## 3 Materials, Methods, and Equipment
### Specim Preparation and Test Method for Calibration of Relative Densities  
Calibrating sand relative densities ensures experimental accuracy by establishing compaction-density relationships. Using a 30 cm \(\times\) 30 cm \(\times\) 20 cm wooden box and precision balance, specific placement methods were developed for target densities (10%, 35%, 50%, 75%, and 95%) by varying sand pouring height and layer thickness. These densities span loose to very dense states, enabling the observation of threshold behaviors such as contractive-dilative transitions at medium densities. The selected densities correspond  
Figure 3: Hybrid steel–concrete/hybrid timber–concrete composite details. The diagram uses different colors to distinguish various sections of the foundation. The blue dashed line represents the groundwater level, while the foundation itself and the surrounding soil are depicted in different shades. to field conditions, with 10-35% representing reclaimed land or hydraulic fills, 50% representing natural sand deposits, and 75-95% representing engineered fills or deep natural deposits. This range facilitates comparison with the existing literature while supporting comprehensive analysis of pile-soil interactions across compaction states. For the loosest state (10% relative density), sand was gently poured from a height of 10 cm without any additional compaction. This method mimics the natural deposition of sand and results in a very loose packing arrangement. To achieve 35% relative density, sand was poured in 15 cm layers, and each layer was compacted using a Proter electro-pneumatic hammer fitted with a steel tamper plate (15 cm \(\times\) 15 cm). A plastic base was attached to the bottom of the steel plate to prevent the crushing of sand particles. Each layer was compacted for 1 s. For 50% relative density, the pouring height was reduced to create 10 cm layers, with each layer compacted for 2 s. This increased compaction time resulted in a denser packing of sand particles. The 75% relative density was achieved by reducing the layer thickness to 7.5 cm and increasing the compaction time to 4 s per layer. This method significantly improves the energy input, resulting in a much denser arrangement of sand particles. Finally, for the densest state (95% relative density), sand was poured in thin 5 cm layers, with each compacted for 10 s. This intensive compaction process results in the very tight packing of sand particles, approaching the maximum density achievable in the laboratory. Figure 4a-d illustrate the laboratory equipment assembly used for sand compaction testing, including the wooden test box, Wudil river sand, an electro-pneumatic hammer, and a steel tamper plate. Figure 4: Laboratory setup for sand compaction process: (**a**) wooden test box (30 cm \(\times\) 30 cm \(\times\) 20 cm), (**b**) Wudil river sand sample, (**c**) Proter electro-pneumatic hammer, and (**d**) steel tamper plate (15 cm \(\times\) 15 cm). Post-compaction, sand was leveled and the filled box was measured to determine total sand mass by subtraction. Sand volume was calculated from box dimensions, while moisture content was assessed using multiple samples oven-dried at 105 \({}^{\circ}\)C for 24 h per ASTM D2216. Dry unit weight was calculated using mass, volume, and moisture data; they were then combined with previously determined maximum and minimum void ratios (ASTM D4253 and D4254) to compute relative density. The process underwent multiple repetitions for each target density to ensure reliability, with averaged values establishing the final compaction effort-relative density relationship. This calibration enabled consistent specimen preparation for subsequent experiments, ensuring the accurate representation of density states from very loose to very dense conditions for comprehensive pile-sand interaction studies. The relative density calibration procedure balanced established standards with model-scale experimental requirements.