Been there, done that. The county specs for driveway tie-ins are minimum six inches of stabilized, compacted base topped with your choice of a) 4" of 3800 PSI reinforced concrete or 2" of hot-mix blacktop. In some instances, depending on use to be determined by the county engineer, the topping can be crushed limestone or decomposed granite.
I met with the engineer and told him I'd be happy to do that if he ripped up the entire length of the county road I was going to tie into and rebuilt it to those same specifications, his choice of topping "depending on use" which is very high and includes five different company's garbage trucks. He studied the pattern on the floor and said well, I think crushed limestone topping on that pile of caliche you got there to cover the culvert will be ok. And so it was.