Micropile to Footing Connections- Findings of the CPF Research-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines the strength and failure behavior of bonded micropile-to-footing connections, drawing on U.S. tests from 2005 and Portuguese tests from 2012. In these connections, the micropile is drilled through and grouted into the footing, so its capacity depends on the bond and confinement around the grout and casing.<br /><br />Axial loading causes the grout and surrounding material to expand radially. This dilation increases normal stress and friction at the interface, while adhesion tends to disappear early. Connection failure can progress through interface movement, cracking or shearing of grout or concrete, or failure of the micropile itself. Surface roughness contributes, but only its short-displacement component is likely to be mobilized.<br /><br />Confinement is critical. In unreinforced footings, concrete cracking can reduce confinement, and proximity to a footing edge strongly affects whether capacity is retained after failure. The presentation identifies full post-failure capacity loss as likely when the edge is close, while greater edge distances may reduce or prevent that loss. Textured casing alone may offer limited benefit in unreinforced concrete.<br /><br />In reinforced footings, reinforcing bars can provide additional confinement and capacity. Proposed estimates account for the force developed in nearby bars, but the methods are preliminary and have not been thoroughly verified. No closed-form method is available for predicting capacity beyond the elastic limit when concrete cracking occurs; numerical analysis may be needed. Designers must also select appropriate resistance factors.<br /><br />Capacity depends on casing size and texture, grout and concrete properties, annulus thickness, edge distance, and footing reinforcement. The presentation recommends limiting annulus thickness, carefully assessing reinforcement and edge proximity, and avoiding unsupported extrapolations. It also notes that shear through grout or concrete and other failure modes require separate checks. Further testing, crack-aware numerical modeling, and design tools are needed.

Keywords

micropile

bonded connection

footing

axial loading

grout

confinement

concrete cracking

edge distance

reinforced footing

failure mode