huge drops around 0.075mm on grading curves

[Tatsu]

Greetings:

I have seen huge drops (e.g.40%) on grading curves around 0.075mm, where sieving test results meet hydrometer test results. The curves are obtained with ASTM standard. I believe that the drops are caused by some sort of miscaluculation, but I could not find how to correct them. If you have experienced such odd curves, please let me how to make correct them.

Thank you in advance.

Tatsu

 

[Ron]

Minor discontinuities do occur in the plotting of the respective results, but you have described a large variance. This is likely due to miscalculation or an improperly run hydrometer test.

 

[iandig]

Based on the methods from the BS, there is a different procedure used to wash and prepare the sample for a sieve test (i.e. the part greater than 0.075mm) and the sample used in the hydrometer test. With a normal grading the sample is first dried to constant mass, then washed to remove the fine particles. The act of drying can cause the clay fines to bond together and/or change shape and when they are washed they do not readily break down, even when using sodium hexametaphosphate which is supposed to floculate the clay particles. This can result in that some of the material retained on the sieves would pass through the finest if it had not been dried before washing. In conjunction with this, for the hydrometer the sample is prepared from natural, and an identical seperate portion is dried and used to calcualte what the dry mass would have been. This means that in the majority of cases, all the clay particles are able to easily floculate, giving a finer grading.
The result of this is that there can be a discrepency between the percentage passing for the grading using the sieves for the upper part, and the percentage found for the hydrometer.
To get around this, there are a couple of options.
1. A quick, yet not entirley accurate method is to use the percentage passing the 75µm from the grading, in the calcualtion of the particle size for the hydrometer.
2. A better option is to retest the grading, and this time wash the sample from natural with an identical portion used to calculate what the dry mass would have been, had the sample undergone the drying before washing. This should mean that any cementing of the particles will have been prevented.
Both of the above assume that the sample used for the grading and the hydrometer are representitive of the entire sample. It is also not unknown for the sample used in the hydrometer to be 'selected' for the test, which can often produce a sample which is not representitive of all the size fractions within the sample. In this instance the option 1 above would give a better assessment, as although the Hydro will give an accuarte breakdown of the particles passing the 75µm, it needs to be 'matched up' with the overall grading.
Anther thing to consider, have you checked the particle density/sepcific gravity of the soil? Does the calcualtion of the dry mass used in the test make sense?
As stated in your thread, the percentage drops after the 75µm, and to me this would indicate that the sample used in the hydro, was either not fully washed (i.e. once they had sufficient fines for the test, they stopped, which can result in only the finest material passing, with the coarser silts still in the original test portion) or the portion used for the hydro was not representitive.

 

[hstanger]

I have seen similar results on Tropical Residual soils where the dispersing agent was not strong enough to completely break the bonding between the particles. However, the abrupt change in the grading curve did not happen at 75 microns, but lower at about 30-40 microns.
A very good indicator as to whether a dispersant is working is the presence of a layer of clear water at the upper part of the hydrometer cylinder.
If this is the case then your dispersant is not working. Also check that the technician is adding Sodium Carbonate to the solution. Another common mistake is to use the same concentrations of Hexametaphosphate as the wash sieving test, this will also produce strange results.

 

[Tatsu]

Thank you for your replies.

Actually, I have had this issue on Tropical Residual soils. 'not enough dispersion' makes a sense..small particles become a larger aggregate and sink faster than they are supposed to....

Tatsu