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Micropile Connection for Fixity

Micropile Connection for Fixity

Micropile Connection for Fixity

FHWA gives guidance of a micropile attaining full head fixity at 24" of embedment. With a top plate attached and threaded bar/nut/ embedded into the pile, could the connection not be designed for fixity with a lesser pile cap embedment? Basically check that breakout on the bottom side of the plate/cap is sufficient to withstand the applied moment? Love to hear some thoughts.

RE: Micropile Connection for Fixity

Presumably, if the pile has a fixed connection to an adequately stiff embed that develops the full moment capacity of the pile through the connection and into the concrete, then it's essentially fixed.

I'm a bit surprised at how much embedment they're calling for. TXDOT did some full-scale testing of H-piles (HP12X53) embedded in a concrete abutment cap and the piles hinged (developed the full plastic moment capacity), without breaking out if the concrete, with 12" of embedment.

RE: Micropile Connection for Fixity

I agree. Another DOT ( I think Oregon) did one with micropiles and the deflections even at 6" embedment mimicked the expected fixed head deflections. Often times when dealing with piles in either a grade beam setting or diaphragm, it is not feasible to get that kind of embedment but it is desirable to have the deflections calculated as fixed head rather than pinned.

RE: Micropile Connection for Fixity

I think you might have that turned around, jwatso33. For a conservative design, I would calculate deflections using the pinned assumption, which would be the most flexible, and design for strength using the fixed head assumption, which would max out the moments.

RE: Micropile Connection for Fixity

For a conservative design I agree...but that is not always the case. Where trying to limit deflections, often the most economical method designing a fixed head connection even when it requires more embedment. From there you just have to deal with the moments with casing size/reinforcement. That is why I am curious about what connection could get that kind of result without such a great embedment into the cap.

RE: Micropile Connection for Fixity

I apparently misunderstood your statement, jwatso33. I agree that with adequate embedment (or other restraint), modeling the pile head as fixed is a reasonable assumption. Of course nothing is truly fully fixed, but you can get pretty close to it with direct embedment or a stout connection plate and anchors.

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