Puni is right, that plan was beyond ridiculously stupid. The walkers were contained within the quarry so there was no goddam reason whatsoever to let them all out. What they should have done, since they're so good at building walls, is to build a new wall a little further up from where that semi was about to fall.
However, their incredible stupidity led to a cool action packed episode and more cool episodes to come. The show sucks when nothing walker-related happens, we saw that when they were at Herschel's farm twiddling their thumbs and talking about feelings and stuff.
RIP Rusty Griswold