dissipation but is among the first to demonstrate it in a field-scale model [54]. The average FOS across all four conditions is 1.119, with a standard deviation of 0.145. This high variability (coefficient of variation = 12.9%) underscores the dam's sensitivity to reservoir-level changes. Furthermore, the low FOS values during drawdown (1.006 and 1.002) translate to just 0.6% and 0.2% safety margins, respectively--far below the 10% often used as a rule of thumb in geotechnical engineering. The stability analyses in Figure 12a-d do not explicitly incorporate the soil expansion effects. This is a limitation of our current study that warrants further investigation. Soil expansion can significantly affect the stress states within the dam and its foundation, potentially impacting overall stability. The expansive behavior observed in the downstream area could lead to several effects. As the soil expands, it can reduce void spaces, potentially leading to higher pore water pressures. This could decrease effective stresses and, consequently, the shear strength of the soil. Non-uniform expansion across the dam body and foundation could induce differential movements, potentially creating tension zones or concentrated stress. The expansion process might alter the soil structure, affecting its permeability characteristics. This could influence seepage patterns through the dam.