My neighbor would like to put 240V, 30A subpanel in his detached garage, which is about 35 feet from the house with a paved driveway in between.

While scoping out the job, we discovered a rectangular, unused metal raceway running between the garage and the basement of the house. Apparently, someone had some forward thinking 70 years ago. The raceway is 2" x 3" at each end, and we were able to fish a 1" diameter rope through it with no problem. We'd like to run 10-gauge THWN through flexible watertight conduit and fish that through the raceway. Trouble is, we don't know for sure how deep the raceway is under the driveway. On each end, it exits at a shallow angle. Probing with a pole leads us to believe it is about 20" below the surface. The wiring inspector seems to only care about ground rods. Any thoughts on using this raceway?