I also built two walls out of rubble stones.
60cm is not really much.
Most of the stones are about 40cm high.
The really big heavy ones in the photos are 50-60cm. For that, it’s enough to just place one of them each.
For me, these stones either came from the excavations of construction pits here in the area and nobody wanted them, or sometimes such stones appear in agriculture, and the farmers throw the stones to the edge of the forest.
If you don’t have any of that, then quarries have plenty of such rubble stones, though they cost money. Best to have them delivered, and if there’s already an excavator on the property, have them placed as well.
I transported my stones to the spot with a tractor and rolled them by hand the last few centimeters to meters (it would have been nicer with two people, it’s easy to strain yourself lifting one). There are also small mobile hydraulic cranes (like a larger car jack), I tried that too, but the stone was too heavy and the crane lifted itself. So I had to roll it by hand again.
Gravel bed underneath (frost depth) and wide enough so that the stones fit on it at depth.
Compact the gravel and place the stones loosely into a small layer of chippings (like paving stones).
Backfill the stones with gravel or a gravel-soil mixture and tamp it down firmly with a tamper.
If necessary, a drainage is needed. For that, lay an appropriate high-quality drainage pipe behind the row of stones and then cover it with the gravel/soil mixture.
At the end of the drainage pipe, you need a soakaway pit.
With this low height and the fact that everything is loose (without concrete) and lies in a lot of gravel, I personally would skip a drainage pipe, because I consider the entire gravel base to be one big soakaway pit.
