Micropiles in Karst - Potential impact of underlying voids?

[motorbiketocrank]

Just wanted to get some impartial opinions. We design and review a lot of projects in karst limestone regions where micropiles are commonly used for foundation support. Personally, I've always been concerned with a potential failure mechanism being collapse of a group of micropiles into an underlying void. I realize that micropiles obviously aren't end bearing and transfer their load in side shear over the bond length into the surrounding rock. My concern is that this overall load added into the rock mass over a void could cause collapse (obviously would depend on the strike/dip, etc of the rock and size/depth of the underlying void).

In my literature research, however, the main opinion I see is along the lines of "because micropiles aren't end bearing and shed their load in friction, there won't be a problem with underlying voids".

What do you all think? My opinion has been that a core boring should be extended below the pile group to make sure there isn't a large void right below the pile tips (or alternatively one of the piles should be extended to check for such voids". I get huge pushback on the issue, however, basically telling me that I don't know what I'm talking about. So I'd appreciate any experienced opinions.

 

[BigHarvey]

In France it's mandatory to check that no void is encoutered 5 m below the bottom of the pile or micropile in such conditions. You don't need to core, destructive drilling with drilling parameters recording will tell you if you have voids or not.

 

[dik]

Air, at low velicities, is terrible at transferring shear...

Not in Karst, but, we commonly drill 10' below major caissons to verify the strata (6" dia hole).

Dik

 

[miningman]

The expression " It was fine untill the moment it failed" comes to mind. Unless you go looking for a void as per previous posters, you always have that risk potential. How big a risk would be a very subjective personal opinion, but unless I have a VERY good feel for the local geology, I would not want to go on the record as agreeing that it is a risk that can be ignored.