For thousands of years retaining walls were successfully constructed without more than a cursory understanding of soil compaction and they didn't have rebar or

For thousands of years retaining walls were successfully constructed without more than a cursory understanding of soil compaction and they didn't have rebar or geotextile to help them. Nobody needs to understand soil compaction if they're willing to move and expend way more material than the bare minimum in order to solve the the problem. This is true in a lot of subject areas. You don't need to understand a lot of things if you're willing to copy what is tried and true and can tolerate some inefficiency. For almost all personal and commercial projects the material is going to be cheaper than paying a real engineer to poke the soil with a calibrated poker and plugging the numbers into a spreadsheet that has some formulas.