IllustriousAd9800：
After sections of root are exposed to the air for more than a month or so they adapt to that, the surface hardens, becomes more like the trunk.
After that point, they are no longer able to handle the moisture levels of being fully underground. Burying them will rot the bark, and eventually the wood, then it spreads through the root system.
An inch or two of mulch is fine over them but wouldn’t do much more than that