Hi George, I agree with @john.kempf comments. We have some very heavy clay soils in the ‘prairie pothole’ region of Iowa where water does not naturally drain off the landscape. These fields are not farmable without drainage, but I have never heard of anyone backfilling with gravel. It might help with drainage near the tile but is cost prohibitive. What spacing do you have on your lateral drains? Spacing of 40 to 80 ft is typical here and the trend is towards narrower spacing and shallower placement. You might consider skipping the gravel and cutting your drain spacing in half instead if you can do it for the same money. I don’t see how the gravel would help draw water any faster from 30-40 ft away as is common here, but maybe your situation is different with the high silt content.

Regarding roots plugging the tile, we have not seen an issue here IF the tile is properly installed. If the tile has a low spot, water will accumulate there and roots can/will plug the tile. As long as the water keeps moving and the tile dries up occasionally it hasn’t been an issue here. Newer fittings where the lateral is stabbed through the side of the drainage main, rather than cutting a hole to insert the lateral, can cause problems. Dead roots can snag on the obstruction and cause plugging because the sidewall isn’t smooth.