Sewer lines should be installed with a pitch to them, (minimum one eighth inch per foot), pitched toward the septic field in a gravity system, but pitched toward the pump chamber in a pumped system. If the pipe gets flattened out, pushed downward, or a root pushes the pipe up to level or stretched in a certain area, it will cause the water to puddle in that area (moving water is less likely to freeze; for example, a stream versus a pond or a river versus a lake).
Once the water starts to freeze it will expand out and up. When more water comes along and backs up behind the small clog, freezes, and expands still further and clogs the pipe even more, it will eventually plug up the pipe.
In a pumped septic field, we install a weep hole in the line inside the pump chamber to let whatever effluent that does not make it up to the septic field during the pump cycle, to drain back out of the line. If the weep hole plugs up for whatever reason, the line will remain filled with effluent, and make the line more susceptible to freezing!
Usually sewer lines are installed between at least one to two feet below the ground surface. Grass and snow help to keep severe cold from driving the frost too far into the ground; normally the two combined are sufficient to keep the frost to only a few inches into the ground.
Pipes that are under walkways, and driveways should all ways be insulated in the cold climates, but we don’t usually insulate them under the lawns. Keep the snow over the sewer lines and the leach fields that are not under the drive and walkways, to help keep them from freezing!