New project for work

[MikeJhn]

If you do use softwood beams I would give the ends that are going into the ground
several coats of Bitumen up to ground level , It will help stop rotting.

Not so, any timber set into earth will rot at ground level, therefore protection needs to extend above that level, heat shrink wrap can be obtained to do this or just simply bitumen, but this is prone to damage when strimming etc:

 

[Stigmorgan]

Would 50mm not be too much into a 100mm (probably actually 90mm) sleeper?
The area will be woodchipped so use of a strimmer would be minimal and by myself so that wouldn't worry me too much, I plan to paint with bitumen then use a heat gun to get it soaking into the surface as much as I can.