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Negative Skin Friction and SPT value

Negative Skin Friction and SPT value

Negative Skin Friction and SPT value

Hi, My name is Tim, I am currently working on a foundation design project concerning the negative skin friction. I used to think that the depth of NSF developed should extend to the whole soil layer described as marine clay. But I was told later that the SPT value would give a hint to choose the depth as a great SPT would mean the soil is stiff and firm and should be fully consolidated. I just want to ask if that is reasonable to say so. And is there any journals about SPT value and NSF having a relationships?

thx alot

RE: Negative Skin Friction and SPT value

It is common to assume that the neutral plane will be approx. 2/3 down the pile.  The neutral plane being the point where the pile settlement is equal to the soil settlement.  

Go to www.geoforum.com and search for negative skin friction, there are a lot of helpful hints.  You can also find some good papers there by fellinius.

RE: Negative Skin Friction and SPT value

Yes, I have read the books that 2/3 would be a good idea but that only applied to the soil profile with clay material only as they have not yet consolidated.
But thanx for your advice

RE: Negative Skin Friction and SPT value

Good question, Tim.  Underconsolidated soils are a special problem.  I have a few questions:

1. Have you run both undisturbed and remolded lab vane shear strength tests to estimate the soil's sensitivity?

2. What is the range in soil strength with depth? (Please use psf or pascal for units, not tsf or kPa.)

3. Where is the site?

4. What is the structure?  What kinds of loads are being resisted?  What are the consequences of foundation failure?

RE: Negative Skin Friction and SPT value

A coorelation between SPT and NSF have no direct relationships.  For a marine clay, an SPT is not a good test to establish the materials stress history in order to determine its consolidation state.  But as a general rule one should be able to calculate the vertical effective stress, then assess the shear strength of the material in question and compare values to see if indeed the material is over consolidated.  The best thing to do is get shelby tube samples and perform 1-D consolidations or trixial tests to get a good handle on the material in question.  Negative skin friction calculations are usually dependent on the post loading (fill) of pile foundations.  That interpretation is best left to seasoned geotechnical engineers with local experience.

RE: Negative Skin Friction and SPT value

Focht3 - whilst I say okay to psf rather than tsf (I like ksf too), I don't think you really want pascals over kPa. - if you have an undrained shear strength, say, of 250 psf - this is approximately 12.5 kPa - do you really want to use 12,500 pascals??  How about if Su = 900psf; then either like 45kPa or 45,000 pascals??  

This is the problem with using Indnonesian Rupiah or Lao kip - there are 10,000 to a dollar and one is a millionaire and still broke!!!  The one I would like them to get away from is using kg/cm2 when in SI units.  Big problem in this SI thing - not everyone can agree on their units.  psf/ksf/tsf - much better to use, in my opinion.

RE: Negative Skin Friction and SPT value

I stand corrected - kPa it is.

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