I can tell you a handy-dandy easy way how to keep your grade, when you're digging your old tile out keep the bottom half to one third of the tile in place after you've dug your ditches far as you want to go get down in the ditch with your dirt and pop the bottom part of the old existing out that will keep your great going and you don't have to do hardly any work with your dirt shovel
I can show you old flat bottom tile that are working fine. My guess is that the outlet was in bad shape. If you run onto concrete tile,replace it. Finding clay tile is like finding money!