The trick to installing a straight sewer line at any depth consist of two very important items. The first is to utilize a pipe laser and the second is spending money on quantity bedding material. We are sometimes a little more expensive than other companies but have never failed a rough inspection. The reason is that we actually over excavate just enough to install 57 rock and check pipe elevations (Via laser) 3 spots for every section. Yes it sounds like a lot of work but the laser makes it very efficient. See picture below
