The least amount of concrete would have been poured if the driveway slabs were flat bottomed and a consistent thickness.
We must assume this as “the worse case” since the contractor would make a higher profit.
But a knowledgeable professional would have made both lengthwise and transverse “ditches” in the subsoil before the concrete was poured to create concrete “cleats” meant to help the slab resist sideways and downhill drift. Since most cracks occur near edges it is good practice to make the concrete thicker at edges, and especially at corners. On automobile driveways it is good practice to make the concrete thicker underneath where the tires will roll.
Combine these two “practical tips” and the unseen bottom of the concrete will not be “pool table flat” but rather “lock and key” hugging the subsoil.
Ultrasonic thickness testers could be used to see if this varying concrete thickness exists. Ultrasonic testers are way cheaper today than they were in the 1980s.
A cheap IR Temperature gun could also be aimed at the driveway on either a very hot or cold day. On a hot day the thicker concrete might read a bit cooler than nearby thinner. After a cool night it might read warmer.
If Superman is flying overhead just put a Lois Lane look-a-like in a skimpy dress to wave him down for a quick X-Ray vision.
Any sub slab work is really going to be too costly and not knowing how thick is not paramount to know, only because the slab looks pretty good and not breaking into small pieces. I thought about different means to core through the slab and pin it to sub soils, etc, but again not knowing the soil types it could be a big waste of $. Easiest is to drill with a small diameter masonry bit until it breaks through and measure depth with a small dowel. Seal with caulk.
I'm with you on the table flat sand or compacted rock bedding. Flat will not provide enough shear to keep the ice (slab) from sliding off the table. the transverse cleats are likely the most economical at this point for o.p.
Brain storming, in case anyone plans construction.
1 - A series of 4" diameter holes could be drilled 24 to 30" deep and filled with concrete and a single #4 or 5 bar that is bent over and tied into the slab reinforcement. One would be effectively building a pier rather than floating docks.
2 - Condtruct 6" wide x 16" deep concrete curbs each side the length of drive with steel at least top and bottom with a few verticals and also e.j.'s every 50 feet or so. The top of curb would be flush with the top of drive slab unless water was to be ducted. After set, a 1/4" foam type e.j. on the face of each curb and pour the slab in 50' segments with e.j. staggered to align mid point between the curb e.l.'s.
