I think you will find that the most popular opinion is to NOT use it mixed in or layered with the clay. That sounds like a bad, bad idea. Using a foot of it on top will greatly increase the growth of grasses however. The soil around my place is pretty poor, clay and rock, but my pond dredgings grow vegetation like nobody's business. In fact, you can obviously tell where I used it to fill in low spots in the yard. Now, it is pretty common to properly build the clay levee and then use the silt on the back side which makes the dam wider and/or with a more gradual slope for mowing purposes.
