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Field Classification of of Soils with < 25% Fines
3

Field Classification of of Soils with < 25% Fines

Field Classification of of Soils with < 25% Fines

(OP)
We have some lab results that classify soil as SC without corresponding Atterburg limits. The sieve analysis show less than 25% passing a #200 sieve. When questioned how the classification was reached the respons was it was based on field observation. Is it reasonable or possible to discern between an SC and an SM soil with this percentage of fines by visual observation of samples during collection or is lab testing required?

Thanks,

RE: Field Classification of of Soils with < 25% Fines

According to ASTM, the answer is, "yes." ASTM D-2488 provides criteria for the visual classification of soils, which may be the criteria that they used (doubtful).  Most likely, however, they just felt "plastic" soil fines and called the soil a clayey sand v. a silty sand, which they would have assigned had they been non-plastic fines.

Just how does the naming of the soil affect your project?

f-d

¡papá gordo ain't no madre flaca!

RE: Field Classification of of Soils with < 25% Fines

one thing I try to get people to understand, if your are a carpenter and you been using 16p nails for twenty years. how do you know its a 16 p nail. Or a mechanic with 1/2 bolt.

You could say you just know whit out having to measure it. It the same with soil classification, although the 100% correct way would be to measure it, but if you been playing with that same soil for twenty years, you just know.

One contractor told me one day, he the expert, dont question his field, that what we pay him to do.

RE: Field Classification of of Soils with < 25% Fines

(OP)
To Fattdad,
  The specification references ASTM D 2487 for soil classification. For our backfill SC soils or any dual classification with SC are not allowed while SM soils are suitable.

 

RE: Field Classification of of Soils with < 25% Fines

(OP)
To Brownbagg,
     I've seen carpenters drive 1/2" bolts and mechanics use 16d nails as cotter pins and learned that experts come in many forms.
     I've also found that 'experts' who are uncomfortable with being questioned usually aren't as proficient as they claim. If the decision may cost me $ I want the measure not the opinion.


 

RE: Field Classification of of Soils with < 25% Fines

Quote:

The specification references ASTM D 2487 for soil classification. For our backfill SC soils or any dual classification with SC are not allowed while SM soils are suitable.

If the specification cites D-2487 then the appropriate testing to qualify fill materials should conform to these criteria.

As an aside, it's a bad spec.  There is likely nothing wrong with SC backfill especially if it's at the proper moisture content and it contains only 25 percent fines.

Excluding SC soils seems arbitrary.  I'd like to have some technical basis to exclude SC soils before I'd cause undue harship to the project (i.e., delay or additional expense).  If it's in your power to accept this fill, I'd accept it without reservation. First, however, I'd be yelling at the testing firm for stirring the pot in the absence of data.

f-d

p.s. to brownbag:  One thing I try to get people to understand, if the specification calls for a test - DO IT!  Any contractor that reacts unfavorably to questions has something to hide. Phooey!

¡papá gordo ain't no madre flaca!

RE: Field Classification of of Soils with < 25% Fines

(OP)
To Fattdad,
Thanks for the help. I'm told that the SC soils are excluded due to liquifaction concerns during a siesmic event.

RE: Field Classification of of Soils with < 25% Fines

??????????????? What does SC v. SM have to do with liquifaction potential? The critical void ratio is not related to Atterberg limits!  Got to wonder who's doing the tellin'.

f-d

¡papá gordo ain't no madre flaca!

RE: Field Classification of of Soils with < 25% Fines

any sort of sand or granular material is potentially liquefiable, doesn't matter whether it is contaminated with silt or clay. Maybe they are confused with quick clays vs quick sand...

http://en.wikipedia.org/wiki/Soil_liquefaction

RE: Field Classification of of Soils with < 25% Fines

Agree with F-D....<25% fines should not significantly affect liquifaction, particularly without testing...Test it.

RE: Field Classification of of Soils with < 25% Fines

If anything, the SC would provide more resistance against liquefaction than SM.

RE: Field Classification of of Soils with < 25% Fines

although the 100% correct way would be to measure it,

Mat, what part of this you dont understand

RE: Field Classification of of Soils with < 25% Fines

There is a standard chart out there that shows the "seismic behaviour" for various fines content - 5%, 15% and 35% (I believe - still on vacation) - check out Kramer's book.  It's in a lot of publications, though.

RE: Field Classification of of Soils with < 25% Fines

there is not much different from a sc a sm depending on atterberg. For anybody that actual done atterberg daily, anything under 20% passing is a diffulcult sample to run. so minus 25 and 20 is not much of windows to play with. I would run a hydrometer to seperate the silt from the clay, to determine weather its a sm or sc. But alot is done by feel. which is acceptacle. you can feel the different between a silt and clay. On a sm/sc is really sandy But most are non plastic material.

RE: Field Classification of of Soils with < 25% Fines

The SC soils will not necessarily liquefy but with the water table coming up during a seismic event, and clays are highly sensitive to moisture.  Would they not be affected by the sudden increase in moisture content.  Are the soils undisturbed or are they disturbed and placed as Fill?  

RE: Field Classification of of Soils with < 25% Fines

Could frost heave be the concern with the SC material?

RE: Field Classification of of Soils with < 25% Fines

Quote:

I would run a hydrometer to seperate the silt from the clay, to determine weather its a sm or sc.

This is incorrect advice. ASTM D-2497 (the basis of this thread) has no provisions on classifying soils based on hydrometer or percent silt or clay content.  It's amazing to me that a geotechnical engineer is unclear on the classification methods that are used in the profession and the basis for our correlation with engineering behavoir.

Soil scientists use weight percent but the ASTM does not provide for this at all.

End of rant. . .

f-d

¡papá gordo ain't no madre flaca!

RE: Field Classification of of Soils with < 25% Fines




This is incorrect advice. ASTM D-2497 (the basis of this thread) has no provisions on classifying soils based on hydrometer or percent silt or clay content.



D 2497 Man-Made Organic-Base Filament Single Yarns

what does yarn have to do with classification of soil.

RE: Field Classification of of Soils with < 25% Fines

D-2487 (typo - refer to the earlier posts).

f-d

¡papá gordo ain't no madre flaca!

RE: Field Classification of of Soils with < 25% Fines

3
brownbagg - Pun intended? tongue

RE: Field Classification of of Soils with < 25% Fines

(OP)
To Risk Taker,
       Soil will be placed as backfill at 95% modified proctor starting well below the water table up to final grade.

RE: Field Classification of of Soils with < 25% Fines

in general i wouldn't expect liquefaction potential to be high or even moderate for rhis fill material placed to 95% modified proctor compaction. natural soils would possibly be more of concern. also, as has been presented before (can't remember the author off the top of my head), fine grained soils may still possess liquefaction potential so don't rule it out just based solely on fines content...but well compacted fill pretty well knocks it out due to the reduction in void ratio. but as a geotech i always get clarification on those "bad" specs before playing cowboy on my own due to the added headaches if i simply disregard the specs (regardles of how right or wrong they are). often a simple phone call and conversation with the team can clear that up.

RE: Field Classification of of Soils with < 25% Fines

Plastic soils generally ARE NOT liquefiable due to their plasticity.  There are exceptions, with low PI and high void ratio (much looser than 95% of modified!!), although sometimes it's hard to tell liquefaction from sensitivity. Assuming there is enough fines that they dominate the behavior, water content of the minus #40 fraction has to be more than 80 - 85% of LL, and PI has to be less than 12 - 18%, according to Bray and Sancio in the September 2006 ASCE JGE.  (The ranges shown here are the "maybe" range, where testing is required.)  Idriss and Boulanger's red book looks at a little differently, but you would get to the same place by their approach.

The original "Chinese criteria" (less than 15% smaller than 5 microns, LL<35, %w> 0.9 LL) have been shown by a number of researchers not to work, in part because of the limited data set used by Wang to develop them.  Likewise, there is no fines content that can rule out liquefaction; there are a number of well-known cases where ~100% fines liquefied (Moss Landing, a loess deposit in Tajikistan, tailings, etc.).

95% of modified would be similar to or higher than what we use for large dams, where we typically require 98% of standard Proctor.  Liquefaction can be ruled out DEFINITIVELY with that kind of density!

RE: Field Classification of of Soils with < 25% Fines

right on dgillette!
(and sancio is the author i was trying to think of)

RE: Field Classification of of Soils with < 25% Fines

I agree. With that density, liquefication should not take place.  But here in Texas the clays I have came across have a possibility of shrink/swell when water is added.

RE: Field Classification of of Soils with < 25% Fines

dgillette - agree about not "liquefying" but what about loss of shear strength?  If the threshold pore pressure is exceeded, there could be a loss of shear strength as pore pressurs "build up" and hence potential shear distress/failures (not liquefaction - but a direct consequence anyway).

RE: Field Classification of of Soils with < 25% Fines

Big H - You're referring to typical clays, NOT those at 95% of modified, right?  At 95% of modified, the material probably needs 15 m or more of overburden pressure to make it be contractive.  We don't worry much about that stuff too much.

But yes, you are correct to pay attention to decrease in strength w/ cyclic loadings if they will push the peak strength.  In general, saturated clays show sensitivity, i.e., remolded strength less than peak, and will show decrease in strength with very large cyclic shear strains and buildup of excess PWP, but just loss of stiffness with smaller cyclic shear stresses that stay below the peak strength.  Sensitivity ratio can be anywhere from 1.5 to VERY high in Scandinavian and Quebec quick clays, with 2 to 4 being most common.  Idriss and Boulanger recently ~2007 published a big report on cyclic failure of clays, sensitive and otherwise.  (I'm fairly sure that it's not in the public domain, so I can't post it for you, unlike that report I promised to post for McCoy last week.)  Like their liquefaction red book, it will probably be considered the state of practice in the near future.  They show cyclic failure eventually occurring with repeated loads that flirt with the monotonic peak strength but don't quite reach it (like 95%).

Funny you should bring this topic up.  In a dynamic deformation analysis, the trick (OK, one of many tricks) is deciding how much deformation it takes to reduce the strength from peak to fully softened to remolded strength.  I was considering posting a new question to see if anybody knew of any well analyzed case histories or centrifuge research or anything else that would provide a good analog.  The one good case history I know of is the 4th Ave, Anchorage AK slide in 1964, on which I have papers by Idriss and by Stark and Contreras, and of course that is different geology, stress history, clay properties, etc. from the case I'm interested in.

Every time I turn around, this s(tuff) gets more complicated.

DRG

RE: Field Classification of of Soils with < 25% Fines

Thanks Dave - and you all have a wonderful Holiday . . .

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