One of the things I learned on my recent experiences on archeological digs in the Holy Land was that not everything that is uncovered stays uncovered. Because archaeology is necessarily destructive (removing layers of civilization from the ground), one way to preserve a site for future study is to cover it. Aren Maier has a good discussion of the process here. Instead of plastic bottles, some use modern coins on top of the “geotech” tarpaulins.
By the way, for those who have opportunity to travel to the Holy Lands, you might want to consider taking a picture or two of ongoing digs at sites you visit (if they are there) because the next time around you might not be able to see it. I was just at Tell es-Safi/Gath this past summer and took a number of pictures. I haven't gone back through them but I probably took some of areas that are now covered over.