What is actually under your yard
Walk the spot where the slab is going and look at what is really there. Most yards are not one uniform material. There is topsoil on top, which is full of roots and organic matter that rots and shrinks over time. Under that there is often fill dirt from when the lot was graded, and fill compacts unevenly no matter how long it has been sitting there. Somewhere in the mix there can be an old stump hole, a filled ditch, or a soft low spot that never quite dried out.
None of that shows up by looking at the surface. The ground can look flat and firm and still have a soft pocket eighteen inches down that a footstep will never find but a loaded slab will.
Why the unevenness is the problem, not the dirt itself
Concrete is strong when it is pushed together and weak when it is pulled apart or bent. A slab sitting on ground that supports it evenly stays in compression and does its job for decades. A slab sitting on ground where one section is firm and the next is soft has to span that difference on its own, the way a board bridges a gap. It can do that for a while. Eventually it cracks right at the boundary between the firm ground and the soft ground, because that is where the bending is worst.
That is why a crack running straight across a driveway or a patio, in a spot with no joint anywhere near it, is almost always a base problem rather than a concrete problem. The slab did not fail on its own. The ground under it moved first.
Stripping, compacting, and building the base back up
The fix is not complicated, it is just work that has to happen before the forms go in. We strip the topsoil and any soft or organic material out of the footprint down to firm subgrade, the undisturbed ground that will actually carry the load. Then we bring the grade back up with crusher run stone instead of whatever came out of the hole.
The stone does not go in as one thick dump. It goes in layers, and each layer gets its own compaction pass before the next one goes down. That is what gives you a base that is uniform all the way across, instead of firm in the spots that were already solid and springy in the spots that were fill. On red clay ground, which is common around here, that stone base also matters for drainage, since clay holds water instead of letting it pass through.
When bare ground looks like it would be fine
People sometimes point at a spot and say it looks solid enough, and sometimes a small section genuinely is. But we do not gamble a slab on a look. A sidewalk that only ever carries foot traffic still needs an even, compacted base under it, because the failure mode is the same even if the load is lighter. The corner cases where prep gets thinner, like a shed pad on already firm, undisturbed ground, are a site by site call, and that is exactly the kind of thing we look at in person before we quote the job.