preventing grass from showing through top soil
 

Hi, I have got some buckets of drainage drillings from a cricket ground.
its good light sandy soil, but as you can guess it has some grass in it.

If I use it in pots for plants, to what depth should I place soil on top of
it (say a mixture of john innes and course sand) so as to prevent the grass
in the drainage drillings from poking through to the surface, please?