Micropile Strain Compatibility

[born2drill]

I'm looking at a micropile project with design loads of 200T in compression and 220T uplift. These piles would be cased to rock with the threadbar grouted into a socket below.

In the past I have seen other engineers limit the strength of the reinforcing to 87 ksi, based on limiting concrete compressive strain to 0.003. This approach is mentioned in FHWA's Micropile Design & Construction Guidelines, section 5.E.7.

Should this prohibit me from relying on high strength (150 ksi) reinforcing steel for a rock socketed micropile? The grout will certainly have confinement in both the cased and uncased sections, so is the grout crushing or fracturing really a legitimate concern?

I'd be interested to hear if anyone has designed micropiles using bar strengths over 87ksi. It seems the geotech engineer on the project who will be doing the review is fine with using 150 ksi steel, but I'm still interested in hearing some outside opinions.

 

[STVU]

Grade 150 has the lighter weight advantage, so it is easier to install it in 10 foot sections. This is specially true where batter micropiles produce obstruction problems.

When you are testing a MP in tension, the casing is mostly 80 ksi, and If the bar is 75 ksi, your compatability is similar, since the lower of the two strengths govern. However, when you use a 150 ksi threaded bar with 80 ksi casing, you are stil limited to the 80 ksi for compatability-so the additional bar strength is not helping you out.

Now, if it is a small job, and you have stock 150 ksi bars left over from previous job, that is another story.

 

[PEinc]

If you are casing only to the top of rock and then drilling a rock socket for the full length threadbar, then the weak link in the micropile for both tension (a tiedown anchor) and compression (a micropile) is the uncased, grouted threadbar in the rock socket. Therefore, the steel casing pipe strength should not matter.

A grouted, cased hole with a threadbar inside should have more load capacity than an uncased, grouted drill hole (rock socket) with the same threadbar. I would not worry about strain compatability unless the casing pipe extended all the way to the bottom of the rock socket and was needed to supplement the strength of the threadbar.

For 220 tons of tension, I assume you have a rock anchor. Therefore, the steel casing pipe is usually not considered to add any tensile capacity. Also, if you tried to use the casing pipe for tension, you would need a strong connection for the top of the casing to the structure. Not very easy.

If a threadbar grouted into an uncased rock socket can support 200 tons in compression, the steel cased micropile with a threadbar should support more than 200 tons. This assumes that the grout area in and above the rock socket are approximately equal and that the threadbar extends for the full length of the mictopile.

 

[born2drill]

STVU:
I didn't mean strain compatibility between casing steel and internal reinforcement, I meant strain compatibility between internal reinforcement and the surrounding grout. PEinc is correct - I have bar in the full length of the pile, so the socket length is the weak section and governs the design. Casing doesn't even need to be considered for strucural capacity. When it is, I have even seen reviewers in my area write "DO NOT USE CASING FOR STRUCTURAL CAPACITY" and make me resubmit.

PEinc:
It would be a traditional pile - case the hole to rock and punch out a socket. The bar is doing all the work in tension (and most of the work in compression). Have you designed using 95ksi or 150ksi on the bar for compression loads?

 

[DRC1]

I had a project where strain compatibility was an issue. Looking into it after, it seems to be more of an issue for uncased pile heads than cased. Essentially as load comes on the pile it is shared between the core steel and the grout. The steel and the grout will defelect equally to share the load. You want to be certian that the deflection is not enough to crack the grout. If you have a casing, it provides confinement of the grout to reduce potential for cracking. Usually by the end of the casing, the pile has shed enough oad to the existing soil so the strains are no longer excessive.Really it is more about bar area vs concrete area than it is about bar strength, unless you are trying to get higher loads. In that case strain compatibility may be a problem. It can be overcome by a casing

 

[PEinc]

born2drill,

Usually, the micropiles I design use either GR75 or GR150 threadbar core steel. A recent job used GR150 in the uncased rock socket with a special coupler to a GR75 bar that ran up the cased micropile and into the pile cap.

I just did a preliminary design for a 335 ton capacity micropile bonded in dense sand with some gravel - no rock socket. I figure on leaving the casing pipe above the bond length. Because I have no bond length casing pipe or rock socket to contain the bond length grout, I considered strain compatibility and I did not use GR150 bars. Instead, I used a larger GR75 bar for the entire length of the micropile.

Shedding of the load down the length of the casing (as mentioned by DRC1) is a function of the casing length, casing diameter, and the soil/grout bond along the casing. If the upper soils have low bond strengths to the grout, the majority of the load, and possibly the entire load, will transfer to the uncased bond length. I designed the full compression load to be supported by the portion of the pile below the casing pipe.

 

[born2drill]

DRC1,
It's not really about the pile head, it's about the socket length, which is uncased and usually governs design since it's the weakest section. As for shedding load along the cased length, it might happen, it might not. But many building codes don't allow it. Some designs involve plunging the casing into the bond length to reduce the load on the socket, but I've never seen any evidence that having a plunge length does anything at all.

I'm not sure what you're getting at talking about bar area vs. concrete area. If you limit concrete compressive strain to 0.003, you can't count on any more than 87ksi for the steel, regardless of what the cross section looks like.

PEinc,
In the past I used to always seen gr75 steel, I never even heard of using gr150 on a micropile until very recently. Of course, the building code I mainly work with also limited bar strength until last year. Rather than using higher strength steel in a rock socket, I'm accustomed to having a cage or bundle of bars in the socket + development length, with a single bar above.

335 tons is huge for a pressure grouted micropile - much larger than anything I've ever done. I sure hope the contractor is a good one!

 

[DRC1]

I have transcribed some notes I had for stain compatibility for an uncased minipile. (Hope this works) I don't have details on the project - would have to dig out later, but do recall minipiles were 8" with steel core no casing drilled I think 50-55 feet in sand.
Not saying this is right, just how I did it.
I agree in some soils, silts, etc. casing will shed little load, In good sands, could shed alot.
335 tons is indeed huge. Largest load I ever heard of was 200 tons. Let us know how the project goes.

 

[hokie66]

Never been involved in a project using these micropiles/minipiles, and after reading the variation of opinions in this thread, think I will do my best to avoid them in the future.

 

[DRC1]

I don't think that is necessary. Minipiles have been around for a long time and have a very high sucess rate. They are exclent for many locations were conventional support systems would be impossible. Although they work, we are always looking to understand them better so they can do more. This bantering here is not unlike structural engineers exchanging differing views on bracing and braced lengths.