Thursday, June 28, 2012
More testing, more data
Another day of testing of the F-60 sand and more of the parameters have been acquired. We figured out that the reason for the differences in the k-values from the previous day was because the sand in the inner column was higher than the sand around the inner column. We tested this and found that by adding more sand into the inner column it changed the k-value. From now on we will figure out our length by how much of the inner column is in the sand by using the inside amount. This change helped us to get more accurate k-values. We then moved on and started running a few tests with the F-65 sand. This sand is consider to be more fine which will mean there will be less room for the water to move through. Due to this, we are predicting that the k-values will be lower than that of the F-60. So far in the few tests we have ran, the data looks that way. Whether there will be a statistical significance is yet to be determined!
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Thanks for the update Matt! I did want to address one part of your post that I think either was worded funny or you're slightly confused about. We do predict that with finer grain sizes we should have lower k values, but it's not a matter of the amount of space between the grains. If you picture stacking tennis balls and stacking basket balls, there is the same amount of space between the balls relative to the volume, the spaces between the basket balls are bigger, but there are fewer of them. (this isn't entirely true of you start considering edge effects) But the sand is fine enough that most of the water is flowing between the sand grains and not between the sand and the glass walls. The fraction of void space you're talking is porosity and we'll measure that for each sand once our graduated cylinders arrive. The reason having smaller sand particles would affect hydraulic conductivity is that by having smaller paths for the water to flow through, there is more resistance, more interference between the water and the sand (more friction) and this is going to make the water flow slower. In our case we have different grain size distributions between these sands too (not nice spheres), and this may result in different packing which could cause more or less space to be between the grains, which is part of why we guessed it would be lower, but aren't really sure it should be.
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