Non Plastic silt soil 1

[KiranDave]

Is it possible that a soil sample that can be drawn to 3mm thread when drawn delicately be classed as non plastic. I think all silt soil samples need to be drawn delicately but if they can be drawn to 3mm from a ball then they cannot be classed as non plastic.
Our lab classed it as NP with a soil described as clayey fine sandy SILT. I sent it for re testing and they came back with same result. Please tell me what to do.
my description for that soil would be fine sandy SILT. It contains little clay but not enough to class as clayey SILT.

 

[fattdad]

If the liquid limit and the plastic limit are the same value then it's non-plastic (i.e., PI=0). It is possible for a silt to be non-plastic. It is not possible (per ASTM) for a non-plastic sample to have a description that includes the word "clay".

f-d

¡papá gordo ain’t no madre flaca!

 

[iandig]

Not sure how the method in the ASTM differs from the BS method, but in the BS, the soil is not "drawn delicately" into a thread, there is a very specific method for forming the thread.
In basic terms, the soil is rolled using two fingers, at a constant pressure, to reduce the thread from 6 to 3mm, between 8 and 12 times on a scrach free glass plate. If you can't form the thread using this method, the soil is non-plastic, even if other handling techniques can form a 3mm thread with the soil. The test is very operator sensitive, and I have seen numerous occasions where the technician uses the palm of the hand to form the thread and even rolling the soil between two palms. Any variation in the testing technique can and will return lower moistures than you can achieve using the two fingers as per the BS(i.e. soils which should be "non-plastic" return values for the palstic limit.
You have to be very careful when undertaking the test to ensure the data accuratley reflects to soil.

 

[KiranDave]

thanks for your reply
My argument was just what you said if there is clay no matter what percentage it cannot be classed NP
Now our lab has performed particle size analysis on the sample. The sample has 55% fine sand, 35% silt and 10% clay. So the sample is clayey very silty fine SAND. Now the lab has come back saying that it is sand hence the sample is NP but my argument still is that it has got 10% clay and hence cannot be NP, probably low plastic.

 

[cvg]

I just reviewed a report today where most of the material was clayey sand and none of the SC samples was identified as non-plastic. I think some training is in order for the lab techies...

 

[msucog]

is the arguement particle size or mineralology?

which classification system you using?

i read "clayey fine sandy silt" to mean a "sandy silt with a trace of clay" and not a "clayey silt". i'm not expert on the classifications and haven't plotted your situation on the classification charts but that's the way i see it from this side of my computer.

i quite often see sandy silts that have some percentage (sometimes rather high amounts) on the clay fraction of the gradation be NP...it just happens to very fine soils but NP. i also see very similar soils have low PI's. heck, sometimes even mica influences the gradation since it hangs up on the coarse fraction artificially classifying it as a SM instead of an ML...or it ends up with 48% passing #200 (and 40%+ mica) and the mse designer talks about "all the sand" on the site when it's definitely a silt.

sometimes even the seemingly most elementary subject can be complex to decipher...maybe it's just me.

 

[fattdad]

Classification of soil fines based on percentages is not consistent with ASTM. ASTM defines the distinction between "lean clay", "fat clay", "silt" and "elastic silt" soley on Atterberg limits. If you were to take a bottle of talcum powder and submit it to a soil laboratory, you'd find it to be a silt. If you ran a hydrometer and did the USDA it may classify as a clay. Why? Because one is based on behavior and the other on grain size. If you were to take a bunch of silt-sized mica flakes it would classify ASTM as a non-plastic silt and the USDA would agree (well there would not be any reference to plasticity). If you were to add 10 percent bentonite, the soil would likely classify as a fat clay, even though there is only 10 percent clay-sized material. ASTM and USDA would not agree.

Which is right? For engineering, I'd agree with the ASTM - behavior is more important for engineering projects. For soil moisture charactistics pertaining to capillarity and plant growth, USDA may be better. Mixing the two (e.g., Burmiester) never made sense to me. I don't like the terms, "trace silt" or "trace clay" - how the heck do you figure that out and what difference does it make - just do a wash 200 and an Atterberg limit and follow the ASTM.

Just a stubborn middle-age engineer, I guess. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

fattdad - one day, I am going to come to the eastern seaboard (I think that is where you are) and we are going to have a nice discussion over many things over many a single malts! - or you can come out to Bali!!
- sorry - I don't mean to highjack the thread:
I am not a big fan of ASTM. In many ways, I think that they complicate descriptions too much - and when you get down to it, their descriptions rely "heavily" on percentages of the constituent parts too - they just couch the wording in a different way.
In thinking that only the ASTM way "works" one sells out many pioneering geotechnical firms - they didn't use the USCS; used the "percentage" way were involved in building very very many very very important projects. I started work for Geocon in Canada (a long established firm) in 1977 and am familiar with almost all of their early reports (1954 to 1977) from before joining them. They never used "elastic silt", "fat clay" or any of the other terms bandied about by ASTM. Still, their history of major projects in the mining, pulp and paper, steel mill, ports and wharves for the St. Lawrence Seaway, oil refineries (Sarnia, for example) were built with success. Most of Golder's founders were indoctrinated at Geocon before starting Golders and in reports that I saw of theirs when I was with them and subsequently, again, no mention in the logs of their reports the ASTM terminology. Both firms and many others lived with clayey silt, trace sand; silty clay; etc. See attached page from Soderman and Quigley as found in the Canadian Geotechnical Journal (CGS - don't be mad!) - notice that they use clayey silt. This is found, too, in the original USCS charts (as found in Lambe and Whitman among others). See also the Canadian Guide to the Field Description of soils - they also indicate the use of clayey silt (a term not found in ASTM). Review the borehole logs and descriptions form the Canadian Geotechnical Journal articles in the 1960s, 1970s; see the example borehole logs in Fang's Foundation Engineering Handbook. I admit that with the younger set "growing up with ASTM" there is a shift towards the ASTM nomenclature (hell, I never even read an ASTM testing spec for 20 years after I started - we used Lambe's Soil Testing book for our testing). Still, we must give credit to the developers of geotechnical engineering. They did fine without ASTM.
One argues that ASTM is a "behavoural" classification system - if so, then why so much reliance on %ages of constituents as I indicated above. See Fig 1a in D2487 - you need to know 30%, 15%, 50%. Say I have a soil that is fine grained. Less than 15% is "plus #200 sieve". I decide to do two Atterbergs on the same sample. One test gives LL = 51; the also gives LL = 49. The plasticity index of the first is 23, and the second is 21. By ASTM the first is to be called a Fat Clay. The second a Silt. Now, to be honest, these two terms, to me, are diametrically opposite, yet they are the same sample. What is the behavoural difference of the two? I couldn't say. It is similar as what is the "line you draw in the sand" to apply the consistency descriptor - or the relative density descriptor. Anyone venture a guess on that? And how do you classify a varved clay? or is it a varved fat clay? or a varved silt? or a varved ???????
The bottom line, and I think that fattdad, I, and others will agree, is that one must have the experience and "feel" of knowing the material at which you are looking, touching (tactile) and smelling to have the right appreciation of whether a material is one way or the other. A common descriptor system is useful in data-base work but as I pointed out in my little example above, very slight differences in the Atterberg limits may very well slant one's view immensely.

 

[fattdad]

BigH - Here! Here! There is truely a practical component to this whole process. Unfortunatly I too started this business many years (and scotches) ago. My first mentors were from Golder, we were in Seattle. We used the USCS with all the terms trace, little, some, and "and". To me it was second nature and we got fairly consistent. Where I have a problem is when folks try using both together in some hybrid-hybrid. If I read a report that says they used ASTM, I think the logs should be ASTM. Otherwise, what do I know? I think alot of this is also from folks reviewing reports from other areas.

I guess to me it's a matter of being consistent. To the original post, I would repeat. If you are using ASTM then anything non-plastic cannot have the word "clay" in the name.

Otherwise, I think I'm with BigH and others.

Enjoy your 4th (i.e., if you're in USA).

f-d

¡papá gordo ain’t no madre flaca!

 

[howardoark]

I don't really care what it's called, but I do want to know what the Atterberg Limits are. Very low plasticity silts scare me (as do very high plasticity clays - and cemented sands (don't get me started on cemented sands)). Liquid limits of 49 to 51 with PIs of 19 to 23 I can live with.