As little as two inches of fill dirt can kill a post oak. Instead of trenching across the roots it is much better to trench directly towards the trunk and then bore underneath and resume trenching on the far side. Do not trench closer than 10 feet from the trunk. After a tree begins to show signs of stress it is very likely to attract opportunistic diseases and insects that will further reduce the health of the tree. It is very difficult to bring an old tree back to health after the decline has become visible.
Prevention is by far the best remedy.
When a tree is grown in close proximity to other trees, such as in the typical nursery environment, the trunk never builds up a thick enough bark layer to protect the cambium from the sun. Trees with thin bark, such as young red oaks, grown in dense shade for the first few years and then suddenly exposed to the hot sun cannot develop the protective layers quick enough to prevent desiccation of this sensitive tissue. The result is a large canker wound that extends from the base of the tree to the first limbs on the side facing the hot afternoon sun. Even a tree that was grown in full sun then subsequently dug up and transplanted facing a different direction can suffer the same fate. The old time remedy was to apply white wash to the trunk which reflected enough of the heat to prevent the tree from developing a canker wound.