Sam
If you are referring to the actual runway (not the strip or boundary) the most common method is to insert white painted concrete slabs flush with the grass surface or alternatively dig out and fill with white stones and again not proud of the surface (there is a minimum size of acceptable stones by the way). The maximum spacing along the edge markings is 60m
Anything you do proud of the surface, whatever you use, must be frangible and of a low height not to be struck by a propeller. Runway edge lighting is explained in detail in this regard.
Last edited by Fl1ingfrog; 14th May 2018 at 12:55.
Reason: add info