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Can I add soil underneath a tree?
You may have noticed over time that tree roots tend to come to the surface, and stick up and get in the way of mowing with the lawn mower. In fact, they can become downright irritating for some people, and certain trees are worse about it than others. The Arizona ash that was overly planted about forty years ago, and sometimes still is, is really bad about bringing those surface roots up. As the root gets larger and protrudes out of the soil, erosion exposes even more of it. You can add soil to that type of root without hurting the tree at all. If you're only adding an inch or two to a small area, it's not a problem. If you add four or more inches, you will harm the tree, in most cases.
But, there is a variation between trees as to which ones are more susceptible. For example, a tree like a cypress can survive being underwater or under asphalt for that matter. They allow themselves to breathe in different ways in the root, so adding soil isn't as stressful to them. But an oak tree, a long-term investment, may be severely stressed by a lot of soil. Try to keep it to about an inch or two whenever you add soil underneath the entire area of the tree. If you're just putting some in small areas you can add a little bit more than that.
Keep in mind though that when roots come to the surface, they're only going to grow taller over time. So you're going to have to gradually add soil over time, or you may want to just turn that area into an ornamental bed, a ground cover bed for example, and quit trying to grow grass in that spot altogether.
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