Field Classification of of Soils with < 25% Fines 3

[Matsuri]

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,

 

[fattdad]

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!

 

[bb29510]

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.

 

[Matsuri]

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.

 

[Matsuri]

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.

 

[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.

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!

 

[Matsuri]

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

 

[fattdad]

??????????????? 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!

 

[cvg]

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

 

[Ron]

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

 

[moe333]

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

 

[bb29510]

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

Mat, what part of this you dont understand

 

[BigH]

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.

 

[bb29510]

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.

 

[RiskTaker]

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?

 

[RiskTaker]

Could frost heave be the concern with the SC material?

 

[fattdad]

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!

 

[bb29510]

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.

 

[fattdad]

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

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

brownbagg - Pun intended?