Robinson I think you have described sorting. Different types of dirt have different grain sizes. When they are mixed and wet they exhibit sorting. I should have taken time to post the pic with the first post.

Well sorted (grains of approximately all one size) materials have higher porosity than similarly sized poorly sorted materials (where smaller particles fill the gaps between larger particles). The graphic illustrates how some smaller grains can effectively fill the pores (where all water flow takes place), drastically reducing porosity and hydraulic conductivity, while only being a small fraction of the total volume of the material.




http://en.wikipedia.org/wiki/Porosity


When dry clay is mixed with other dirt the aggregate has grains of different sizes. Water flows through the pore spaces between the grains not the dirt. When the mixture gets wet the grain size of the clay increases (swells as it absorbs water)reducing the pore space volume and the smaller other dirt grains sort to clog the remaining pore spaces to form a seal. Compaction helps because it binds the grains and reduces the pore spaces.

The water in a new/renovated pond may look like it is leaking for a while as its clay/soil mix absorbs water reducing pore space and goes through sorting and sealing. Once that process finishes it should be sealed and not lose water except through evaporation (unless there is a real leak ).