You can place soil over the roots of trees with no adverse effect. Certainly better than chopping them up with a mower blade. What you don't want to do is install a lot of heavy, clay soil. Both water and oxygen are necessary for a tree to have healthy roots. You can build a beautiful stone wall on the down hill side, backfill it with good, sandy, loamy soil that has plenty of air in it. Generally grass and shade don't go together and your soil has both eroded away from the lack of sod and the roots have grown healthier in size.

Trees don't really care about the weight of something on the soil around them as much as they care about the surface being modified or compacted in such a manner that air and water penetration is limited. Parking a car under a tree repeatedly for example will compact the soil to the point it might as well be asphalt.

On the other hand sometimes it is necessary to raise the level of soil around a tree three feet or more. To save the tree, place large rock, 6" or larger around the root zone, build a stone tree well to keep the soil off the trunk of the tree, install some 2" or 4" pipe into this layer of stone so that air and water can reach the original grade. Install filter fabric over the stone and then add topsoil to the desired new height. By doing this you are still allowing air and water to reach the original soil level. This is not always 100
% successful but if properly installed and maintained good success is usually occurs.