Thanks for the pic links, pats1! I've been searching for pics online instead of working today
Those will be for drainage, depending on the install contractor they would be filled with a flexible drainage "tile" (essentially a fabric covered plastic pipe with holes in it) first or the hollow would be filled with the final grade material (which from the pics looks like this is what they are doing). The black stuff is a geotextile fabric which is still permeable but they'll run the french drains to help move large amounts of rainfall quickly to the drainage around the perimeter of the field.