Using field N values vs. corrected N60 values for drilled pier foundation design

[Vtnguyen]

My question: Is it normal practice to use corrected N₆₀ values for drilled pier foundation design? Are there cases where I should be conservative and use the uncorrected N_field values?

Background: I'm designing a concrete pile foundation with an anchor bolt cage using the Hansen method. I have a soil boring report (attached) that lists the SPT results in one column along with corrected N₆₀ values in another column.

The soil is fat clay for the first 5 ft or so, and then silt or silty sand for the next 20 ft. At 25ft, I start finding lean clay. There is a note saying water is encountered at 5 feet below the surface.

Factored structure loading @ groundline (resultant forces):
Axial force= 28.1kips
Shear force= 57.6 kips
Moment= 3392.42 kip-ft

I'm getting results (approximately) between 25ft to 28ft for the total anchor bolt length depending on whether or not I use corrected N values.

 

[oldestguy]

If it was me doing this, I'd be using unconfined compressive strength data rather than blow counts. I'd have undisturbed samples for that.

 

[Vtnguyen]

@oldestguy
Well, the reason I'm using blow counts is becuase I use an internal company program that designs the foundation using those values. I input layer depth, designate whether the layer is a sand or clay, and then input blow count. It calculates the soil properties based on those inputs.

 

[oldestguy]

Interesting attempt to simplify things. I've seen these come and go, for obvious reasons.

 

[oldestguy]

While we are at it with this program, a question. Does this program account for ground water buoyancy of the anchor you are sizing? Was there an engineer inspecting the boring work? Who verified the sample data, preferably in a lab? Is the company's professional liability insurance up to date?

 

[BigH]

OG - unconfined compression for "silt or silty sand"? Whether 25 ft or 28 ft - the cost for additional steel is minimal so I'd use the 28 ft (presuming that your computer programme is verifiable) Have you done a hand calc to see what it gives? Most of the correlations that you find (say pre-1980) were based on whatever N value was reported - usually, then, it was cathead and rope (and depending on whether your driller used 1-1/2 wraps or 2 wraps) - the correction values seemed to come in more for liquefaction and then, with more efficient automatic hammers a much higher efficiency - which is whey they would be corrected to the "old" N values . . . and why N60 and not N55 as some correlations now use? OG gives good advice in his post about buoyancy . . . I'd say that you should be "comfortable" with what you use - and for sure, do a hand calc (might take you some research) to verify. I hate relying on computer programmes especially non-commercial unless they have been substantially verified.

 

[fattdad]

I agree with the hand calc. I haven't read the attached geotechnical report. If the silt (i.e., of the silty sand) is highly elastic, I'd be interested in the unconfined compressive strength also. Of course if we don't know about saturation levels, I'd want a UU TXC at multiple confining stresses (i.e., to see if phi is really equal to zero).

I'm always suspecious of water table information solely at the time of drilling. So, I'd want a piezometer.

I'd adjust the N-values for 60 percent efficiency.

Brinch-Hanson is the dude's last name. Had to do a double take on the citation of Hanson. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[FixedEarth]

Usually the corrected SPT values are higher than raw N values in the upper 15 ft and lower than raw N values below 15 ft. Given your embedment is about 25 ft, the correction factors will cancel out anyway. I recall that Hansen's method assumed one soil layer with groundwater table location (GWT). In your case, it is a four soil layer system with GWT and you need to verify if Hansen's method (1961) is still applicable. The "fat clay" layer has a moisture content similar to the rest of the soils underlying it and am wondering if Atterberg limits were done on it.

Anyway, I checked it with other methods and am coming up with 24 ft embedment, 4 ft diameter shaft and 3% vertical steel. This yields about 1/3 inch vertical settlement & about 1/2 inch lateral deflection.

http://www.soilstructure.com/