## 3 Geotechnical Analysis of the Dam  
### Specific Gravity  
The specific gravity (Gs) of soil is a fundamental geotechnical property that reflects the soil solids' density ratio to water density [13]. It is a dimensionless quantity that provides insights into soil composition, particularly the mineralogical makeup [17]. In this study, specific gravity tests were conducted on twelve soil samples from the downstream area of the Tiga Dam. The results are shown in Figure 5. The specific gravity values range from a minimum of 2.11 (sample 9) to a maximum of 2.63 (sample 4), with a mean of 2.34 and a standard deviation of 0.21. This variability (coefficient of variation = 8.97%) suggests moderate heterogeneity in soil composition across the downstream area.  
To better understand this variability, it is helpful to categorize the samples based on their specific gravity ranges: 2.00 \(\leq\) Gs \(<\) 2.20: samples 2, 7, 8, 9, 10 (41.67% of samples), 2.20 \(\leq\) Gs \(<\) 2.40: samples 5, 6 (16.67% of samples), 2.40 \(\leq\) Gs \(<\) 2.60: samples 1, 3, 11, 12 (33.33% of samples), and Gs \(\geq\) 2.60: sample 4 (8.33% of samples). Comparing these results with the established literature reveals intriguing soil composition insights. According to [PERSON] and [PERSON], typical specific gravity ranges for common soil minerals are quartz: 2.65-2.67, kaolinite: 2.60-2.68, illite: 2.60-2.86, montmorillonite: 2.65-2.80, orthoclase feldspar: 2.57-2.62, calcite: 2.72-2.87, and organic matter: \(<\)2.00 [44].  
Sample 4 (Gs = 2.63) closely aligns with the range for quartz and kaolinite. This suggests a soil rich in these minerals, indicative of a residual soil derived from granitic or quartzitic parent rock. Such soils often exhibit good drainage characteristics due to  
Figure 4: A flowchart that summarizes the methodology in this study.  
Figure 5: Specific gravity on twelve soil samples from the downstream area of the Tiga Dam in Kano State, Nigeria.  
quartz's non-plastic nature. Still, they may be susceptible to erosion, especially if the clay fraction is predominantly kaolinite, which has lower cohesion than other clay minerals. Samples 1, 3, 11, and 12 (\(2.51\leq\) Gs \(\leq 2.58\)) fall within the range of orthoclase feldspar. This mineral is common in igneous rocks and suggests that the soil is relatively young, not having undergone extensive weathering. Feldspar-rich soils can be problematic as they weather to form expansive clays, potentially contributing to the downstream soil's expansion behavior.  
Interestingly, many samples (58.34%, combining the first two categories) have specific gravities below 2.40. Such low values are atypical for inorganic soils and strongly indicate the presence of organic matter. [PERSON] and [PERSON] note that organic matter can have specific gravities below 2.00, which aligns with samples 7, 8, 9, and 10 (\(2.11\leq\) Gs \(\leq 2.15\)) [44]. The presence of significant organic matter in downstream soils is noteworthy. Organic soils exhibit high compressibility, low shear strength, and substantial volumetric changes upon wetting or drying [13]. This suggests that a considerable portion of the downstream area may be prone to settlement under loading and experience significant expansion-shrinkage cycles in response to the pronounced wet-dry seasons. Moreover, the low specific gravities might also indicate the presence of diatoms or volcanic ash. Diatoms, microscopic algae with silica shells, are known to have specific gravities as low as 2.0-2.3. Their presence would imply past or present lacustrine conditions, not unexpected given the dam's long-term impoundment. Alternatively, volcanic ash, with specific gravities often below 2.4, could be present, suggesting past volcanic activity in the region [44].