Behaviour of Hollow Bar Micropiles Under Uplift and Lateral Loading-Article

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The study examined the uplift and lateral performance of hollow bar micropiles (HBMPs) installed in cohesive soil at a site in London, Ontario. Eight full-scale micropiles were constructed using 178 mm or 228 mm drill bits and tested under uplift and lateral loading. The research also assessed steel-fiber-reinforced grout through laboratory testing and numerical analysis.<br /><br />Uplift tests showed plunging capacities of approximately 575–656 kN. Fuller and Hoy’s interpretation method best matched observed failure loads, while FHWA’s recommended grout-to-ground bond values underestimated capacity. Average unit skin friction was about 1.5–1.6 times the soil’s undrained shear strength. Uplift capacity differed only slightly between the two drill-bit sizes, likely because higher grouting pressure with the smaller bit increased bond strength and hole enlargement. Capacity was primarily transferred through shaft friction.<br /><br />Lateral tests indicated that micropiles behaved as flexible piles, with response strongly influenced by soil conditions in the upper 8–10 pile diameters. Larger drill bits generally improved lateral performance. Testing revealed two deformation mechanisms: separation and gapping at the soil–pile interface, followed by radial cracking within the grout.<br /><br />Numerical analyses found that fiber-reinforced grout increased lateral capacity at 25 mm displacement by about 30% and improved ductility and resistance at larger displacements. Fixing the pile head also substantially increased capacity: by approximately 80% for conventional grout and 160% for fiber-reinforced grout.<br /><br />Overall, HBMP performance depends on installation methods, drill-bit and nozzle specifications, grouting pressure, and head restraint. The findings suggest that standard FHWA bond-strength assumptions may be overly conservative for these micropiles in cohesive soils.

Keywords

hollow bar micropiles

cohesive soil

uplift capacity

lateral loading

steel-fiber-reinforced grout

shaft friction

grout-to-ground bond

drill bit size

flexible piles

numerical analysis