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# Side Friction in Limestone

## Side Friction in Limestone

(OP)
Hi Everyone,

I've been trying to find the correct equations to use to determine side friction in Limestone in a Karstic Environment with a 30" dia. drilled shaft.

Initial tests show an RQD of 50% and qu = 5000 psi.

Any insight on the correct formulas to use would be great. I've referenced the FHWA manual and equations 13-18 and 13-19 seem to be what I'm looking for but I want to see what everyone's thoughts were.

Thanks!

### RE: Side Friction in Limestone

While not directly addressing side support of pile in karst - you might want to look at a Geo-Strata article by Griffith, et.al., "A Probabilistic Approach to Karstic Foundation Variability". Can give some other insight other than "forumulas".

### RE: Side Friction in Limestone

With an RQD of 50, I don't know how much I would want to rely on side friction. Is the qu value from a reassembled section of core, with the joints tacked back together? Or, does it represent the best piece in the run? I agree with BigH, insight other than formulas will serve you well on this.

### RE: Side Friction in Limestone

the AASHTO LRFD Bridge Design Specs has equations for it but with an rqd of 50% you won't have much capacity. I'd suggest getting a geotech to give you a value. We will calculate something so we can do some prelim work but always end up getting a final value by the geotech. They always end up with a value much lower than what I come up with.

The soil guys understand it much better than just some equations in a book that anyone can plug and chug. After all they do the borings so they have the info they need.

### RE: Side Friction in Limestone

Hi CJ, do a pile testing and I am sure you might get the right answer.

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