poland_jack,
I actually had in mind a skid loader such as a bobcat. Preferrably one fitted with rubber tracks over the tires. With the tracks they will dig pretty well. You usually come out a bit cheaper since you can tow these generally with a 1/2 ton pickup. So for a 50x50 area with a 1ft blanket your talking 2500 ft^3 or about 95 yds^3. Or a 50 hp tractor with a good front end loader could do that job. Or you might do some testing on the material that is there now. If it has enough clay you may just be able to compact what is there now with a sheepsfoot and form a seal.
