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# How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

## How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

(OP)
Greetings, fellow Eng-Tippers.
Numerous authors suggest assume 10D for loose soils, 15D for medium soils, and 20D for stiff soils, for critical depth.
I am analyzing an existing 60-ft long pile that was driven thru sand, and am trying to determine the SKIN FRICTION capacity using empirical methods.
I plan to later verify the results via A-pile or other software.
For now, I am trying to understand the theory and its practical application.
The textbooks don't seem to cover realistic scenarios where the pile goes thru various soil stiffness properties.
What critical depth should one assume for a 60-ft long pile where the upper 20 feet goes thru loose soil, the middle 20 ft is medium soil, and the lower 20 ft is stiff soil?
Thank you.

### RE: How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

FHWA Driven Piles Manual Vol I has some good information on static design. The better way to determine the capacity of an existing driven pile is to run a dynamic or static load test.

### RE: How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

Looks like critical depth isn't really addressed anymore.

### RE: How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

For pile capacity, I suggest using effective stress ("beta method") for all soil types for static loading conditions.
I suggest reading some Fellenius papers, which many are free to download. Search "unified design piled foundations Fellenius".
Critical depth is an inaccurate and flawed concept, but will usually result in a conservative pile depth calculation.

### RE: How to Determine Critical Depth for Design of Pile Thru Mixed Soil Strata?

(OP)
I appreciate the fast replies. Thank you, all.

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