Cindy, It's great you didnt cover the roots. However oaks are very sensitive to root disturbance and soil compaction. Just driving a truck over the root zone can eventually kill roots. Also the swale changed the water dynamics in the root zone. Plus they hate any limestone that may have been associated with the driveway. I would suggest contacting a local arborist and get the tree on a good maintainence program of watering and fertilizing. good luck, Sandy