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Quick Sand Demo
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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