Micropile and Lagging

[MTNClimber]

Is there any published guidance as to the minimum strength of grout for drilled micropile and lagging walls (fully cased piles)? I’ve seen 4000 psi in the past, which seems overkill.

We always spec 100 psi CLSM for drilled SPL. I’m guessing traditional flow fill CLSM might be difficult to pump through a 1” tremie.

 

[JedClampett]

Why mess with this? Is it a money thing? The difference between CLSM and 4000 psi concrete is what, about $30 a yard?

 

[MTNClimber]

I’m not messing with anything. My question is regarding standards of practice. Plus, if the contractor decides to flush grout through the rig, it may end up on the outside of the casing. Chipping 100 psi CLSM off the casing is easier than 4000 psi grout. Talk about added costs!

Batch plant concrete and grout mixed on-site for micropiles likely have different costs. Especially with a union driller, laborers, and operators all on-site while waiting for the grout to be mixed. If the contractor can save cost by just having the CLSM or low strength grout mix delivered from a batch plant, then I don’t see why they can’t.

That said, the difference in price for an on-site mix of 100 psi and 4,000 psi will come down to the bags of cement + delivery + the small amount of time it takes to mix the extra bags for the 4000 mix. Not that I’ve seen 100 psi lean grout mix before.

Jed - Do you have experience with specifying lean grout mix for MPL walls?

I know PEinc will. But his section in Ratay doesn't mention it.

 

[MTNClimber]

Talking to my colleagues at other firms and contractors, it appears everyone still specs 4 ksi. Not sure if anyone sees anything different in their area.

 

[PEinc]

If the grout is needed to contribute to the composite strength of the pipe pile soldier beam, I would not use lean mix, low strength, flowable fill. If the bending strength of the soldier beam is provided by the pipe pile alone, weak cementitious fill would be OK. The bigger design consideration for a drilled-in "micropile" soldier beam are the reductions to section modulus and moment of inertia due to flush threaded casing pipe joints. Bending strength can easily be reduced by 50% to 60% at the joints.

For micropile SOLDIER BEAMS, I have usually seen other design-build contractors ignore the strength of the grout. I usually ignore it also, unless I need to squeeze a bit more strength out of the pile. In that case, I call for 5000 psi grout only because a common, pumpable grout mix can usually reach 5000 psi when samples are properly mixed, sampled, cured, transported, and tested - which in my experience does not happen consistently.

http://www.PeirceEngineering.com

 

[MTNClimber]

Thanks John. I’ve always used 50% reduction in the casing’s Z and I, which I assumed was conservative since the reduced sections are only at approximately 6”-8” zones every 3.5’ to 4’. I haven’t had any issues yet. I haven’t used the composite section but that’s a good approach to keep in the pocket for some situations.

Thanks again!

 

[PEinc]

I have always used 50% reduction without any problems. However, GEI did some research testing, for Nucor Steel and presented their findings in a 2021 Nucor webinar.

A different paper, see attached PDF, from Schnabel Foundation Company, and others, says that the threaded joint caused a 40% to 50% reduction.

www.PeirceEngineering.com

 

[Milliontown]

In that case, I call for 5000 psi grout only because a common, pumpable grout mix can usually reach 5000 psi when samples are properly mixed, sampled, cured, transported, and tested - which in my experience does not happen consistently.

I can second that. I am working on a large post-tensioned anchor job with 5000 psi grout for the bond and unbonded zones. Our biggest issue has been the consistency of curing and testing from the contractor's QC lab; their lab would sometimes give lower strengths compared to our QA lab. Even after a few audits there were still issues. It's gotten better but it was a struggle. Overall we feel fairly confident the grout mix design and production (batching and mixing) will give 5000 psi after ~3 days, but the lab sometimes tells a different story. There was also a few anchors where a different cement silo was used which may have been contaminated or had some partial hydration of the cement which slowed strength gain to closer to 7 days. This didn't affect the anchor quality but did impact the schedule. 5000 psi pumpable grout is possible, like you said it takes care on the sampling and testing side to have assurance.