When you say a significant amount of grading was done can you be specific. Like was the area built up with soil, or scraped down?
The majority of tree roots reside in the upper layer of soil, probably within the top foot. Tree roots extend out well beyond the drip line of the canopy. It is these fine hairy tree roots that do a lot of the work, taking up water and nutrients. These fine roots require air as well as the water etc, building up or scraping down along with driving heavy machinery takes its toll on these roots ... consequently the tree suffers, no more resources going up the trunk to feed the leaves etc.
It can take trees many years to slowly decline and die. In essence, what the solution is, is to encourage new root growth as well as some TLC for the soil area around the tree. There is commercially available root hormones and stimulants you can try to treat the soil with, also watering and mulching will help. But by the sounds of it, it appears it may be getting a little late for the save.
The older and larger and more established the tree is the more likely decline and death. I wish you well, try to save, but it sounds too late to me. If possible, send some pics in.
