How to distinguish silt from clay in the field 1

[asb2011]

When it was being excavated the observations showed that the particles (sand and gravel) were sticking together and were excavated often as lumps. Does it suggest that we have clays or could silt be the same.

I am doing moisture content test and see that the (microwave) dried sample is still as lump and can be broken between the two fingers with moderate to high finger pressure.

Can I assume at least C = 5 kPa for this materials. The soil is clayey silty gravel and it is difficult to have some pure fine materials to carry out UCS or vane testing. Thank you.

 

[Ron]

Take a small, clear bottle with a lid (like a sample jar or a small mayonnaise jar (without the mayonnaise of course )) to the field with you. Put about 200 g or so of your soil in the jar. Fill about 2/3 full with clear water. Shake rapidly for about 30 seconds, then let stand in an area where it will not be affected by vibration. The sand will sink to the bottom rapidly, followed by the silt. Clays will remain suspended for much longer. After the water clears and the material has settled in its stratified layers, you'll see the differences. Pay attention to how long the water takes to clear. If more than a few minutes, your fines are most likely clay, but you can gage the relative amount after the stratifying takes place.

Many silts have cementing qualities. This is often called "hardpan" or some similar colloquial designation. Your description is consistent with such a material.

 

[Ron]

Another way to distinguish clay and silt in the field it to check the dilation properties of the soil. Put a small pat of soil in your hand with the moisture content just slightly over the plastic limit. Cup your hand so that the soil looks shiny wet, but no latent water is on the surface. Now slowly open the cupped hand and watch the sheen disappear. With your hand open, now tap the side of your hand until the shiny appearance returns. If this happens, the fines are probably silts. Clays do no dilate quickly enough to show this sheen/no sheen/sheen return condition.

 

[asb2011]

I carried out the test and seems that fines are 1/3 clay and 2/3 silt.

Sieve analysis showed that I have 11% fines, 26% sand and 63% gravel.

Prior to sieve analysis, substantial number of cobbles and boulders were discarded and the above percentages may be almost half if the weight of the cobbles and boulders are taken into account. For liquefaction analysis, should I still say that we have 11% fine or indeed we would have 6% fine?

Of course the cobbled and boulders are not still in contact and are within the finer soil matrix.

With the stickiness discussed can I assume a C = 5 kPa for the cohesion? Thank you again.

 

[Ron]

If your sieve analysis is correct, I would assume c=0. I'm not so sure you can count on cohesion for analysis purposes. Besides, it will be variable, so better to assume low value.

 

[BigH]

You can always chew it.

 

[asb2011]

BigH

It mixed with aggregates and chewing is not possible

 

[fattdad]

read and apply the measures published in ASTM D2488.

(not to suggest the obvious that is. . . )

f-d

¡papá gordo ain’t no madre flaca!