Definitely slang... It's also super strange if you take it as the literally meaning as a non-native speaker, lol, ouch, a torn a**!? It has nothing to do with a tear like paper. And it has nothing to do with the body part , like a torn hamstring. It means moving quickly, in this sense, past tense (tear, turns to, tore). Also indicating that the workers moved quickly and stupidly without thinking, angering the foreman, in this context. In another context, when Ive heard it used, like you're in a race car.... "Whoohooo, I'm tearing a**! = I'm driving so fast, wow! Or sports, soccer...." How did that player get down field and score that goal like that?? "I don't know, he tore a** down the field past everyone and came out of nowhere to kick it in. = He ran extremely fast and scored the goal. Hope that helps. That was an interesting question. It wouldn't let me right the a** word in question, so I hope you understand.

The first clue to this ridiculous statement is, The « dozen idiots ». This guy is frustrated! they drove dump trucks into the tunnel, basically they were very destructive in the process of going into the tunnel. This is slang, very broken english. Tore a$$ up is a crude way saying how dumb and haphazard the idiots were in the process of driving the dump trucks.

In this context, it's a slang way of saying "to move/drive very fast." Generally speaking we'd understand it to be moving at a very high, and unsafe, speed.