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Correction Factor use for SPT

Correction Factor use for SPT

Correction Factor use for SPT

I have a question regarding the use of the Sampler Correction factor when normalizing SPT N values to N60 values.  My question is this:  The ASTM specification D-1586 shows a "standard" sampler as one having no liner.  Skempton (1986) suggests using a correction factor of 1 (no correction) for a "standard" sampler, and 1.2 for a sampler with no liner.  The ASTM spec also has a note that indicates a 10 to 20% increase in N values can be expected when a liner is used.  Although some associates say there is some ambiguity with all the wording, I am certain that using a 1.2 correction factor when no liner is used is appropriate.  Some experimentation with the Skempton method of correcting to N60 shows that using the 1.2  versus 1 correction factor with no liner makes sense as it is shown in the table.  I would appreciate your opinion on the matter.  Thanks.

RE: Correction Factor use for SPT

Be very careful in making corrections to SPT results.  While the SPT is a useful test, it is at best a comparative test.  The test is also not very precise, as so many factors other than the properties of the soil layer of interest can influence the uncorrected N value.

If you are struggling to add another 5 or so to the N value, you are probably barking up the wrong tree.  Keep in mind whether the corrections you are applying have "only just" brought the N value to the lower end of an acceptable range.  If that is the case, then you should consider whether you are demanding more of the soil than it has to give.  You should review the risks involved, and the benefits of some of your alternative solutions, using less optimistic values of N.

RE: Correction Factor use for SPT

Thanks for your response Sean2.  I have a few more thoughts to add:

Through my technical literature review and experience, I believe the SPT test can be used strictly as a comparison test (on a single site), or it can be used to actually estimate preliminary insitu soil properties such as relative density provided the proper corrections, correlations, and engineering judgments are used.

Even if the SPT is used as just a comparison test, no two sites are ever the same, so you really can’t compare N values from one site to the next until you normalize the variables of the testing, soils, and site as mentioned above.  The danger with using the SPT strictly as a comparison test is that some believe the standard penetration test is actually standard around the world, when in reality there are many variables of this “standard test” from driller to driller and how the engineer interprets the results.

I've performed hundreds of soil boring projects in my area (relatively uniform water and wind deposits of fine to medium sands are frequently found in this area).  I've had the opportunity to retest some of the same areas we drilled during the soil boring program within the excavations at various levels below original grade after construction starts using a nuclear gauge or sand cone.  I've found that the insitu relative density measurements are, in many cases, within 10% of my original estimates based on the soil boring program with the properly interpreted and corrected N values.  Of course as you mentioned, there are a lot of variables to consider such as type of hammer used, condition of equipment, sampler liner or no liner, 2" sampler or 3" sampler, soil type: silts-sands-gravels, overburden, particle shape, gradation, etc.  It is important that engineering judgment still be used throughout the process.  The risks and overall site variability also need to be considered in the final foundation recommendations.

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