Grain Size Distribution by Hydrometer
Grain Size Distribution by Hydrometer
(OP)
The hydrometer test, according to the ASTM guidelines, says to air dry your specimen. After the test you dry and weigh the specimen, correcting the whole deal by applying a hygroscopic moisture correction factor and correcting for the additional mass of the defloc salts. Ok, that's fine. It'll work.
My question is why not dry the sample completely using a lower oven setting, use the actual dry weight so that no hygroscopic moisture correction factor and defloc correction factors are required, place the sample in a container with defloc solution and allow it to soak for several days prior to mixing, testing, etc. Does anyone see any problems with my proposed method?
Thanks for your input!
My question is why not dry the sample completely using a lower oven setting, use the actual dry weight so that no hygroscopic moisture correction factor and defloc correction factors are required, place the sample in a container with defloc solution and allow it to soak for several days prior to mixing, testing, etc. Does anyone see any problems with my proposed method?
Thanks for your input!





RE: Grain Size Distribution by Hydrometer
The same is true for Atterberg limits - we used to oven dry, then grind them up with a Waring blender. (Isn't that special!) But that method was shown to consistently result in Liquid limits that were 5 to 10 points lower than the current "don't dry it back" approach. Perhaps the hydrometer change was influenced by the Atterberg change; I don't know.
But I'd stick with the current method unless you have a very good reason to alter it...
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Grain Size Distribution by Hydrometer
Even if you did dry the sample in the oven at a lower temp (60c) you would still have hygroscopic moisture in clay. Clay has bound water that makes up part of its structure. If you cook the bound water out of the clay at a higher temp (110c) the size of the clay particle will change, hence the air-drying procedure. The deflocculant, meniscus and temperature will always have to be corrected for. I think it is time to come in to the 21st century and start using laser diffraction. I’m not very knowledgeable in soil chemistry; I hope I got it right.
David Cooper
Cooper Testing Labs, Inc.
RE: Grain Size Distribution by Hydrometer
I saw a laser diffraction system demonstrated in the mid to late 1980's. I wasn't impressed: very small sample volume, didn't deal well with "coarse" particles (i.e silts), easy to clog, and incredibly expensive!
I'm already uncomfortable with the small sample size of the current hydrometer test method; and I have a real tough time calling only 2 to 5 grams of soil "representative."
Improvement is needed, but the results must be reasonable, reproducible and "representative."
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Grain Size Distribution by Hydrometer
RE: Grain Size Distribution by Hydrometer
I have enough of a challenge in convincing clients that they should let me obtain Atterberg limits or get grain size by hydrometer for another $100. I can only imagine what they'd say if I suggested they give me another $1,000 or so to get grain size dist. by laser diffraction. As I've written in other threads, as more and more people look at geotech engineering as a commodity, and more and more geotechs are providing recommendations based only on gut feelings (without any testing whatsoever) and praying for a good outcome, it is getting extremely difficult to properly evaluate soils. That's a whole other thread though!
davecooper
I had a thought on your response and was wondering if you could help me out. If I were to air dry my specimen only and place the specimen in a defloc solution of some kind, what guarantees do I have that the cations (and maybe some anions too) that are locked in the bound water layer (double layer) would not interact, and possibly disassociate from that double layer, causing the clay particle to change size anyway? I would think it would depend on the actual makeup of the double layer, makeup of the defloc solution, exchangeable cations, temp, pH and everything else. But maybe this effect would be more pronounced if the specimen were completely oven dried and then subjected to a defloc solution. Any experts on clay chemistry or electric double layer interaction out there?! I think it's somewhat beyond me.
RE: Grain Size Distribution by Hydrometer
Good thread!
RE: Grain Size Distribution by Hydrometer
RE: Grain Size Distribution by Hydrometer
Let us know how you make out.
RE: Grain Size Distribution by Hydrometer
This will give you the wt of dry soil used in the test directly (after subtracting out 5 grams for the defloc.). Then there is no assuming the Hygroscopic MC correction.
We have run our Hydrometer tests on the -#200 fraction for years as permitted in ASTM D422 and have always had excellent proficiency test results.
Steven Grimaldi
Geotechnical Testing Services, Inc.
AASHTO and ACOE Certified Laboratory
http://home.comcast.net/~gtslabs
RE: Grain Size Distribution by Hydrometer
If the difference in results between the two methods is minor than that between two operators using the same method, then it will not make sense to be confined to one method.
RE: Grain Size Distribution by Hydrometer
So my test is still run on air dried not oven dried material.
Steve
Steven Grimaldi
Geotechnical Testing Services, Inc.
AASHTO and ACOE Certified Laboratory
http://home.comcast.net/~gtslabs
RE: Grain Size Distribution by Hydrometer