It seems to me that if the quarry was keeping them safe, they should do everything they can to keep using the quarry. Your telling me they couldn't reinforce the barriers and build a new barrier further behind the truck. Or even knock down the rest of the cliff road that collapsed making it impassable. If it's a quarry, then there's probably bulldozers and digging equipment around. Use those to build barriers and then start pushing dirt into the quarry to burry them, although noise might be a problem. It also appears to me that someone put those trucks there on purpose in the first place. Who? The Wolves I'm guessing....
I was thinking the exact same thing... That quarry was an awesome zombie blocker. I would have built a wall around it and made it like a roach motel... zombies check in but they don't check out. That thing about hydration sounds logical too. If they don't drink water, they would eventually whither to nothing and die anyway. Zombies aren't immortal... sure they are easy to kill but they are still faced with the same limitations that humans are. If they don't eat, they will die on their own! Just because the virus reanimates dead tissue, doesn't mean that the tissue doesn't require the basic nutrients to continue to function.