## 5 Conclusions  
This study investigated the performance of hybrid steel-concrete and timber-concrete composite pile systems using classical bearing capacity methods and numerical simulations. The key findings revealed significant improvements in load-bearing capacity, material efficiency, and adaptability to varying soil conditions. The integration of numerical modeling highlighted the complex interplay between soil density and material interfaces. Below are the conclusions and a future recommendation derived from this research. 1. This study demonstrated the systematic relationship between sand relative density (Dr) and geotechnical properties, where increasing Dr from 10% to 95% reduced void ratios from 0.886 to 0.476, increased dry unit weight from 14.1 kN/m\({}^{3}\) to 18.0 kN/m\({}^{3}\), and elevated internal friction angles from 28\({}^{\circ}\) to 41\({}^{\circ}\). Permeability decreased by three orders of magnitude (0.01 cm/s to 0.0001 cm/s), underscoring the critical role of compaction in enhancing soil strength and stability. 2. Skin friction angles at pile-soil interfaces exhibited material-dependent behavior: concrete-steel interfaces increased from 12.8\({}^{\circ}\) (Dr = 10%) to 32.8\({}^{\circ}\) (Dr = 95%), while concrete-timber interfaces showed higher values, rising from 20.2\({}^{\circ}\) to 37.4\({}^{\circ}\) over the same Dr range. The 7.4\({}^{\circ}\) difference in very loose sand (Dr = 10%) highlights timber's superior interfacial performance in low-density conditions. 3. Load transfer mechanisms shifted significantly with soil density. For hybrid steel-concrete piles, skin friction contribution (Q\({}_{\text{s}}\)) increased from 23.3% (Dr = 10%) to 46.5% (Dr = 95%), while base resistance (Q\({}_{\text{b}}\)) decreased from 76.7% to 53.5%. Timber-concrete piles displayed a more variable response, with Q\({}_{\text{s}}\) dropping from 43.7% to 9.90% under [PERSON]'s method, demonstrating density-dependent interaction dynamics. 4. Ultimate bearing capacity (UBC) predictions varied widely across methods, with steel-concrete piles ranging from 353.1 kN (Vesic, Dr = 10%) to 14,379 kN (Vesic, Dr = 95%), a 40.7-fold increase. Timber-concrete piles showed capacities between 244.1 kN (Vesic, Dr = 10%) and 9537.5 kN ([PERSON], Dr = 95%). PLAXIS 3D simulations validated theoretical results, yielding 14,202 kN for steel-concrete at Dr = 95%, closely matching [PERSON]'s prediction (14,379 kN). 5. Field implementations in Kano, Nigeria, confirmed hybrid systems' efficacy: steel-concrete piles achieved a 22% higher UBC than conventional bored piles, while timber-concrete systems exhibited minimal differential settlement (\(\leq\)5 mm over 18 months). Volumetric strain values (0.050-0.006) and interface friction angles (\(\delta\) = 32.8\({}^{\circ}\) for steel-concrete at Dr = 95%) aligned with performance metrics, validating the designs' adaptability to regional soil conditions. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15115868/s1](https://www.mdpi.com/article/10.3390/app15115868/s1)]([https://www.mdpi.com/article/10.3390/app15115868/s1](https://www.mdpi.com/article/10.3390/app15115868/s1)). Figure S1: Bearing capacity factor Nq as functions of the angle of internal friction and depth-to-width ratio (redraw after [PERSON]'s method (1961)); Figure S2: Bearing capacity factors Nq and N\(\gamma\) as functions of the angle of internal friction and sand density, showing the influence of soil compaction on pile load-bearing capacity (redraw after [PERSON] method (1966)); Figure S3: Bearing capacity factors Nc and Nq as functions of the angle of internal friction and depth-to-width ratio, Illustrating the influence of soil compaction on pile load-bearing capacity (redraw after [PERSON]'s Method (1976)); Figure S4: Bearing capacity factor Nq as functionsof the angle of internal friction and depth-to-width ratio, Illustrating the influence of soil compaction on pile load-bearing capacity (redraw after [PERSON] (1981)).