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Name: Michael P.
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: December 10, 2002


Question:
Please..if it is possible...how can I make a pot full of quick sand for a classroom demo ( to show supersaturation and change of density )....but really to show a density demonstration...I already have prepared a few liters of salt(kosher) solution and added food coloring and have had the students "stack" the lesser density solution above each other...but I would like to try the quicksand demo IF IT IS feasible...I do not know...maybe yes-maybe No...I go with what you say...thanks.



Replies:
Quick sand is entirely a mechanical phenomenon and has nothing to do with solubility of materials in water or layers of variable density. Civil engineers who specialize in soil behavior (called geotechnical engineers) refer to the total stress in soil as the pressure at a given depth caused by the soil above that point. The effective stress in the soil is the difference between the total stress and the pore pressure inside the soil mass. This concept is key to understanding quicksand.

For instance, say you had a soil with a saturated unit weight of 150 pounds per cubic foot. At a depth of one foot, the total stress in that soil mass would be 150 pounds per square foot. At two feet, the total stress is 2 times 150 psf, or 300 pounds per square foot. However, since the soil is fully saturated, the water in the soil will cause a hydrostatic pressure that will tend to force the soil grains apart somewhat. Since water weighs 62.4 pounds per cubic foot, at a depth of one foot, the "pore pressure" in the soil is 62.4 pounds per square foot. So, the "effective stress" in the soil mass will be the total stress minus the pore pressure. In our example, the effective stress is 150 psf minus 62.4 psf, which is 87.6 psf.

The reason engineers care about this is that the soil's ability to carry load or remain stable in a slope is variable and depends on the effective stress. The higher the effective stress, the more tightly the soil grains are held together, generally resulting in higher strength. A soil is said to be in a quick condition when the effective stress drops to zero.

In the quick case, the water pressure is counteracting the weight holding the soil together and the grains in the soil are essentially floating. Since the specific gravity of water is 1.0 by definition and most rock from which soil grains are derived has a specific gravity of about 2.7 (i.e., rock is 2.7 times denser than water), there is no way a normal soil can be made quick under static conditions. "Quick sand" occurs in nature when water is being forced upward under pressurized conditions. This can occur during earthquakes and over artesian springs. In this case, the pressure of the escaping water exceeds the weight of the soil and the sand grains are forced apart. The result is that the soil has no capability to support a load from the physical interaction between grains.

As we can see from this explanation, there is no "sucking" or downward force associated with quicksand. Consequently, it is literally impossible for a person to be sucked, screaming, into quicksand and disappear, leaving only a floating pith helmet, as is so popular in the movies. In fact, since humans are made mostly of water, the specific gravity of the sand/water mix is substantially higher than the human body. So, a person walking into quicksand would sink to about waist depth, then float. The screaming would be optional...

You could demonstrate this by pumping water upward through a clean sand (such as that used for swimming pool filters) at such a rate that the sand floats and roils. This could be accomplished with a bucket of loosely packed sand and a submersible pump. However, I am afraid that, much like the real thing, it probably would not be too exciting to behold.

Actually, quicksand is something of a curiosity but is not really significant from an engineering standpoint. Of more importance is when soils are turned temporarily quick from earthquakes. This is referred to as liquefaction, which is a very big deal because it can destroy buildings. Check out this site on demonstration of liquefaction at:

http://wusceel.cive.wustl.edu/ucist/Projects/Web-Controlled%20Shake%20Table /liquefaction.pdf

It may give you some ideas, although a vibrating table is necessary to perform the exact demonstration illustrated. Also, check the following site for a list of professors who are in a consortium to demonstrate earthquake engineering principles. One of them may be nearby and would probably be willing to assist you.

http://wusceel.cive.wustl.edu/ucist/

Select the Members link to get to the faculty list.

Good luck,

Andy Johnson, Ph.D., P.E.



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