Concrete terrace slabs on a gravel bed with or without edge restraint

Hello,

we have laid 5cm thick concrete slabs on a gravel-sand mixture, dug a little deeper at the edges and made a concrete wedge that goes up about 1.5cm on the concrete slab, and then soil & turf on top of that. We have been doing it like this for 3 years and the slabs have not "moved," and the grass grows perfectly there (despite only 3.5cm of soil/turf above the wedge).
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(and before anyone says it: Yes, I need to clean the joints again )

Do 6cm curbstones/edging stones have a height of 20cm and we made sure that we still had 5cm of coverage with soil above the concrete wedge. Our landscaping guy, who also wasn’t the best, even suggested using regular rectangular paving stones with a concrete wedge as edging as an alternative to the 20cm edging stone. But I didn’t really trust that idea because I couldn’t imagine how to make a proper concrete wedge for an 8cm or 10cm paving stone that still has enough coverage for grass to grow.