## 3 Materials, Methods, and Equipment
### Interface Shear Test (IST)  
The interface shear test evaluated the interaction between sand and two pile materials--timber and closed-end steel--concrete composite piles--using a modified direct shear apparatus. Pile material specimens were precisely fitted to the lower half of the shear box (6 cm \(\times\) 6 cm). The hybrid timber-concrete composite and closed-end steel--concrete composite pile specimens were fabricated with high precision, each comprising two 6 cm \(\times\) 6 cm \(\times\) 3 cm sections. The timber section utilized construction-grade Douglas Fir softwood, while the steel section was cut from ASTM A36 structural steel square tubing. Concrete sections were cast in wooden molds with reinforcing cages of four 6 mm diameter steel bars positioned at corners for structural integrity. A 30 MPa Portland cement-based concrete mix (1:4 cement-to-aggregate ratio with water-reducing admixture) was used. Specimens were cured for 28 days at 23 \({}^{\circ}\)C and 50% relative humidity, and were then kept moist with wet burlap for 7 days. This was followed by 21 days of air curing. Figure 6a,b depict the composite pile specimens prepared for interface shear testing. Figure 5: Direct shear test (DST) machine. Surface preparation for bonding included the light roughening of timber and steel by belt-sanding and wire-brushing concrete surfaces. Fabricated specimens achieved dimensional tolerances of \(\pm\)0.2 mm for the cross-section and \(\pm\)0.5 mm for the length. Destructive testing confirmed bond integrity, with sections joined using high-strength epoxy adhesive (20 MPa shear strength). The direct shear apparatus was modified by replacing the lower half of the shear box with fixed pile material specimens, positioning the sand-pile interface at the shear plane. Sand specimens were prepared in the upper half to target relative densities of 10%, 35%, 50%, 75%, and 95%. Normal loads were applied at three stress levels (54.5 kPa, 109 kPa, and 163.5 kPa) for each density-material combination. After consolidation under normal stress conditions, interface shear testing proceeded at 0.5 mm/min under drained conditions until we reached peak stress or constant stress at approximately 10% of specimen length. The comprehensive testing program included 30 individual interface shear tests. Analysis involved plotting peak shear stress against normal stress to establish interface shear strength envelopes, with the slope yielding the skin friction angle (\(\delta\)) between sand and pile material at specific relative densities. This investigation revealed the relationship between friction angle, sand compaction state, and pile material type, providing insights for modeling pile foundations and understanding load transfer mechanisms. The specimen preparation process emphasized rigorous quality control to ensure valid interface shear testing. The selected two-component epoxy adhesive (20 MPa shear strength) demonstrated resistance to moisture and temperature fluctuations, with a glass transition temperature exceeding 60 \({}^{\circ}\)C. Adhesive performance was verified through lap shear tests, confirming that there were bond strengths exceeding 18 MPa under ambient conditions and 15 MPa after water immersion. Surface preparation involved degreasing with acetone, grit-blasting to achieve uniform roughness (Rz \(\approx\) 50 \(\upmu\)m), and epoxy priming. Bonding occurred under controlled conditions (23 \(\pm\) 1 \({}^{\circ}\)C, 50 \(\pm\) 5% RH) with adhesive thickness maintained at 0.5 \(\pm\) 0.1 mm. Quality verification included ultrasonic testing for voids and pull-off tests, yielding average bond strengths of 2.8 MPa for timber-concrete and 3.2 MPa for steel-concrete interfaces. Durability assessment through accelerated aging showed a <10% reduction in bond strength after 200 h of temperature and humidity cycling.