Micropile Design - Casing Required? 1

[jdonville]

I have a situation where we would like to install completely uncased elements, likely with a full-length bar, to support relatively light compression loads (15 kip allowable). We would use a temporary casing (duplex system) to drill the hole and grout as the drill is withdrawn.

I have been perusing the micropile literature, and just about every reference I can find uses a cased upper section (presumably through the relatively poor upper soil profile).

What advantages/function does installing a permanent casing offer besides corrosion protection and confining the grout?

Or should I be treating this as a small-diameter drilled shaft?

All comments/advice welcomed.

Jeff

 

[Mccoy]

jdonville,
as far as I've seen uncased micropiles are the rule here in Italy (and they're used a lot!), unless serious hole instability sets in (usually in shallow unconsolidated soil below water table, or in horizontal holes).
They're expensive enough without a casing...

 

[jdonville]

McCoy,

Well, I did a preliminary design based on drilled shaft criteria and neglecting end bearing, but I think it still way underestimates the grout/soil bond acheivable using standard tieback design guidance - especially if grouted under pressure.

Jeff

 

[Mccoy]

Jeff,
that should make up a lower bound for shaft resistance, the furthest from real value the greater the csoil coarseness, the porosity, the lower the cementation et cetera....

Have you read the classic work by Bustamante and Doix (1985 in French). Pressuremeter readings are used in design but...Who the heck has ever used a pressuremeter in the real world (aside the French geotechs) ????

The following is an interesting case history in English by the Italian authors of a book on micropile design:

http://www.ce.utexas.edu/prof/tonon...foundations, a case history in Costa Rica.pdf

Probably you already know the FHWA manual on micropile design FHWA-NHI-05-039, which I could not find online...

Maybe teh best info available online is the following, you probably fmiliar with

http://www.iti.northwestern.edu/projects/NDE/cg_ch4.html

 

[jdonville]

McCoy,

Thanks for the references. I don't think I've seen them before.

The FHWA manual I have in both printed and PDF. It does a poor job, IMO, explaining how the load transfer mechanisms work and why you would want to select a construction method over another. No doubt because it was largely written by specialty contractors who don't want to give away the farm...

I will look at your other references more closely after the Holidays.

Many thanks,

Jeff

 

[Mccoy]

... and why you would want to select a construction method over another

In Italy a favourite application field for micropiles is foundation consolidation, due also to the small operating room...

some other times, micropiles are just a more expensive way to proceed

(last month an excavation consolidation - a pilesheet- was carried out by drilled shafts, after ascertaining the micropiles would have costed 1.5 times more).
The job came out just good.

Probably, there isn't a general rule, depending on specific soil/design/field conditions.