Dynamics of an Earth Ring

Before anybody gives me that look...this question is totally in the
context of a "what if" kind of hypothetical scenario.

Suppose an asteroid somehow, through some super, far future
engineering achievement has been captured into orbit around the Earth.
Now suppose we want to carve it out by detonating a series of missiles
that incrementally hollow their way into the body of such an asteroid.
The material excavated out of the body would create a thin ring system
around the Earth, as I try to illustrate he-

http://uk.geocities.com/aa_spaceagen...arth-ring.html

My main concern with such a project is one of SAFETY. I'd like to know
what a *safe* perigee (minimum) altitude would be necessary to prevent
orbital decay of ring material. I don't want any material from my
hypothetical ring system coming down toward the Earth under any
circumstances. Would the ring material be contained in a narrow plane
of fixed orbital incline, or would it scatter over time? What about
interactivity with particles trapped in the Van Allen radiation belts?
Is there any learnings from the Voyager studies (and now Cassini
studies) of Saturnian rings that could be used to predict the long
term stability of such a *hypothetical* ring system around the Earth
in the future?

Abdul Ahad