Bury a Retaining Wall
Covering concrete with soil will not affect the structural integrity of the concrete.  If anything, doing so will enhance structural integrity because the soil will protect the concrete from the extremes of weather.

Building a 6' high retaining wall is not something you want to DIY.  You really want the design (and probably the construction too) to be done by somebody with suitable liability insurance.

One point about covering the wall with soil, is that it allows the wall to be banked, rather than vertical.  That will mean you can have a much less substantial wall.