## 1 Introduction  
Foundation systems play a pivotal role in ensuring the stability and longevity of infrastructure, particularly in regions with heterogeneous soil conditions [1, 2, 3, 4, 5, 6, 7, 8]. In Kano State, Nigeria, where loose to medium-density sandy deposits interbedded with clay lenses dominate, conventional foundation solutions often struggle to address challenges posed by fluctuating groundwater levels and variable soil compaction [9]. These geotechnical complexities demand innovative approaches that balance structural performance, cost-effectiveness, and environmental sustainability. Hybrid composite piles--combiningmaterials such as steel, concrete, and timber--emerge as promising alternatives, leveraging the synergistic strengths of disparate materials to optimize load transfer and adaptability across soil strata [10; 11]. Traditional pile design methods, while well-established, frequently overlook the nuanced interactions between composite material interfaces and soil density variations. Existing studies on bearing capacity often focus on homogeneous materials or static soil conditions, leaving gaps in terms of understanding how hybrid systems perform under dynamic density gradients [12; 13]. This study addresses these gaps by systematically evaluating the mechanical behavior of hybrid steel-concrete and timber-concrete composite piles across a spectrum of sand relative densities (10% to 95%).  
Foundation engineering continues to evolve through innovations that address the complex challenges of soil-structure interactions while incorporating sustainability considerations. Among these advancements, hybrid composite pile systems represent a significant development that merits comprehensive analysis [14; 15; 16; 17; 18; 19; 20]. The integration of numerical simulation into this study offers profound benefits by enabling the comprehensive analysis of complex soil-structure interactions that cannot be fully captured through traditional analytical methods alone. Numerical modeling provides valuable insights into the mechanical behavior at critical interfaces between disparate materials--steel-concrete and timber-concrete--under various loading conditions and environmental factors. This approach allows for the examination of properties such as stress distribution patterns with unprecedented detail, overcoming the limitations of classical bearing capacity methods, which often rely on simplified assumptions regarding material homogeneity and isotropic behavior.  
The research aims to integrate experimental, analytical, and numerical methodologies to quantify skin friction dynamics, ultimate bearing capacity (UBC), and load distribution mechanisms methods across five soil density conditions ranging from very loose to very dense. Key objectives include (1) characterizing the geotechnical properties of Wudil River sand under varying compaction states, (2) assessing interfacial shear behavior between composite pile materials and sand, and (3) comparing classical bearing capacity theories (e.g., [PERSON], [PERSON], Coyle and Castello) with finite element simulations (PLAXIS 3D). Laboratory investigations adhered to ASTM standards (D422-63, D854-23, D4254) for sand characterization, while interface shear tests employed electro-pneumatic compaction to calibrate relative densities. This introduction contextualizes the research problem, outlines objectives, highlights methodological rigor, and previews key contributions, setting the stage for detailed discussions in subsequent sections.