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Compaction of solids under weight of water

  1. Apr 22, 2014 #1
    Hi there, first time poster here but I've been following the forum for years - what an outstanding resource! Thanks to all who contribute. Hopefully one day I can add my 2 cents.

    I have a general question (more of a curiosity) that arose from an observation at work. It has sparked a debate amongst my co-workers which I am hoping to settle.

    The ultimate question is whether or not the size of a column of water has an effect on the solids which have settled at the bottom. The debate has come from observing flocs of tiny iron particles settling out of containers of water. The mixture typically settles to approx 1/4 of the initial bulk volume. For example, in a 1 L container, after settling the solids on the bottom are roughly 250 mL with 750 mL of clear water on top.

    Someone suggested that in a larger container with more of the initial mixture the weight of the extra water on top of the settled particles causes them to compact tighter, thus changing the relative volume proportions (say 1/5 solids & 4/5 water). Does this make any sense? To me it doesn't because the mud at the bottom of the ocean would be rock hard. Does the actual quantity (volume) of water you start with really have any bearing on the final outcome?

    Would the shape of the vessel (tall and narrow vs short and wide) have any effect? Why do people prefer to decant supernatant from tall and narrow vessels? Do the particles settle any faster?

    I always thought particle settling was dependent only on buoyancy, gravitational, and drag forces. I remember calculating pressure at various water depths in my education but does that pressure have any effect on the compression/compaction of the solids? What determines how close the particles get to one another or how much water stays between them? At this point is it more of a particle-particle interaction?

    We can assume that in each case, the ratio of iron to water in the bulk (mixed) solution before settling is the same.

    Any input would be greatly appreciated!
  2. jcsd
  3. Apr 22, 2014 #2


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    Just consider sediment settling to the bottom of a sea ... after a while it gets compacted enough to turn into sedimentary rock.

    So you need the extra weight from the water column, and time.
  4. Apr 22, 2014 #3


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    But is that because of the weight of water? Because of time? Because of the weight of all the other sediment layered on top? Because the interstices between the original sediment particles themselves fill up with sediment?

    Consider, instead, a bag of marbles on the sea bed. Does the pressure of all the water above cause the marbles to be crushed into a single spherical clump? No. It does not. It compresses each marble individually.
  5. Apr 22, 2014 #4
    If you are forming a floc, then the bulk density of the flock must be much lower than if the floc were compacted. What would be the thickness of the precipitate if it was fully compacted, or at least compacted with a void fraction of, say, 50%? Are there electrostatic repulsion interactions between the floc particles involved?

  6. Apr 22, 2014 #5
    It gets tricky in this particular case because the particles are actually flocs of sub micron particles. Nano particle behavior as I understand can be a whole different animal. Factors such as Zeta Potential need to be considered. For example a simple pH adjustment can change whether the flocs form and settle, or not. In general it takes several hours for the particles to settle out, and then they continue to settle tighter down for another day or two before finally reaching steady state. As far as their size when completely compacted I really have no idea, as when they are dried there is hardly anything left.

    Perhaps this needs to be addressed differently than if it were sand or soil particles, but the concept should be close. Like I say, its more so a curiosity about the potential forces due to the relative column of water above the particles.

    Thanks everyone for your input so far!
  7. Apr 22, 2014 #6
    If this is the case, then there must be repulsive forces between the particles that prevent them from settling all the way. This will not be affected by the column of water above the particles. So the fraction that fill the column after the settling is complete should not be a function of the length of the column.

  8. Apr 22, 2014 #7
    Thanks Chet,
    So how would things be different if these were ordinary solids such as sand? Does the water overhead actually compact these particles such as in the case of mud at the bottom of a body of water? Do the particles even "experience" the weight of the water above? Or is their separation solely based on the specific gravity between the particles and the fluid?
  9. Apr 22, 2014 #8
    The latter. There is, of course, a buoyant force on the particles. And each individual particle is compressed slightly by the pressure of the water, but this is typically negligible. As far as the interaction between particles is concerned, for sand or marbles, it's pretty much the same bulk density of solid as if the water was not even there. (For more extreme situations involving large overhead amounts of solid, such as in deep aquifers, you need to employ poroelasticity theory to consider the deformations of the particles and the changes in the void space between particles).

  10. Apr 23, 2014 #9


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    For an introduction to sedimentary rock formation see: http://gomyclass.com/geology10/files/lecture6/html/web_data/file43.htm
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