I once had a load of prepared sand on my trailer for ice removal purposes when I lived in the country. Since it was salty sand, I first put a sheet of plastic on trailer. In going home I went up a road at about 10 degrees slope. Then I turned onto a side road at no more than 20 degrees lope, probably 15. Getting home I was much amazed to see the trailer empty. Going back there was a neat pile of sand on the plastic in the middle of the road at the steeper slope. I suggest you do a similar test.
OG, very nice example. I also consider the value to be zero. For example, if we have a plastic moisture barrier sheet underneath the floor slab, I recommended no consider contribution of lateral resistant from the slab friction with the soils.
To which I'll ask, what's the end game? I mean If I was some researcher and claimed to know the answer was 8 degrees, what would you then do with the number?