This is the black pipe with the holes in it? Some the newer stuff is covered with a fiberglass felt. They say you don't need anything with this. I put in a 8" black pipe for my leach field at the weekend place, just covered with dirt, no problems. Remember the holes go down. Sometimes there is a like going down the pipe, this line should be pointing to the sky.
The old way was to fill the trench with rock, lay the pipe, cover with rock, then cover with hay, then fill with dirt. Whether or not you need to do this I would guess depends on your soil. Sand and you are probably ok, heavy clay maybe not.
Good Luck,
Rob