how strong is 4x6 Pressure Treated wood laid flat on top of drain rock?
Where I live this is a standard way of building short retaining walls for small garden plots.  1/2&quot; or 5/8&quot; rebar at least 2 feet into the ground every 3/4 feet.  pack the gravel and if it's deep you may need longer rebar.  Large drain rock can be very hard to pound through.  Usually it's not necessary, but you can put a point on the rebar by cutting it with an angle grinder.  Pre drill the wood ties with an augur bit for a good pressure fit.