I'm looking for a reference to determine an appropriate friction factor between a treated wood sill plate (square shape) and gravelly sand soil with no backfill restraint.
Lindeburg's Civil Engineering Review Manual, Third Edition, Table 10.4 says the friction angle for masonry on wood is 26 degrees. If f = tan(26 degrees), f ~ 0.49. I'd say friction for untreated wood on gravelly sand should be about f = 2/3 tan(35 degrees) = 0.46. For newly treated (slippery) wood, I'd probably use even less, maybe 66% x f = 0.30. That's just my guess.