USCS Determination for Soil w/ >50% Non Plastic Fines
USCS Determination for Soil w/ >50% Non Plastic Fines
(OP)
In ASTM D 4318, Test Method for Liquid Limit (LL), Plastic Limit (PL), and Plasticity Index (PI) of Soils, there is the option to state that the sample was tested and determined to be Non-Plastic, NP (section 19.1.1). This is done if either the LL or PL could not be determined or if the PL is equal or greater than the LL (ie: PI is negative).
The USCS method does not make mention of what to do when a soil is tested and determined to be NP but has a significant franction that passes a #200 (75 micron) sieve. For example we have had samples with greater than 50% passing a #200 sieve where the fines were determined to be NP. In some of these cases the LL could be determined but the PL could not (or the PL was greater than the LL).
How do you characterize this soil? If the LL is less than 50 and the PI is essentially zero, it would seem to be ML (silt). If the LL is greater than 50 but the PI can not be determined or is zero, would that be MH? What if the LL can not be determined?
The USCS method does not make mention of what to do when a soil is tested and determined to be NP but has a significant franction that passes a #200 (75 micron) sieve. For example we have had samples with greater than 50% passing a #200 sieve where the fines were determined to be NP. In some of these cases the LL could be determined but the PL could not (or the PL was greater than the LL).
How do you characterize this soil? If the LL is less than 50 and the PI is essentially zero, it would seem to be ML (silt). If the LL is greater than 50 but the PI can not be determined or is zero, would that be MH? What if the LL can not be determined?





RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
The samples we've tested recently, several of which have tested similar to what I described, are from a gold mine in Nevada (no gold in the samples).
Any other thoughts??
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
Good point about the USCS. I think it and the plasticity chart would look different if we could go back and redesign them. The A-line is where it is because Casagrande drew it there one day (in the 1940s?), somewhat arbitrarily, before we had nearly as much experience with the mechanics of those soils. MH with LL=51 and PI=21 has a lot more in common with CH with LL=51 and PI=24 than it does with your samples, which may legitimately classify as MH. There are some extremely plastic montmorillonite-rich clays (determined by grain size and mineralogy) that classify as MH on the chart because they fall below the A-line. The cut between plastic SC and sandy CH is made at 50 percent fines, but with more than 35% clay fines, the sand pretty much floats in the matrix of fines (so the fines dominate the soil's behavior), so it might make more sense to make the cut there. We do Atterberg limits on the -#40 fraction (because Mr Atterberg the agronomist started out approximately that way), but nobody ever seems to know what the -#40 percentage is for their soils. My former boss once proposed changing that to the -#200 fraction, and got a whole lot of blank stares from the audience at the conference.
Oh, well, it's what we have to work with. Just shows the importance of not generalizing too much about behavior of broad groups, particularly MH or OH or OL.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
As has been pointed out, you have to be careful in determining the behavior of the material, regardless of what the numbers tell you. Personally though, I think that even if you got the material to flow (has LL), if it is basically non-cohesive (unless moistures are at or above the LL) then it will act and classify as a ML. As BigH said, there is little meaning to the LL in a NP soil.
Actually, now that I think of it, I probably had soils that would get a LL and be NP, when I was out east. The thing of it is, you can add quite a bit of water to a sample, turn it to jello, and run a LL, but that doesn’t mean it will hold together to get a PI.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
Geopavetraffic you mentioned to be careful with soils with an MH designation. What were you referring to? Also, you mentioned that these soils should be looked at repeatedly to make sure that all of the bases are covered. With respect to this, we've tested several samples of this soil and have come up with the same results.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
It may have a weird compaction moisture-density relationship and making compaction control more difficult,
It may be more compressible than is typical,
It is probably VERY easily eroded, and
Typical strengths may not apply, if for example, there is a lot of mica that could reduce the friction angle.
That very high LL means it can absorb/adsorb a lot of water somewhere before it can start acting "liquid." Could this make freeze-thaw problems worse?
This is strange enough material that you have piqued my curiosity, and probably that of others. Is it diatomaceous earth? Decomposed volcanics? Does it shrink and swell on drying and wetting? Is it residual soil, rather than lacustrine, alluvial, etc.? What part of what country?
DRG
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
dgillette, pretty much covered the specific items that would be of concern. My point was "don't call this soil xxx (ML, MH, Non-plastic, whatever) and think about it just like all other soil of xxx type. This is an unusual material that, since you asked here; you don't see very often. Keep that in mind as you go through the design and particularly during construction.
Make sure that the personel in the field know about this unusuall material, that they are looking for it, and carefully monitoring how it responds to construction activities. You may find that it has some properties that cause problems or unexpected behavior during construction.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
From the description of the source (gold mine in Nevada) I am wondering if these are mine tailings?
As another question, would you be able to post (to add grist to the mill) the raw sieve and hydrometer data, along with the raw Atterberg data? Then we could all crunch the numbers and offer more interpretation on this unusual material.
Jeff
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
mm % Passing
100 100.00
50.0 100.00
37.5 100.00
25.0 100.00
19.0 100.00
9.5 100.00
4.75 100.00
2.00 100.00
0.850 99.73
0.425 97.37
0.250 89.08
0.125 72.58
0.075 57.94
0.042899903 52.95
0.030722147 49.49
0.022214588 43.17
0.020036663 40.73
0.014478856 34.21
0.012039593 28.51
0.008728438 20.37
0.006246618 16.29
0.00312723 6.52
0.001344744 4.07
The other important piece of information is the liquid limit equal to 60% and no plastic limit.
The samples are from a waste dump in a gold mine in Nevada. This dump has been experiencing long term strain softening (time span over 10 years) that has led to recent major displacements. The samples in question are in fact weathered volcanics as you guessed dgillette. This material is at the base of the dump and as it turns out, is the weakest material in the dump profile. I'm not sure of the mineraloy of the samples, but I suspect that there is some montmorillonite and rock flour in these samples.
In addition to the sieves and PI's, we've also done some slow speed direct shear tests to determine the residual shear strength. Based on back analyses and the results of the testing, we've been able to come up with a model that reflects what has happened in the field. With this model, we've forward analyzed dump configurations that should be stable for the long term.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
I am not sure why this sample should see additional scrutiny beyond the fact that ML soils are difficult to construct with and are generally weak materials. Are the properties different than the other classifications? Sure. Of course it doesn’t take long to figure that out when you look at the material or watch a contractor try to compact this “fine grained material” with a sheep’s foot roller.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
I'm always impressed by the insight of those of you that regularly submit posts on this website.
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
RE: USCS Determination for Soil w/ >50% Non Plastic Fines
If you follow through ASTM D 4318, specifically 19.1.1, you would not have a reported value for a LL. This section indicates to "report the soil as nonplastic, NP" when the either the LL or PL cannot be determined or the PL>=LL. The same think is reiterated in section 20.
So, I run the tests in my lab for you. 60% passing the #200, LL=52, PI could not be determined. According to the standard, I report to you that the soil has 60% passing the #200 sieve, and is NP. How do you classify this?
My point is that the concentration appears to be on the flow chart of how to classify, and that people are trying to use data that "is not there". That is, it is not reported.
Don’t get me wrong, I believe in using ALL the information available to help determine the engineering properties, including information that is not always reported; however, I do not feel that this soil gets lost in the classification system. It is a matter of interpreting the index-testing standard correctly.