The first test that we ran using marbles was the of a layer of 100 marbles below the column in F-65 sand. We ran this test twice a weekend apart since we suspect that the rate in which this particular sand settles is rather slow, and that different degrees of settledness effect our measurements. Below is a graph that depicts the two times the sand was run (trial 1 and trial 2) along with 3 different models, of what we might expect the K values to have been, from MODFLOW.
The model with marbles was meant to replicate the conditions of our experiment exactly. The model with a raised boundary is a comparison of having a layer of marbles with sand below or having the bottom of the glass container right below the column. The model without marbles is just the F-65 sand run through MODFLOW with no obstructions.
The first thing we noticed is that the two times we ran this experiment we got graphs with similar shapes, but allowing the sand to settle brought the average K value down quite a bit. The models indicated that there is very little effect on the measured K value unless you are right above the layer of marbles or glass bottom (boundary) of the container. The second time we ran this experiment was more similar to the models than the first time as it shows a drop right where the models showed a drop at 0.15 meters (right above the marble layer).
The next test we ran once with both sands (F-65 and Lake Superior). This experiment involved a layer of marbles in which the column was going through. There were a total of 97 marbles in this layer, 7 of which we had placed within the column. Below is a depiction of our experimental data vs our models at this point.
The two models show the same shape and are at different K values as the two sands have different K values. The accuracy of our model is in question and we are working to develop the most accurate one we can. The two experimental runs showed some what different shapes, but both did indicate an initial drop in K value, a leveling off and an increased value at approximately 0.13meters (more noticeable in the Lake Superior sand).
We plan on running some more different marble set ups as well as perfecting our means of modeling what should happen.