Design Considerations for Bonded Micropile-to-Structure Connections-PDF

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The report develops a preliminary method for estimating the capacity of bonded micropile-to-existing-concrete-footing connections, which are commonly used in structural retrofits but lack widely accepted design standards. It draws on U.S. testing by Gómez and Cadden and research by Veludo, alongside analyses of connection behavior.<br /><br />Connection resistance develops mainly through friction at the interfaces between the micropile, grout, and concrete. Surface roughness causes radial expansion as the micropile is loaded. The surrounding concrete and footing reinforcement restrain this expansion, creating the normal stresses that generate friction. If concrete cracks or confinement is inadequate—particularly near a footing edge—the connection may lose capacity or fail in a brittle manner.<br /><br />The proposed conservative procedure separates resistance into two components: concrete confinement, estimated from the connection’s elastic limit and concrete compressive strength, and reinforcement confinement, estimated from strain induced in nearby footing bars. Their contributions are added to obtain the estimated connection capacity. The report also identifies key influences, including micropile diameter and texture, grout properties and annulus thickness, hole texture, reinforcement layout, and distance to footing edges. Textured micropiles can substantially increase resistance when adequate reinforcement provides confinement.<br /><br />The procedure is intended for reinforced footings and for unreinforced footings where the micropile is sufficiently far from the edge. It is not recommended for specified combinations of small edge distance and limited reinforcement; these cases require redesign, detailed analysis, or testing. Wider grout annuli also fall outside the method’s recommended scope. The report emphasizes that the proposed estimates are preliminary and conservative, resistance factors must reflect connection ductility, and field testing or advanced numerical analysis may be appropriate. More research is needed before the method can support a comprehensive design standard.

Keywords

micropile

concrete footing

bonded connection

structural retrofit

connection capacity

friction resistance

concrete confinement

reinforcement confinement

grout annulus

edge distance