That's interesting. In areas that get deep frosts, rocks of that size often seem to "grow" out of the soil, like you've already described.

My guess is that you need something that will resist the slow and steady compaction from people stepping on them as well as undermining by soil organisms, in other words, a foundation. Where I live, we also have the problem of sand that is placed on icy streets and sidewalks in the winter, some of which finds its way onto lawns, causing them to rise over the years.

I've done brick walkways in front of a couple of homes I've owned. In each case I prepared the base by digging out the extant soil (clay and silt) to a depth of several feet, replacing it with coarse gravel that I machine-packed, and capping with 1/4"- sand that I could screed to a uniform surface. Neither of these walkways have shown substantial movement in many years, and this is an environment that gets deep frosts that often cause heaving in wet, heavy soils. I think that the environment I have created under those bricks is well drained and inhospitable to most creatures except ants. It was lot of work, but I think it was worth it.