Relation of Pile unit side resistance to SPT N 2

[PantheraX]

Hi all,

I recently have been involved in a research on deep foundation engineering.

One of the issues that came up was:

Is there currently a relation of Piles side resistance to SPT N?
Or for that matter, CPT values?

Note that side resistance,skin friction, side-shear resistance mean basically the same thing.

Any help will be appreciated!!

Thanks,
Ben

 

[fattdad]

qs=N/50 [tsf]

Meyerhoff's Terzhagi lecture, 1976.

Taken from my course notes, 1986, Prof. Duncan.

f-d

¡papá gordo ain’t no madre flaca!

 

[emmgjld]

Displacement piles with an slightly oversized foot (toe?) plate would create some difficulties in interpretation. Define your pile system very carefully.

 

[PantheraX]

thanks fattdad, I'll look up terzaghi's lecture!

emmgjld, in fact i'm looking into different types of piles thats available and currently trying to pick up equations relating to different types of piles, and different piling methods. Thereby, I would be able to, hopefully, tabulate a table that would correlate these data.

 

[Guest262653]

Hi there,

I've been using the relation between the CPT point resistance qc and NSPT established by Robertson & Campanella (1983) taking into account the average particle size (D50). I then use the Bustamante Gianiselli method to determine qs=qc/alpha and integrate the side friction throughout the pile length.

I enclose an excel spreadsheet that I used to determine the expression that relates qc/N to D50 based on the Robertson & Campanella chart.

Hope it helps.

Best regards,
André

 

[PantheraX]

Hi André,

Yep that correlation would be plausible. What made you choose the relation establish by Robertson & Campanella as well as Bustamante & Gianiselli?

I'm interested to get the file that you uploaded, but when I click the link, the file does not load. Is there any other way to get the spreadsheet?

Thanks,
Ben

 

[Guest262653]

Hi Ben,

Sorry for the file issue. Forgot to check if it worked. I've uploaded it to my Public Dropbox folder so it'll work fine now.

Regarding the method chosen, it is our standard practice to use the Bustamante Gianiselli method here in Portugal (sometimes called the LCPC method, I think) and the Robertson correlation I've uploaded is the one they taught is in college in the Soil Mechanics classes. I really don't have any special insight on the reasons to choose this correlation, but it has measured well against in situ pile load tests in sandy soils in a couple of projects we had.

I can try to find the original Bustamante Gianiselly method article and send it to you directly if you need it.

Hope it helps,
André

 

[PantheraX]

Hi André,

The link works fine now, just downloaded the spreadsheet. Don't worry about the Bustamante method article, I should have a book for reference lying around.

What about cohesive soils? Got any idea as to how the relation measures up?

Thanks,
Ben

 

[Guest262653]

Hi Ben,

Regarding cohesive soils, I guess the relation still holds as the diameters range that is included in the chart goes down to the silty clays. Much lower than that diameter and if you're talking about soft soils (unless you've got stiff clays), the SPT values are always to low to be relied upon and to be of any use.

I found the original Robertson article and I've uplodaded it to my dropbox public folder as well. It is freely available at Prof. Robinson's website (

http://www.cpt-robertson.com/pub.html).

I don't know if I can distribute it freely here so if anyone complains, I'll remove it immediately.

Kind regards,
André

 

[Guest262653]

I meant "too low" and not "to low"...

 

[PantheraX]

Hi André,

I agree SPT for soft clays would be impractical to use for such correlations.

Yep, I would guess that it would be a good route to go about estimating the pile shaft friction from SPT N using CPT, considering the fact that .

While a formal direct correlation between the pile shaft friction and SPT N may not be proven(i may be wrong), does the stiffness of the soil actually affect the shaft friction?

Considering that the undrained shear strength of cohesive soils is used almost exclusive to estimate the pile shaft friction, for sands on the other hand, what parameter most important? Grain size?

P.S. While reading through reference texts, i realise terms such as: skin friction, side-shear resistance, unit side resistance, side friction all mean basically the same thing. Just in case it gets confusing.

Thanks,
Ben