So my experience is only with sandy soil > clay > bedrock or topsoil > clay bedrock but I assume actual sand or otherwise no topsoil would only make this worse but any time you have concrete above any of the above, voids will always form. Be it from rain cycles or freezing, there will always be a void under the slab in places and unevenly but mostly on the lower grade where water increases the void over time. If it's cracking or otherwise out of grade since it was poured, there really is no easy fix. They're able to do it on concrete foundations for houses because the property is expensive enough to make filling voids financially feasible or worth the effort. Take a 16 x 12 or similar back patio. It's not tied into anything even if poured against the house. It will either start to slope more towards the house or away from it based off the soil and general direction of water. You can't just jack it up either direction or fill in the voids because the process of digging up that will disrupt the underlying further. The only "real" solution if you have a water build up problem is just cutting into the concrete and making a french drain in the middle of it to save it from completely flooding or redoing it. That's just one example that comes to mind.