If you're referring to backhoes and cars/trucks and dump trucks, 1-2" is NOT gonna do it.
What type of heavy equipemtn are you referring to??? I bury 4" pipe a minimum of 12", gravel and all, when there's sufficient down-sloping. The frost line doesn't matter as long as the water can move in this particular case. You can drive a VW Beetle over this and you won't crush it. The price is good but I'm wondering about the depth. If the contractor is burying the pipe only 6", that's a lot of $$$ assuming the work is being done in a lawn. Plez say more. I can't see what your lot looks like.