Dear Ronald,

field observationof surface cracks on the topsoil and observation in soil pit of slickensides or wedge-shaped soil aggregates are good indicators.

Best regards

Dear Ronald:

The recommendation of Dr Juilleret is the best. In addition, you can search for a dry Vertisol where the cracking pattern is evident. This is an interesting visual evidence for undergraduate students.

All my best

Dr. Millan

Activity describes how expansive a soil is.  It's the PI/%clay https://en.wikipedia.org/wiki/Atterberg_limits.  We used a silty sandy clay to cap a rocky ramp to stop interstitial flow, and it had an activity of less than 0.5.  Means it won't crack with it dries.  When picking out the clay, I found a 'puddle of mud' where erosion had washed the fines out of the pile of soil, and saw that the surface cracks were shallow and narrow, and it had been wetting periodically for 3 years.  So you'll want just the opposite - massive, deep cracks in the dry soil.  They will tend to be very plastic, high water holding capacity, and relatively low density (due to water content).

There is general trend that fine grained expansive soils tend to be high plasticity clays. Here are a few quick field tests that may help your students locate some suitable soils other than looking for desiccation cracking....

1.     Plasticity test - Take a small sample of soil - approximately 1 cubic inch, which about the size of a half a golf ball - and roll it into a thread.  

a.      If it is very difficult or impossible to roll into a thread, you likely have a silt of low plasticity.

b.     If you can roll a thread, and it can be done quickly and easily and quickly, then you likely have a low to moderate plastic clay.

c.      If you can roll a thread but it is difficult and the soil is very stiff, you likely have a highly plastic clay.

2.     Dilatency test - Take a small sample of your soil (again about 1 cubic inch) Shape the soil into into a cube or spehre and place it in the palm of your hand. Take your other hand and hit the side of the hand that is holding the soil sample. Repeat the hitting action several times which will impart a vibration to the soil sample.

a.      If the vibrations cause water to come to the surface of the soil (i.e. soil surface starts to shine) and the soil starts to slump it is likely a low plasticity silt.

b.     If water appears slowly and the soil retains its shape, it is likely a low plastic clay.

c.      If the water does not appear and the soil holds its shape very well, it is likely a highly plastic clay.

3.     Dry Strength – Take a dry sample of fine grained soil, if available, and try to break it.

a.      Soils that crumble very easily are likely to be low plastic silts.  

b.     A soil that takes some effort to break up is likely a low to moderately plastic clay.

c.      Those soils that are difficult to break are typically highly plastic clays.

Thank you all a lot. I'm sure this will help them. Though some have been used, there are other concepts that are practical and will help them perform their tasks faster.