NP Non Plastic Soil Designation 4

[cap4000]

I have the ASTM description of NP as: a 3mm thread cannot be rolled at any water content. For example an SM soil is a silty sand classification typically with a PI of less than 4 or below the A-Line. Does this mean there is "none to very little clay content" in the soil?? Any help would be appreciated.

 

[jdonville]

cap,

It just means that there aren't enough cohesive fines (the fines present may not be cohesive) to make the soil as a whole remain together when the thread is that thin.

Jeff

 

[cap4000]

Thanks for the quick response. I am really interested in the clay content percentage or as you indicate the cohesive fines. I see the SM classification sometimes with ML fines associated with the silt particles. My feeling is that an NP has no appreciable clay fraction in the soil.

 

[jdonville]

cap,

Well, as I understand it, the clay particles by definition are cohesive, and the silt particles by definition are not, regardless of the diameter of the particles.

Think of it this way: the silt particles are just really, really fine (generally spherical, and electrostatically relatively inert) sand grains. The clay particles are those that have a high enough surface area to exhibit cohesion when sufficiently hydrated - up to the Liquid Limit.

The nominal particle sizes for clay and silt defined in the various classification systems are oversimplifications that tend to be accurate for the most part. Hoever, they should not be relied on as an absolute indicator of the relative composition of the soil - the cohesive or cohesionless behavior of the soil is a better measure of the relative abundance of the clay minerals.

So, depending on the coarser particle size distribution, it may take the presence of a larger or smaller amount of the clay minerals (whatever their actual diameter) within the soil matrix to raise the Plastic Limit above NP (i.e., induce the soil to behave cohesively overall).

Please note that even when the PI is small (but non-zero), the soil does behave cohesively. It's just that there isn't enough of a cohesive fraction present to act over a significant enough range to classify the soil overall as cohesive, except when it is within the very narrow range defined by the LL and PL. However, if the expected moisture condition is within this range, then the soil should probably be classified as cohesive for the purpose of the analysis.

Hopefully, that was fairly clear.

Jeff

 

[cap4000]

Jeff, Thanks again for the info. I have the NAVFAC DM-7.02 manual and on page 39 it indicates that an SM soil has a cohesion value of 1050 psf when compacted and 420 psf when saturated. I am wondering how an NP soil can have such a high cohesion value with apparently no clay particles. I have now had 3 soil reports on three different jobs indicate an SM soil type and also with NP.

 

[msucog]

fines content does not necessarily mean plastic. if it is plastic it would probably be kicked down to a ML or SC depending on the particular gradation.

 

[BigH]

You might want to read the following threads - as they would be of interest to you with respect to NP designations. CHEERS

thread261-114846
thread261-161797

 

[dgillette]

Hi Cap4000.

What you see in tables of "typical" strengths can vary tremendously in definition. It's context dependent. The cohesion (i.e., y-axis intercept of the strength envelope) you cite sounds like it came from a compacted dense sample that may not have been completely saturated at the time of testing, so there is some pseudocohesion from capillarity. Or, if you do a suite of triaxial tests on very dense material at relatively low confining stresses, the Mohr circles at failure (which is also not defined consistently from lab to lab, job to job), you might see avery low friction angle and a hign cohesion. If you look, for example, at the tables in USBR's Small Dams or Hunt's book, the numbers can't be used easily because the test conditions varied so much. (Told to me by the old USBR tech, now deceased, who did much of the testing cited.)

The term cohesion is used in different ways on different days. When used in the context of strength, it's generally the intercept on the tau axis of the Mohr-Coulomb strength envelope. When used to describe soils, it's whether the soil coheres, as clay does.

Every day I realize that this stuff is more complicated than I thought the previous day.

Incoherently,
DRG

 

[jdonville]

dgillette,

"Every day I realize that this stuff is more complicated than I thought the previous day."

Glad to see that I'm not alone in thinking along those lines...

Jeff

 

[fattdad]

The original question is whether a non-plastic sample (i.e., PI=0) is free of "clay". The ASTM answer is yes. In engineering the presence or absence of "clay" is determined by the behavoir of the sample (i.e., the Atterberg limits). In agronomy (i.e., Soil Science) the presence of clay is determined by the diameter of the soil grain). In mineralogy, the presence of "clay" may be determined by x-ray defraction.

One of the replies correctly points out that the distinction between "cohesive" and "cohesionless" soils is determined by sand content and Atterberg limits. That led a followup post on the term "cohesion" as it relates to the Mohr's strength envelope.

The term cohesive soil typically relates to clays and silts where there is some measure of plasticitity. For the case that you are dealing with "rock flour" you may have clay or silt-sized particles, but no "activity" (i.e., the PI is zero, which in some cases means the liquid limit and the plastic limit are the same value).

When you take the time to do a series of triaxial strength tests, you will get an effective stress envelope that includes a "Y-axis intercept" along the shear strength. This is defined as the cohesion intercept. DM-7 presents representative values for this value for "cohesionless" soils (i.e., sands, silty sands, etc.). Most engineers don't account for this value in their engineering analyses. I don't.

Good luck in this. Hopefully I have not typed too much that you don't already understand, but if I did I was on a roll.

Then again. . . maybe I've just confused matters - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[casimmons]

What's the real issue here? Do you just want to know the "clay content?" If so, run a hydrometer. But it sounds like you are trying to derive some shear strength parameters. If that's the case then we can't answer your question on this forum without a lot more information. But ignoring any apparent cohesion for an SM soil which is non-plastic would be a good start!