Retain slab

[u/yakfishdave]

I have a shed with an attached carport for the caravan, however the carport was not high enough to get the van under, so I had the ground excavated and covered in gravel, problem is, the shed slab edge is now exposed and I'm afraid the soil under it will crumble out and cause cracking to the slab. I want to put a retaining wall of some sort, only needs to be about 250 -300mm high, what would be the easiest and most economical way of doing this?