soil classification 3

[ohyeh]

Based on sieve and hydrometer, the soil is classified as silt. But based on the LL and Plasticity Index, the soil is CI. Anybody has an idea where the inconsistency comes from.

 

[fattdad]

USDA classifies soils strictly on the basis of grain size, hence silt. ASTM (i.e., USC) describes soils on the basis of behavoir. It doesn't matter whether the grain size is in the silt or clay range, if it has a plasticity akin to silt - it's a silt, if it has a plasticity akin to clay - it's a clay.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[dirtsqueezer]

Doesn't L and H designate the plasticity, not grain size, ie. CL versus ML? That's what I just read today in my intro geo-engineering textbook today.

 

[BigH]

dirsqueezer - in Britain and other countries (e.g., India), they use an intermediate designation that ASTM doesn't - i.e., I meaning LL between 30 and 50. If you look in Terzaghi and Peck, they indicate that this is what Casagrande proposed in 1932. I don't know when or why ASTM and USBR dropped the intermediate level.
As fattdad indicated, it is the plasticity properties of a fine grained soil that lead to actual classification - the gradation helps in adding things like "some fine sand", or "trace sand" to the descriptive name. I typically don't do hydrometers for fine grained soils especially if more clayey - it is not that accurate due to the hydrometer "assumption" of round balls falling whereas clayey particles are platy. Still, the hydrometer has merit if trying to determine potential for expansion (i.e., activity - where you need the %clay) - although there are other tests that can be used (free swell test for example).

 

[SiteMonkey]

Hi ohyeh

I've encountered this issue before and the explanation as given by the BS5930 (Section 41.4.4.4) is:

1. the presence of silt size aggreagates of the clay minerals themselves
2. the fact that the A-line is a poor way (although most commonly accepted way) of distinguishing between clays/silts.

As mentioned in the other responses I'd rely on the CI classification as this is how the soil will behave, which is the crux of the issue.

 

[BigH]

sitemonkey: with respect to point 2, please give recommend a better way of distinguishing between silts and clays. We have to have some manner of making a distinction even though we know that there are "fuzzy" zones around any definition. It is easy for BS or whoever to say something is "a poor way" - but they always seem to fail to go that next step and give a recommendation as to the better way.

 

[fattdad]

I like the ASTM soil classification and think the A line is just fine as it is. If I were to make any refinement to the ASTM soil classification I'd include some expression on the plasticity for clayey sands and silty sands - something along the lines of fat clayey sand or lean clayey sand; silty sand or elastic silty sand.

f-d

¡papá gordo ain’t no madre flaca!

 

[Stego1]

Lab classification gives you one view of a soil's make up but there is no substitute for getting your hands dirty and poking your finger in it, moulding it into shapes and all the other fun stuff you used to do when you were a child!
Sometimes it's easy to forget how inaccurate the lab methods of classification are. Try getting 5 people to come up with the same plastic limit!

 

[dgillette]

Big H wrote "...Please recommend a better way of distinguishing between silts and clays. We have to have some manner of making a distinction even though we know that there are "fuzzy" zones around any definition."

Can a material with PI>30 act like a silt (unless we define silt by the A Line, in which case the definition ends up being circular)? There are plenty of materials, heavy on the clay minerals and having PI over 60 or even over 100 that plot below the A Line. I don't think I would describe them as silt-like, in terms of dry strength, dilatancy, undrained strength quite sensitive to small changes in %w, etc.

Is a material with LL=80 and PI=40 more like a material with LL=70 and PI=40, or one with LL=50 and PI =10?

My point here is that at high LL and high PI, the A Line is too high to distinguish between clay behavior and silt behavior, and we should not expect it to be meaningful projected out to very high LL. My first suggestion for a better way would be to put a dogleg in the A Line so it is not so high at high LL, or maybe even flatten it at PI=40 or so, where the high PI indicates the presence of a lot of clay minerals.

By the way, why do they call it "elastic silt"? It never seems to behave elastically when I kick it.

 

[BigH]

dgillette - your points are well taken. Personally, it is quite difficult at the best of times to distinguish the difference between a silty clay and a clayey silt - one just above or one just below. What we are really looking for, is it not, is an idea of whether the material is cohesive or not cohesive - and if it is - would we expect it to behave as highly plastic - a large range in which it deforms without volume change or low plastic.
It brings a question that has always begged an answer: What is the cut-off before a mixed soil is defined in terms of relative density (loose, compact(medium dense), dense) or in terms of consistency (soft, firm (medium stiff), stiff). I once saw, if I remember correctly, a "definition" that a fine grained soil with PI <5 would be judged in terms of relative density and >5 would be judged in terms of consistency. What is your take on this?
With respect to dgillette's suggestion as to modification of the "A" line - perhaps this is something for the ETJPE?? - perhaps, too, my point on behavioural PI?

 

[SiteMonkey]

BigH - I'm not suggesting there is a better way of determining the clay/silt divide. I agree the BS5930 are just putting in clauses to cover themselves without giving any other recommendations.