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Damp dirt under great, sudden, pressure, acts like liquid?

  1. Jul 20, 2013 #1
    I was digging a trench next to a building today so rain water would not back up to the buildings wall. Most of the material was soil but there were some large chunks of concrete. The largest chunk of concrete that was too heavy to move was pryed up and supported on the left and right sides with smaller stones as well as being supported by the ground. The idea was to beat the concrete in the middle with a sledge hammer and break it into smaller pieces. About a half to an inch layer of damp dirt lay on the top of the concrete. I swung a 20 pound sledge hammer with good effort and did little to the concrete but I compressed the dirt to such an extent that the dirt seemed to turn into a liquid and splattered with great velocity in a narrow fan shaped jets whose velocity was mostly perpendicular to the sledge velocity just before impact. After the shock of being sprayed by high velocity dirt wore off, subsequent blows to the concrete broke it up.

    Was the damp dirt changed into a liquid like material by the hammer blow? If the dirt were dry I don't think the same effect would have occurred (unless under much higher pressures?)

    Thanks for any help!
     
  2. jcsd
  3. Jul 21, 2013 #2

    mfb

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    That could be an example of shear thinning. A liquid with solid elements in it is a great way to get that effect.
     
  4. Jul 23, 2013 #3
    It's also an example of why buildings collapse when they're based on dirt, and there's an earthquake. Dry dirt does the same thing - but in your situation dry dirt wouldn't have held its shape for long enough to get hit by a hammer.

    http://en.wikipedia.org/wiki/Thixotropy
     
  5. Jul 23, 2013 #4
    When I was a young man, I took a piece of equipment out to a land seismic crew using vibroseis trucks.
    When the sweep frequency passes the modulus of elasticity of the weathering layer
    of the soil, the ground looked like it went liquid.
    A ripple formed and shook trees down a fence row as far as I could see.
    I later wrote a patent application for using similar constructive interference waves
    for clearing land mines. My company published rather than patented the idea.
    My point is that at the right frequency the ground does in fact go into a somewhat
    liquid state.
     
  6. Jul 24, 2013 #5
    I have googled "soil liquefaction" and the required conditions look similar to my situation. Simple physics but I'm still trying to wrap my head around it.

    Thanks to all for the replies!
     
  7. Jul 26, 2013 #6
    You can make a great demo of this with corn starch.... very effective physical example.
     
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