## 4 Results and Discussions
### Ultimate Bearing Capacity (UBC) Variability in Hybrid Composite Pile Foundation Systems
For the steel-concrete composite piles, field load testing demonstrated a 22% increase in UBC compared to conventional bored piles, aligning closely with Vesic's method predictions (14,379 kN simulated vs. 14,202 kN PLAXIS result at Dr = 95%). The timber-concrete system exhibited \(\leq\)5 mm differential settlement over 18 months, consistent with interface shear test results (\(\delta\) = 37.4\({}^{\circ}\) at Dr = 95%). These measurements validate the analytical and numerical models, particularly the efficacy of [PERSON]'s method and PLAXIS simulations for high-density conditions. Comparative analysis explicitly correlates UBC and settlement data with predictions from eight classical methods. For instance, [PERSON]'s method overestimated the timber-concrete UBC by 18% compared to average measurements, while [PERSON] and [PERSON]'s approach showed the closest match (\(\pm\)5%). These discrepancies highlight the influence of soil heterogeneity and installation effects, which classical theories often simplify. Future work will expand field instrumentation to include pore pressure and strain gauges for granular validation. The hybrid composite pile systems presented in the case studies represent a significant advancement in foundation engineering when compared to conventional monolithic foundation solutions. These innovative designs, featuring steel-concrete and timber-concrete combinations, offer distinct performance characteristics that can be evaluated against established foundation alternatives and employ analytical methodologies beyond the classical bearing capacity approaches documented in the study. Drilled shaft foundations, which typically exhibit shaft resistance contributions of 70-90% in cohesive soils according to research by [PERSON] and [PERSON], demonstrate fundamentally different load transfer  
Figure 12: Comparative analysis of UBC for hybrid timber–concrete composite piles with methodological variations across different relative soil densities: (**a**) 10% Dr, (**b**) 35% Dr, (**c**) 50% Dr, (**d**) 75% Dr, and (**e**) 95% Dr.  
mechanisms compared to the hybrid composite piles analyzed [44; 45]. Regarding the steel-concrete composite pile in very dense sand, [PERSON]'s method indicates a shaft resistance contribution of 46.5%, which is substantially lower than that of typical drilled shafts but significantly higher than the 9.9% predicted by [PERSON]'s method for the same conditions. This variation highlights the unique load transfer characteristics of hybrid systems, which cannot be adequately captured through conventional monolithic foundation analysis. Driven precast concrete piles, commonly analyzed using wave equation methods such as GRLWEAP or CAPWAP, typically demonstrate ultimate capacities 15-20% lower than those predicted by static formulas due to installation effects [46; 47; 48]. This contrasts with the observed performance of hybrid steel-concrete piles in Kano's irrigation project, which demonstrated a 22% increase in ultimate bearing capacity compared to conventional bored piles. This performance differential suggests that hybrid systems may mitigate some installation-related capacity reduction factors through their composite construction. The \(\beta\)-method for estimating skin friction, which employs effective stress principles and empirical coefficients, represents an alternative analytical approach not covered in the study. Research by [PERSON] and [PERSON] indicates that \(\beta\)-values typically range from 0.25 to 0.35 for medium-density sands--a range that would yield shaft resistance values approximately 15-30% higher than those calculated using the Coyle and Castello method for similar conditions [49]. Applied to the hybrid timber-concrete pile in medium-density sand, this would potentially increase shaft resistance from 180.7 kN to 207.8-235 kN. The CPT-based methods developed by [PERSON] and later refined by [PERSON] offer direct correlations between cone penetration data and pile capacity that circumvent many assumptions inherent in classical theories [50].