Sorry this is so long.
Ideally the design would look at the total expansion and contraction from the hottest to oldest temperature rance and then take the length of the drive and make sure it is divided into adequate segments the harness the flex.
Having designed been involved with design of all sorts of roads, parking areas and airports areas with asphalt and concrete surfacing I've seen the god and bad.Not knowing the exact grade (steepness) of the drive there are at least two big forces working on the drive.Loading (dead load (weight of slab) and live load (weight of vehicles)) and thermal expansion. Concrete expands 0.0000055 per linear inch of concrete per degree F temperature change. This totals about 0.66 inch of movement per 100 feet with a 100° F temp swing from summer to winter. Thus 600' will try to move nearly 4".
If the pours were made in 33' lengths with transverse expansion joints it would still create problems. Each time the slabs expand and contract (especially on a slope) they will move down hill every year. If the original slab had no transverse e.j.'s it would be much like standing a wet noodle on end, it would move about as it chooses..
If the structural section is adequate, not just the gravel, but the underlying native material as well, I have a concept that you may talk with the contractor.
Think Bulldozer tracks. Segmented ribbon with cleats the dig into the earth.
Cut the 600' length drive into 5 100' lengths. Each cut will consist of two cuts 12" apart. Remove that concrete and trench to at least 18" below and if frost line is deeper, at least that depth.
Each trench needs to be at least as wide as the drive. Place 2 horizontal rebars and 8 or 10 vertical in the trench attach 1/4" styrofoam to the vertical faces of cut surfaces of the slab and fill with good structural concrete and finish the top. Once set scratch down the styrofoam and caulk with a top quality elastomeric caulk.
Beside the thermal and grvity, there is the resultant forces of tire rotation uphill and braking downhill that is pushing (creeping) the concrete down hill.
The above will not work is the navive soils are yielding (wet, clay or high organic. It needs a good road bed material of high "R" value.
Again not seeing or knowing the soil type I can only imagine.
For a quick test sift your native soils through a piece of window screen (fine screen) take that dust and mix in a bowl with some water until it's like putty. Put a 1/4 or half a teaspoon in your hand and roll it into a tube shape and when it gets 2 to 3" in length see if you can pick it up by one end or the middle.
https://engineeringtraining.tpub.com/14070/css/Roll-Or-Thread-Test-394.htmI don't want to advise, nor cause you wasteful spending. The right guy on the grade will save you.
fwiw, if mine I'd look at the worse area and try something like I explained above. If it is widespread (entire length) it is definitely a design issue and up to you for approach to corral it in.
