Micropile design

[Alekk]

I'm checking a building foundation on micropiles. Soil is a mixture of sand and gravel, weight about 19kN/mc, phi= about 40°; design lenght is about 11m and water table is about 7m under pile cap.
The designer computed an allowable load of about 43tons according to Bustamante & Doix, but I think it is reallly too high for a micropile of 11m, even in gravel.
Depending on different theories I get 15tons to 20 tons allowable load.

Anyway it is the 1st time I heard about Bustamante & Doix. Does anyone know something about it ? Is 43 tons consistent with micropiles ?

Thanks

Ale

 

[Mccoy]

As far as I know, Bustamante & Doix is the classic reference for micropiles here in Italy/Europe. That's all I can say (I'm not much into micropiles design), there might be more recent or reliable methods, but those charts are still used a lot.
Besides, sand & gravel is a typical lithology where, by moderately hi-pressure grouting, you can create lateral extrusions along the tubing's shaft which will boost the micropile's capacity.

 

[born2drill]

Alekk:

You really haven't given enough information to evaluate the situation, such as the pile diameter and drilling/grouting procedures. Is 11m the overall pile length, or the bond length?
Today in the US a 43 ton minipile is peanuts. That kind of load shouldn't be hard to achieve in a good gravel/sand layer.

 

[Alekk]

Mccoy & born2drill, thanks for your reply. I forgot to say drill hole is 180mm, the bond lenght is about 9m. About grouting procedure: these are a "radice" micropiles (type A in the US according to FHWA guidelines).
About Bustamante & Doix: their method seems not to take care about water table and it looks quite strange to me.

Greetings

Ale

 

[born2drill]

FHWA type 'A' piles are only gravity grouted - not pressure grouted. I'm not all that familiar with Bustamante & Doix, but I don't think it applies for strictly gravity grouted piles. Also, unlike traditional drilled shafts where groundwater can be a problem, a high water table isn't an impediment for minipiles.

 

[Mccoy]

Alekk, you might want to try this link out:

http://129.105.121.69/projects/NDE/cg_ch2.html#eq2.6

there is some reference to Bustamante & Doix, although not extensive.

By the way, I just discovered micropiles are an italian invention (that true?); sure enough, they are used very extensively in Italy, with substantial profit by drilling operators (sometimes cost per lenght unit is greater then in large diameter drilled shafts!)

 

[whyun]

Web site I encountered a while back talked about Dr. Fernando Lizzi who appears to have come up with the concept of micropiles in the '50s. He may be Italian...

I'm also not much into micropile design. But anyone know how "popular" this system is in the United States?