Hydrometer/Sieve and Atterberg Limits results disagreeing

[ahgeo11]

Hi there,

I recently completed a sieve/hydrometer and Atterberg limits test on the same sample. The sieve/hydrometer classifies the material as ML with approx. 12% clay (<0.002mm) and 44% silt while the Atterberg results in LL=27, PI=8 for a CL classification. For what it's worth, checking the remainder of the sample it does appear to roll out to 3mm so my gut is that the hydrometer may be incorrect.

My question is: do these results always have to match up meaning there was a testing or calculation error? or is it natural for these to not totally agree for some reason.

Thanks.

 

[TigerGuy]

Did you verify the specific gravity of the material? That is the first thing I check when things get crazy.

 

[geotechguy1]

It also depends on the mineralogy of the clay particles. 50% silt and 5% montmorillonite might still be CH for example.

 

[DanielSgeo]

USCS/ASTM defines fines based on Atterberg limits not particle size. Because of this, the hyd and atterbergs don’t necessarily agree.

 

[EireChch]

The only thing that decides of a soil is CLAY or SILT is Atterbergs and the A line.

 

[dik]

I didn't know that Eire... I always thought it was the particle size. Silt being a quartz like material, and clay being a feldspar material.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TDAA]

You are comparing apples and oranges. Particle sizes define what the particles are: Boulder / Cobble / Gravel / Sand / Silt / Clay. The limits are used to describe how the tested, "finer portion" of the soil behaves. USCS and AASHTO use those limits to help define a classification, based on the behavior.