Large-Scale Physical Modeling of Micropile Group Behavior in Sandy Soil

This study presents a comprehensive experimental investigation into the axial compressive behavior of single and group micropiles in sandy soils, employing a large-scale physical modeling approach. A systematic series of tests were conducted on steel micropiles with a constant diameter of 42.4 mm and varying length-to-diameter (L/D) ratios of 20, 25, and 30. The pile groups were configured in linear, 1×2, 2×2, and 2×3 arrangements with center-to-center spacing ratios (S/D) of 3 and 6. Uniform Firoozkooh sand was prepared at relative densities (Dr) of 45~50% within a large calibration chamber to simulate loose and medium-dense field conditions. Load was applied under displacement control following ASTM standards, and load-settlement curves were meticulously recorded until failure. The analysis quantifies the group efficiency factor (η) and reveals a significant finding: positive group efficiency (η > 1) can occur in medium-dense sand at close spacings, contradicting conventional design assumptions that often presume reduced efficiency. The results provide a high-resolution dataset that elucidates the interplay between micropile configuration, spacing, angle, and slenderness, offering critical insights for refining predictive numerical models and optimizing the design of micropile groups in granular soils.