unit weight of "silt"
unit weight of "silt"
(OP)
for unit weight of "silt" used in calculating stability of a dam or floodwall - anybody care to submit a guess in lbs/cu ft?
silt defined as "the sediment deposited by fluvial action against the dam, consisting primarily of loose deposits of sands, gravels and cobbles"
silt defined as "the sediment deposited by fluvial action against the dam, consisting primarily of loose deposits of sands, gravels and cobbles"





RE: unit weight of "silt"
RE: unit weight of "silt"
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RE: unit weight of "silt"
Some folks have commented on lateral earth pressures of silt (i.e., Ka or Ko). First, the definition of silt that you've presented is nothing I've ever seen. Using words like "sand, gravel and cobbles" to define silt is oxymoronic! Silt in the engineering business is defined by size (i.e., passing the No. 200 seive) and activity (i.e., Liquid Limit and Plasticity Index). There are silts that are essentially rock flour (e.g., in glacial terrain) and behaive as cohesionless and frictional materials. There are also "silts" that have liquid limits of 40 and borderline on being a fat clay.
I'd assume for an ML with trivial sand content, a friction angle of 20 to 22 degrees would be a starting point. That would result in a Ka value of 0.45 and a Ko value of 0.63.
Hope this helps.
f-d
¡papá gordo ain't no madre flaca!
RE: unit weight of "silt"
I agree with the term "silt" being used loosely - that is why I defined it as I did. I am reviewing some calculations and questioning the value of unit weight that was used for the "silt". The material in question (at least the visible portion of it that I have seen) is sand, gravel and cobbles although I'm sure there will be a small silt fraction. Below the armor layer, there could be deposits of more fine grained material including some silt, but that has not been investigated or sampled.
Regarding moisture content, I would assume nearly saturated as this is located in a river bed.
RE: unit weight of "silt"
It is interesting to learn that "There are also "silts" that have liquid limits of 40 and borderline on being a fat clay."
Is that a silty clay, or cohesive silt (is there a such thing)? Can you elebrate it and provide more info. Thanks.
RE: unit weight of "silt"
(Just to avoid confusion, if you come from the USDA tradition of soil judging where soils are classfied by weight percent, then silty clay has a different meaning, which I'm not going to further reference.)
The USC and the ASTM soil classification methods define "fine-grained" soils (i.e., silt and clay soils) as those soils with greater than 50 percent of the soil grains finer than the No. 200 sieve (i.e., 0.074 mm). Anything coarser than 0.074 mm is a sand (well, or a gravel).
The USC and ASTM soil classification methods use behavoir to classify silt and clay. There is no consideration for grain size in classifying silt or clay. Rather, silt and lean clay have liquid limits less than 50 and elastic silt and fat clay have liquid limits greater than 50. The distinction between silt and lean clay is determined by the "A" line. If the plasticy index is greater than the value 0.73(LL-20) than it's a lean clay if it's less than it's a silt. The distinction between elastic silt and fat clay is also determined by the "A" line and if the PI is greater than 0.73(LL-20) it's a fat clay and if it's less it's an elastic silt. So, there is not that much real estate between a silt with a LL of 49 and a PI of 21 and a fat clay with a LL of 51 and a PI or 24.
Confused? A picture is worth a lot of words, eh? Please note the small area for CL-ML soils (i.e., clayey silt) - a widely misclassified soil type!
http:
f-d
¡papá gordo ain't no madre flaca!
RE: unit weight of "silt"
Thank you again. I am not a practing Geo, just curios on the statement you made earlier. I am confused at this moment, but will take time to understand all these.
RE: unit weight of "silt"
Another option is to run a hydrometer to determine if the grain size classifies as silt (smaller than sand, bigger than clay) for the standard you are using.
Or you could just go with the layman's definitions, such as these from Merriam-Webster Online:
1 : loose sedimentary material with rock particles usually 1⁄20 millimeter or less in diameter; also : soil containing 80 percent or more of such silt and less than 12 percent of clay
2 : a deposit of sediment (as by a river)
RE: unit weight of "silt"
RE: unit weight of "silt"
RE: unit weight of "silt"
www.PeirceEngineering.com
RE: unit weight of "silt"
RE: unit weight of "silt"
If it's closer to Ko (which I doubt because of the compressibility of the silt), Ko would be 0.58. The total would then be 91 lb/ft^3, a little bit more.
85 sounds pretty reasonable, but I'd love to see "equivalent fluid pressure" dropped from the lexicon, since it's not really fluid pressure and that term sometimes confuses nongeotechs, especially when there is friction on the wall, controlled by EFFECTIVE stress. ("So, tell me, how come it's so different above the water table, and why does a fluid apply a vertical force on a vertical wall?" "Well you see, the pore-water pressure gets subtracted...")
RE: unit weight of "silt"
www.PeirceEngineering.com
RE: unit weight of "silt"
to dgillette - I don't quite follow your force determination. Unless the face of the embankment is impermeable the horizontal earth pressure acting on the embankment owing to the "silt" (whatever that means) would not inclued hydrostatic pressure. I'm presuming that the embankment is under effective stress also and has a phreatic surface just as the "silt" does.
f-d
¡papá gordo ain't no madre flaca!
RE: unit weight of "silt"
RE: unit weight of "silt"
RE: unit weight of "silt"
f-d
¡papá gordo ain't no madre flaca!
RE: unit weight of "silt"
www.PeirceEngineering.com