Silty-Clay Visual Classification 1

[kumaran856]

Can a soil be classified Silty lean Clay(CL) by visual observation in the field. One of my friend argues that this must be only lab determined and not in the field. Is this acceptable by ASTM standard? or should this be classified as Silt and Clay.

 

[moe333]

A CL-ML has a PI between 4 and 7, and a LL between 10-28. This is pretty difficult if not impossible to estimate visually.

 

[msucog]

a visual classification is just that (but i hesitate to call it a silty clay unless it's very clayey)...depending on the project and what you're trying to do, it may need to be backed up with index testing.

 

[civilperson]

Rolling noodles of 1/8" diameter can be done in the field, with moisture estimated with Speedy meter or heated on hot plate. After a few thousand Atterburg limits done both in the laboratory and in the field, estimating can be assumed to be close, (unless exactly on the A line). I will play the guess LL and PL with anyone for ten dollars paid to the correct classification or vice versa.

 

[BigH]

Typically, one knows the strata in one's area of work and based on his experience he will know by feel if it is lean or fat. In many areas, the soils lie pretty damn close to the A line and you will get some above and some below. In visual, I usually would call it clayey silt (Toronto area - Ontario). You can play out a pat of remoulded soil, let it dry and test how hard it takes to break between the fingers (put between the first two fingers and press up the "crack" with the thumb. If very hard to break, it is clay (silty clay); if hard but breakable, it is clayey silt; if it breaks easily it is silt, likely trace clay.

 

[fattdad]

As far as I'm concerned, you cannot determine CL-ML from a visual-manual method. I agree it's very likely you can discriminate between a CL, ML, CH and CL using visual-manual methods (and local geologic context).

All this is contingent on using ASTM D-2487/2488 classifications

¡papá gordo ain’t no madre flaca!

 

[BigH]

I agree fattdad - but only Moe seems to have brought up the dual symbol CL-ML. The PI is typically so low as to be hard to tell. Main reason why in the field we used to say "silty clay to clayey silt" - unless we knew from experience (or pat test observations) that it was one way or the other.

 

[fattdad]

BigH (and to the OP): I didn't read the question correctly. From my perch on the tree, a classification "Silty Clay (CL)" doesn't comply with ASTM in the first place. That's where I should have made my initial comment. "Silty Clay (CL)" is a mixed classification between modified Bermeister and USC with an implication that the percent silt and the percent clay determines the fate of the soil's classification and the Atterberg limits determine the USC designation "CL".

I hate this mixed classification nonsense. If you want to classify on the basis of percent silt and percent clay, use the USDA triangle and grow crops (sorry to be so blunt). It just annoys me that countless geotechnical firms choose their own methods for soil classification and then harp on ASTM standards for all sorts of other tests.

Here's where the body's burried. If you take a can of baby powder, the USDA (i.e., via hydrometer testing) would likely classify it as "clay", but the ASTM would classify it as silt. If you add 10 percent bentonite, the USDA would be unaffected, but the ASTM would (likely) call it fat clay. If you add 20 percent bentonite, the modified Bermeister would call it silty clay??????? even though the USC (i.e., via Atterberg limits) would remain CH.

Who cares? If it BEHAVES as a fat clay, call it a fat clay!! The ASTM recognizes this and assignes soil descriptions based on behavior NOT grain size. That's the way it should be - need I say IMHO?

f-d

¡papá gordo ain’t no madre flaca!

 

[civilperson]

Burmister is a New Jersey, (and New York phenomena), not known very well in the rest of the geotech community. It has a few devoted advocates elsewhere.

 

[DarthSoilsGuy]

fine clayey SILTs can be very tricky. in the region i worked previously (area between mountains and sea of NC of USA), the standard operation was:

1. Field observation --- "It's a Clay"
2. Lab testing follows --- "ok, so it's a Silt."

my gut reaction is assume all "field observed clays" are silts unless they are fat clays until proven otherwise. i agree with your friend. but, maybe you work somewhere where there are a tremendous amount of CLs out there.