Loess tunnels in China’s Loess Plateau face significant stability challenges due to the collapsible nature and moisture sensitivity of loess. This study investigates foundation behavior and reinforcement design for plain concrete pile composite foundations through integrated laboratory testing and three-dimensional numerical simulation. Laboratory tests quantified moisture-dependent mechanical properties: cohesion decreases from 43.33 kPa (w=14%) to 36.26 kPa (w=20%), following c = 59.5 – 1.17w (R²=0.99), while friction angle decreases from 33.7° to 29.4° following φ = 43.7 – 0.72w (R²=0.996). Interface shear tests revealed loess-concrete adhesion ratios (ca/c) of 0.25–0.30 at natural moisture, critical for pile shaft capacity calculations. Three-dimensional finite element models simulated excavation-support processes across burial depths of 5–46 m. Contact pressure exhibits non-uniform distribution with peak values of 1.32–1.42×γH at arch feet. The empirical relationship σ_base,peak = η×γH, where η = 1.12 + 0.0077H (R²=0.89), provides a practical tool for foundation pressure prediction. The three-bench seven-step method with 3 m footage limits crown settlement to <0.15% of tunnel height. Interface effects reduce pile shaft capacity by 32% compared to conventional calculations, and seasonal moisture variation causes 23% capacity fluctuation. These findings provide validated design parameters and practical methodologies for foundation reinforcement in loess tunnel construction.