## 2 Materials and Methods  
### Study Area  
The Tiga Dam, a significant hydraulic structure in northern Nigeria, is situated in Kano State, geographically positioned between latitudes 11\({}^{\circ}\)15\({}^{\prime}\) to 11\({}^{\circ}\)29\({}^{\prime}\) N and longitudes 8\({}^{\circ}\)16\({}^{\prime}\) to 8\({}^{\circ}\)38\({}^{\prime}\) E (Figure 2). This substantial earth dam spans approximately 40.40 km in length and 24.40 km in width and reaches a depth of 40.00 m. With a surface area of 178.00 km\({}^{2}\), it forms a reservoir capable of storing about 1978.49 \(\times\) 10\({}^{6}\) m\({}^{3}\) of water, making it a critical water resource for the region [32]. The study area's climate is classified as humid tropical, characterized by a distinct bimodal seasonal pattern: rainy and dry seasons. This climatic regime is typical of many West African regions, where rainfall distribution is governed mainly by the Intertropical Convergence Zone (ITCZ) movement. The northward migration of the ITCZ during the boreal summer brings moisture-laden southwesterly winds, resulting in the rainy season. At the same time, its southward retreat in winter leads to dry, dusty, northeasterly Harmattan winds, marking the dry season [33]. A comprehensive field sampling and laboratory testing program was undertaken to assess the geotechnical properties of the downstream area of Tiga Dam. The study area, spanning approximately 122 m along the downstream section, was systematically sampled at 10 m intervals, making 12 samples for each test to ensure adequate spatial coverage and capture potential variations in soil characteristics. While this systematic sampling  
Figure 2: The map of Nigeria showing the Tiga dam. approach provides a good overview of the area's geotechnical properties, it is essential to acknowledge that the spatial variability of soil properties might be more complex than what is captured by our 10 m interval strategy. Local variations in topography, microclimate, and land use can significantly change soil properties over short distances. To address this limitation, we ensured no potential anomalies or rapid changes in soil characteristics by visual inspection or geological history at that interval. These locations included areas with noticeable changes in vegetation, surface water accumulation, or variations in surface soil texture. Furthermore, we utilized geostatistical techniques, specifically kriging interpolation, to estimate soil properties between sampling points. This allowed us to create continuous maps of crucial soil parameters across the study area, helping to visualize and quantify spatial variability, which shows minor differences across the intervals, and that is why the interval data were strategically ignored. It is worth noting that the downstream area of Tiga Dam exhibits some topographic variability, with elevation changes of up to 3 m across the study area. These variations in topography can influence local drainage patterns and soil moisture regimes, potentially affecting soil properties. Land use in the immediate downstream area is predominantly grassland with scattered shrubs, but there are small patches of cultivated land within 50 m of the dam. These areas of human intervention may have somewhat altered the natural soil properties. The soil samples were carefully extracted, preserved, and transported to the laboratory for standardized geotechnical tests. These tests were selected to thoroughly understand the soil's physical, mechanical, and hydraulic properties, which are critical for evaluating the dam's downstream region's stability, deformation behavior, and seepage characteristics. The laboratory tests that were conducted on each sample were specific gravity test (ASTM-D854) [34], Atterberg limits test (ASTM-D4318-00) [35], compaction test (ASTM-D698-12) [36], sieve analysis (ASTM-D6913-04) [37], direct shear box test (ASTM-D3080) [38], and falling head permeability test (ASTM-D5084-16a) [39].