Description

Water flows into soil at an uneven rate causing what the scientific community calls gravity fingers.

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The interaction is analogous to water flowing down a window pane. The explanation for these formations is the surface tension between the water and window pane (or for our case, water and soil). Before the water can flow down the window pane it must build up enough energy to overcome the tension holding it in place.

One common question to this problem is why do these fingers form even if the soil is homogenous? One would expect the water to flow down as a horizontal line passing through the soil rather than the finger-like paths. The answer lies in the surface tension of the entire finger of water, which may be several centimeters in width, as opposed to the tension existing at the micron-scale of pores between soil particles.

This causes problems in the saturation of geotechnical models because the fingers may create pockets of unsaturated sand if the surface tension between liquid and soil is too great. It is important to understand this concept when saturating your sample.

Works Cited

http://medescape.wordpress.com/2008/12/14/gravity-fingers-mathematically-explained/?referer=sphere_search http://www.sciencedaily.com/releases/2008/12/081211141934.htm

http://cee.mit.edu/node/2659