Scale Model Investigation of the Effect of Vertical Load on the Lateral Response of Micropiles in Sand-Article

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Kershaw and Luna investigated how a constant vertical load affects the lateral response of micropiles in sand, addressing limited and conflicting evidence on combined loading. They tested 17 instrumented, one-eighth-scale micropiles in a laboratory pit containing prepared loose or dense fine sand. The program included axial compression tests, lateral-load-only tests, and combined tests in which an axial load—about half the measured ultimate axial capacity—was maintained while lateral load increased. Two casing thicknesses were used, and measured responses were compared with LPILE analyses.<br /><br />Sand density had a stronger influence on lateral behaviour than micropile casing stiffness: piles in dense sand were substantially stiffer than those in loose sand. For most load increments, adding vertical load caused little change in lateral deflection. At higher lateral loads, however, combined-loaded piles generally deflected more and could fail more suddenly, particularly in loose sand.<br /><br />The clearest effect of combined loading was increased bending moment, observed across the tested lateral-load levels and attributed in part to the P–Δ effect as lateral displacement makes the axial load eccentric. At comparable lateral loads, maximum moments were about 22–40% higher in loose sand and 5–30% higher in dense sand in the reported comparisons. The authors emphasize caution where loose sand occurs near the ground surface and note that increasing casing thickness may not adequately control deflection in such soils.<br /><br />LPILE generally overpredicted maximum bending moments; its deflection predictions were slightly too stiff for loose sand and slightly too flexible for dense sand. Because the findings come from 1-g scale-model tests, they should not be directly extrapolated to full-scale foundations. The authors recommend validation through full-scale tests in sand.

Keywords

micropiles

lateral response

vertical load

combined loading

sand density

bending moment

P-Delta effect

laboratory scale model

casing thickness

LPILE analysis