soil classification
soil classification
(OP)
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.
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RE: soil classification
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
RE: soil classification
RE: soil classification
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).
RE: soil classification
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.
RE: soil classification
RE: soil classification
f-d
¡papá gordo ain’t no madre flaca!
RE: soil classification
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!
RE: soil classification
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.
RE: soil classification
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?
RE: soil classification