Roads need to be excavated and then built to the maximum expected future loads they might see. Simple car or pick up access requirements are a whole lot less stringent than something that might see 5-10 ton or more loads. But fwiw I've built and maintained more than a few miles of road that were fully expected to support 24,7,365 loaded 400+ ton haul trucks or much more rare heavy earth moving equipment moves in excess of 1,000 tons. And do so in all weather conditions from +90F to -50F. I also had pretty much an unlimited supply of blasted hard rock to do so. By the sounds of it your shop access road was never done correctly to begin with if there's any soft spots like that in it. Basic road building 101 is that water drainage and removal of any biological type soil (anything that will support plant growth) is fully removed. And excess water or poor/no drainage will destroy any road no matter how well built it is or what materials are used. The good part is you now know where those soft spots are and roughly how large they are. But your only choice is to completely excavate what's there and down to below what's incorrect to the native hard pan that's below any of the biological soil. If you were to do this yourself? That hard pan starting surface or if your really lucky, high rock content needs to be left completely undisturbed since due to hopefully eons of normal ground vibrations it's already at it's maximum compaction. Frankly I'd highly recommend paying pros to do the work whatever it cost.
Ries mention of using that road cloth is a good one, it's designed to allow water to only go one way (drainage) and continuously drain it out of your roads sub base. But that water needs a place to drain to or it's useless. You also CAN'T just dump loose material into any deeper hole that's been excavated. Depending on just how deep you might have to go, that sub base can be an un-crushed rock, gravel and sand mix, but it also needs to be put in and then vibratory compacted, or a bit less effective, heavy track or wheel packed in no more than 6"-8" deep layers. Then your final 6" or so of road surface is at least crushed gravel because it will bind together and compact harder than anything else. Most people tend to think roads are flat across there width, when correctly done there not, on something like multi lane interstates there either sloped a degree or two towards the best drainage, or when narrower with a center crown to each side to promote surface water drainage. And a good all weather road surface does require at least half decent maintenance. Any potholes or areas with standing water need to be kept filled or drained. And for best compaction you should have approximately 8% to absolutely no more than 9%-10% moisture content. All of the above are just general points and your exact situation for your road area could come with tougher to solve side issues of course. There are for various reasons areas that could be called almost bottomless as far as excavating to that natural hard pan or native rock. One trick for that is to excavate deep enough to maybe 4'+ and then add what they did in the old logging days that's still used sometimes even today. Simple 12"+++ diameter logs placed side by side cross ways to the direction of road travel and then back filled on top of those. I'm not sure about everywhere, but up here it's generally called a corduroy road for the obvious reason. Basically it spreads the loads out over a far larger area allowing it to support much heavier weight than without it. I've seen that type of road built through really soft areas that were then supporting 100+ ton off road logging trucks when it's done right. Much better but almost out of the question due to costs, but closely spaced vertical steel or wood piles driven into the ground will do this even better since the support then comes from the frictional drag on each pile. As long as that's above what the psi loads applied to the road surface is it will remain stable. But just like good concrete foundation work, good solid road building isn't cheap. Start cutting corners and your only throwing your money away. It's a whole lot cheaper to do it right once than to have to tear out what you've already done and then do what you should have the first time. Even more so if your planning on any future hard surfacing of that road and expect any longevity out of it.
I've worked around and had to further compact areas going by there test results, but have never personally used these,
Nuclear densitometry - Wikipedia Quite specialized and you'd need to hire a contractor with one if there's still any areas your unsure of. But tbh, with that previous delivery already showing up the problem areas it's probably not necessary. I also think but don't know for sure that these density checking machines are a bit limited in just how deep they can check and still be 100% sure of there results.
