What do I need to do to these drain access covers to make vehicle resistant?

In my gravel drive I have two drain access covers. They are black circular plastic rings with a concrete infill. My problem is that as vehicles drive over them the drains below are being damaged - in particular the plastic riser rings are being unsettled one from another. The covers seem to be designed not to sit into the risers but to sit on the surrounding soil - they are considerably larger diameter than the risers. Should I build a firm bed around the risers or should I make a mortar bed to support the covers which is independent of the risers? Anyone know these covers? or where I might get info? thanks, Davy