## 3 Geotechnical Analysis of the Dam
### Atterberg Limits  
The Atterberg limits (liquid limit, plastic limit, plasticity index) and shrinkage limits for twelve soil samples collected from the downstream area of the Tiga Dam in Nigeria can be seen in (Figure 8). These parameters are fundamental in assessing a soil's plasticity characteristics, which profoundly influence its engineering behavior, particularly its susceptibility to volume changes and erosion. The liquid limits range from 18% (sample 5)  
Figure 7: The histograms show the distribution of particle sizes. to 45% (sample 10), with a mean of 32.55% and a standard deviation of 9.26%. This variability (coefficient of variation = 28.45%) suggests significant heterogeneity in the soil's fine-grained fraction across the downstream area. Notably: Low LL (LL < 30%): samples 1, 2, 5, 9, 12 (41.67%), medium LL (30% \(\leq\) LL < 40%): samples 3, 6, 7, 11 (33.33%), and high LL (LL \(\geq\) 40%): samples 8, 10 (16.67%). Sample 4 is non-plastic, indicating a predominance of silt or very fine sand, aligning with its classification as SC (sandy clay) in the grain size analysis. The plastic limits range from 17% to 29%, with a mean of 22.18% and a standard deviation of 3.74%. The lower variability in PL (coefficient of variation = 16.86%) compared with LL suggests more consistency in the soil's lower plastic range. The plasticity indices, however, show more significant variability: PI = 0: sample 4 (non-plastic), low PI (PI < 7): samples 1, 5, 9 (25%), medium PI (7 \(\leq\) PI < 17): samples 2, 3, 6, 7, 8, 12 (50%), and high PI (PI \(\geq\) 17): samples 10, 11 (16.67%). According to the Unified Soil Classification System (USCS), these results corroborate the grain size analysis: CL, SC, CL-ML, and ML. The shrinkage limits range from 2% to 8%, with a mean of 5.08% and a standard deviation of 2.35%. Low shrinkage limits are typical of many inorganic clays, particularly those rich in kaolinite or illite. The variability (coefficient of variation = 46.26%) is the highest among all Arterberg limits, suggesting diverse mineralogical compositions. The plasticity index is a crucial indicator of a soil's expansive potential. According to [PERSON]: Low: PI < 15 (samples 1, 2, 5, 9, 12)--50% of samples, Medium: 15 \(\leq\) PI \(\leq\) 30 (samples 3, 6, 7, 8, 10, 11)--50% of samples, High: 30 \(<\) PI \(\leq\) 50--None, and Very High: PI > 50--None. At the same time, no samples show high or very high expansive potential and half exhibit medium potential. This aligns with the prevalence of CL soils identified earlier [44]. Notably, the absence of CH (high-plasticity clay) soils does not preclude significant volume changes, as even CL soils can experience substantial expansion, especially in regions with pronounced wet-dry cycles like northern Nigeria. In conjunction with the liquid limit and plasticity index, the shrinkage limit offers insights into shrink-swell behavior. The shrinkage index (SI = LL \(-\) SL) is particularly informative: SI < 15: low shrink-swell potential (samples 1, 2, 5, 9, 12), and 15 \(\leq\) SI \(\leq\) 30: medium potential (samples 3, 4, 6, 7, 8, 10, 11). Interestingly, while sample 4 is non-plastic, its absence of a shrinkage limit suggests it might still undergo volume changes, likely due to capillary forces in its fine-grained (silty) matrix [17]. Figure 8: Afterberg limits on twelve soil samples from the downstream area of the Tiga Dam in Kano State, Nigeria. [PERSON] found that soils with low plasticity (PI! 10) are more susceptible to erosion [13]. This category includes samples 1, 5, 9, and 12: high erosion risk and sample 4 (non-plastic): extremely high risk. This finding corroborates the grain size analysis, highlighting these samples' high silt content. The combination of low plasticity and high silt percentage makes these soils particularly vulnerable to surface erosion (raindrop impact, sheet flow) and internal erosion (piping, suffusion) processes. Skempton's activity index (A = PI / Clay Fraction) is often used to infer soil compressibility.