A backfilled ditch contains four different soil conditions:

The original texture and drainage of a soil can be adversely affected in a backfilled ditch.

The use of the original soil as backfill can allow organic material from the top to be scattered throughout the ditch contributing to the growth of bacteria.

Cinders, originally from the soil near the top of the ditch, can be buried next to the pipe in the moisture zone in the bottom of the ditch and aid the growth of bacteria.

The semi-dense area above the pipe will allow rapid moisture percolation to the pipe depth and also allow better gas diffusion with the possible establishment of differential aeration cells.