## 4 Results and Discussions
### Ultimate Bearing Capacity (UBC) Variability in Hybrid Composite Pile Foundation Systems
Comparative analysis shows that in very dense conditions, the steel-concrete pile's ultimate bearing capacity is approximately 1.8 to 2.3 times higher than that of the timber-concrete alternative, depending on  
Figure 11: Comparative analysis of UBC for hybrid steel–concrete composite piles, methodological variations across different relative soil densities: (**a**) 10% Dr, (**b**) 35% Dr, (**c**) 50% Dr, (**d**) 75% Dr, and (**e**) 95% Dr.  
the computational method used. The [PERSON] and Hansen methods consistently produce higher ultimate bearing capacity estimates compared to other analytical approaches, suggesting potentially conservative design considerations, while the Vesic, Coyle and Castello, and FEMs tend to yield more moderate capacity estimates, potentially reflecting more nuanced soil-structure interaction modeling. This remarkable variability in ultimate bearing capacity emphasizes the importance of comprehensive geotechnical investigation and multi-method analytical approaches. The divergence in predictive models highlights the complex mechanical behavior of hybrid composite piles, influenced by intricate interactions between material properties, soil conditions, and computational methodologies. These findings validate the innovative potential of hybrid composite pile systems and demonstrate their adaptability across varied geotechnical environments, while underscoring the necessity of site-specific analysis, sophisticated computational modeling, and interdisciplinary approaches in foundation engineering design. Field performance data from Kano's infrastructure projects validates theoretical calculations, with steel-concrete piles demonstrating a 22% increase in ultimate bearing capacity compared to conventional bored piles. This is particularly beneficial in the region's interbedded sandy deposits with clay lenses. Timber-concrete piles built using locally sourced Afzelia Africana hardwood showed minimal differential settlement (\(\leq\)5 mm) over 18 months despite seasonal moisture variations, confirming their durability in semi-arid conditions. The interface between composite materials emerges as a critical design consideration, evidenced by challenges in achieving uniform adhesive bonding between timber and concrete under humid conditions. This complexity is reflected in Table 1\({}^{\prime}\)s skin friction angles, where timber-concrete interfaces consistently demonstrate higher values than steel-concrete across all density conditions, with the most pronounced differential (4.6\({}^{\circ}\)) seen in very dense sand. The circular cross-section adopted for both pile types provides significant advantages in stress distribution and installation efficiency, which is particularly relevant to Kano's soil profile. The modular nature of these hybrid designs offers practical benefits for remote construction sites while addressing region-specific challenges through locally available materials integration. Bearing capacity calculations show marked sensitivity to soil density. This is especially evident using the Vesic method, where steel-concrete pile capacity increases from 353.1 kN in very loose sand to 14,379 kN in very dense sand--a 40.7-fold increase. These findings demonstrate that hybrid composite pile design requires the careful consideration of material properties, soil conditions, and installation methods, while substantial variations in predicted capacities across analytical methods underscore the importance of employing multiple calculation approaches during design and validating predictions through field testing. Future implementation would benefit from continued monitoring to assess long-term performance under cyclic environmental conditions and optimize interface design. The research does incorporate case studies I-II geometries from two infrastructure projects in Kano, Nigeria, as detailed in previously.