I plan on building a garage next summer. In trying to lay out how the garage might sit, I am faced with possibly placing the slab over an existing sewer line. It is the line that runs from the tank to the leachfield. About 24 feet would be covered by the slab. The line is at least 4 feet below grade. Is this a common practice? The only thing I am terrified of is having to ever dig up that sewer line, though it is nearly new. Thoughts?