unit weight of "silt" 4

[cvg]

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"

 

[SirAl]

For granular fluvial deposits in my neck of the woods I would 'guess' 2100 +/- 100 kg/m3 (total unit density). Sorry about the metric units.

 

[PEinc]

According to Design of Gravity Dams, US Dept. of the Interior, BUREC, 1976, Page 29, Section 3-12: "Horizontal silt pressure is assumed to be equivalent to that of a fluid weighing 85 pounds per cubic foot. Vertical silt pressure is determined as if silt were a soil having a wet density of 120 pounds per cubic foot, the magnitude of pressure varying directly with depth. These values include the effects of water within the silt.

http://www.PeirceEngineering.com

 

[fattdad]

I'd use a dry density of about 100 to 105 pcf. For moist unit weight I'd consider a 15 percent moisture content or about 120 pcf.

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!

 

[cvg]

f-d
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.

 

[cntw1953]

fattdad:

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.

 

[fattdad]

The USC (unified soil classification), which is mostly as adpoted by ASTM includes four principal fine-grained soils: Silt, Elastic Silt, Lean Clay and Fat Clay. There is a fifth soil class called, "Silty Clay," but it's very rare (notwithstanding the use shown above).

(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://www.acipco.com/international/engineering-data/p8image2.gif

f-d



¡papá gordo ain’t no madre flaca!

 

[cntw1953]

fattdad:

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.

 

[TDAA]

The classification that f-d provides is based on the behavior of the fine-grained material. The fat / elastic material is considered to be more plastic. The material can take on a pretty fair amount of water before entering a liquid state (liquid limit). The plastic limit is determined where the fines become cohesive and can be molded per the test. Subtract the two for the Plasticity Index (PI) and use the A-line equation f-d provided to see whether they behave as a silt or clay.

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)

 

[cntw1953]

Wow, like going back to classroom again. Thanks, guys, my pleasure to learn something I should have picked up long time ago.

 

[BigH]

fattdad - not sure I ever saw in the USC system the use of the term elastic silt - that came, I believe, with the ASTM takeover . . .

 

[PEinc]

The silt discussions and definitions are all good but if cvg is "calculating stability of a dam or floodwall," why not use the information given in BUREC's Design of Gravity Dams?

http://www.PeirceEngineering.com

 

[cvg]

PEinc - we are taking up the issue with our geotech on the project and will see what they recommend. However, I appreciate all the interest and this has been a good discussion and learning experience for me as well.

 

[dgillette]

Say the total unit weight is 112.4 lb/ft^3, so the bouyant unit weight is 50. The dam is rigid, but the silt is compressible, so Ka might work. Assuming 25 degrees, Ka is 0.41. Total force per foot of depth per foot of length is 0.41*50 + 62.4 = 83 lb/ft^3, pretty close to 85, considering I was just pulling numbers for gamma and phi' out of my...uh...hat.

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...")

 

[PEinc]

So, just replace the term "equivalent fluid" with "triangular." I don't know why everyone worries so much about the term "equivalent fluid pressure."

http://www.PeirceEngineering.com

 

[fattdad]

to BigH - you are correct. The USC does not include the term "elastic silt." I'm sure I knew that, sigh. . .

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!

 

[dgillette]

On a gravity dam or flood wall, the face is concrete and approximately vertical. Therefore, it can be treated as a simple retaining wall, on which there is the effective pressure from the soil (H * [Gamma-t minus Gamma-w] * [Ka or Ko]) and the water pressure (H * Gamma-w).

 

[cvg]

this is a relatively small concrete diversion dam, there is no embankment being analyzed

 

[fattdad]

o.k.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

I assume that the "silt" is what has built up against the upstream face of the dam. The 85 PCF "fluid pressure" would be a reasonable number if the silt has built up to the top of the dam. 85 PCF would include the horizontal load due to water and silt. Even if the silt is not to the top of this "small" dam, you may need to analyze it as such to account for future silt build-up.

http://www.PeirceEngineering.com