Verical Oak sleeper lawn edging advise please.

[LLOYD]

Hi,Im wanting to intall some lawn edging in our garden made from Oak sleepers. I plan to install them vertically, quite low, staggers height between 150mm - 300mm. My question is how deep into the ground should i sit them, i was thinking half the longest piece,i.e 200mm deep trench, 50mm gravel drainage into trench then concrete front and back to hold them in place.Is this about the right depth would you think? Or is that going overboard, i really dont want them to move at all,but there in the other hand, Oak sleepers are pricey.Any advise greatly appreciated.