## 3 Geotechnical Analysis of the Dam
### Compaction Parameters  
The compaction test results for twelve soil samples were collected from the downstream area of the Tiga Dam in Kano State, Nigeria. Soil compaction is a critical geotechnical process that increases soil density by mechanically reducing air voids, enhancing strength, reducing compressibility, and decreasing permeability. In earth dam engineering, proper compaction is paramount for ensuring structural integrity, minimizing settlement, and controlling seepage [17]. The result presents two key compaction parameters in Table 3. Maximum dry density (MDD): The highest dry unit weight achieved at optimal moisture content. Ranging from 1.65 Mg/m\({}^{3}\) (sample 8) to 2.21 Mg/m\({}^{3}\) (sample 12), with a mean of 1.89 Mg/m\({}^{3}\), a standard deviation of 0.16 Mg/m\({}^{3}\), and a coefficient of variation of 8.47%. Optimum moisture content (OMC): The water content at which maximum dry density is achieved. Ranging from 10.27% (sample 4) to 18.94% (sample 7), with a mean of 14.34%, a standard deviation of 2.88%, and a coefficient of variation of 20.08%. The compaction curves (Figure 9a,b) visually represent the relationship between dry density and moisture content for each sample. The parabolic shape, typical of compaction curves, reflects how soil density initially increases with water content as lubrication facilitates particle rearrangement, then decreases as water starts to displace soil particles. Using the USDA Soil Survey Manual's classification: low bulk density (!1.40 Mg/m\({}^{3}\)): none, medium bulk density (1.40-1.80 Mg/m\({}^{3}\)): samples 7, 8, 10 (25%), and high bulk density (!1.80 Mg/m\({}^{3}\)): samples 1-6, 9, 11, 12 (75%). Most samples achieve high maximum dry densities, suggesting good compatibility. Sample 12's exceptionally high MDD (2.21 Mg/m\({}^{3}\)) is particularly noteworthy, falling within the range typically associated with well-graded  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Sample No** & \multicolumn{2}{l}{**Maximum Dry Density (Mg/m\({}^{3}\))**} & \multicolumn{2}{l}{**Optimum Moisture Content (\%)**} \\ \hline
1 & 1.84 & 12.81 \\ \hline
2 & 1.92 & 14.52 \\ \hline
3 & 1.91 & 14.21 \\ \hline
4 & 1.99 & 10.27 \\ \hline
5 & 1.86 & 12.32 \\ \hline
6 & 1.88 & 14.28 \\ \hline
7 & 1.72 & 18.94 \\ \hline
8 & 1.65 & 18.51 \\ \hline
9 & 1.81 & 15.99 \\ \hline
10 & 1.67 & 18.26 \\ \hline
11 & 1.94 & 13.59 \\ \hline
12 & 2.21 & 13.73 \\ \hline \hline \end{sandy gravels [44]. This aligns with its classification as SC (sandy clay) in the grain size analysis. The three samples with medium MDDs (7, 8, 10) coincide with those having the highest liquid limits (LL > 40%) from the Atterberg limits tests. [PERSON] et al. explain that high-plasticity clays often have lower maximum dry densities due to their greater specific surface area, which requires more water for lubrication, leading to higher OMCs and lower achievable densities [10]. The OMC values show more significant variability (CV = 20.08%) than MDDs, reflecting diverse water retention characteristics. Using guidelines from [PERSON] et al. [10]: low OMC (<12%): sample 4, medium OMC (12-18%): samples 1-6, 9, 11, 12, and high OMC (>18%): samples 7, 8, 10. Sample 4's low OMC (10.27%) corroborates its non-plastic nature and likely high sand content, as sandy soils require less water for compaction. In contrast, samples 7, 8, and 10, with OMCs above 18%, are consistent with their high liquid limits, suggesting significant clay content with high water affinity. Figure 9: (**a**) Compaction curve on 1–6 soil samples from the downstream area of the Tiga Dam in Kano State, Nigeria. (**b**) Compaction curve on 7–12 soil samples from the downstream area of the Tiga Dam in Kano State, Nigeria. The generally high MDDs indicate that most downstream soils can achieve good compaction. This is crucial for earth dams, where well-compacted fill material ensures low compressibility, high shear strength, and reduced seepage [45].