Deep Foundations - Grout Injection, Hollow Bar Micropile vs. Traditional Cased Micropile

[FndMech]

I am a contractor and would like an engineers opinion as to the pros and cons of each method of micropile installations. In the engineering community, is one method preferred over another? If so, why? Are there any publications you recommend on this topic?

I refer to a Grout Injection, Hollow Bar Micropile as a hollow threaded rod, typically installed and coupled together in 10ft sections, in which neat cement is pumped through the center through then out the tremie holes on the tip of the bit which is installed on the lead section of the first bar. The tremied grout displaces the drill cuttings to the surface and is drilled into competent bearing strata, and grouted in place.

I refer to a Traditional Cased Micropile as a pile in which is drilled using down the hole hammer and air to displace drill cuttings. Once reaching a design depth, drill is removed from the casing and grout is tremied to the bottom of the bore hole. Casing is advanced upward as grout fills the hole through the bond zone and casing typically stays 1' into the bond zone and up through the surface.

 

[PEinc]

For hollow threaded rod micropiles, check IBC for design requirements. IBC states that if the micropile is installed without a casing and if you can't inspect the sides of the drill hole for stability, the micropile should be designed so that the bar (not the grout) is carrying the full load. Also, with hollow threaded bar micropiles, you have a limited selection of relatively small drill bit diameters.

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