Hmm. The AA and forts across the highway seem to indicate a military presence but there is almost no security between the highway and the portals. Odd.
Thanks for the link Dania. One picture on the link shows large stone blocks being removed. I'd suggest those pictures are taken in one of two quarries at: 33-40'; 36-52'.
I found this site with a quarry in this area:
http://tinyurl.com/cknaav6