Your OP was a little general (reason for my general, short response), and few specifics were provided. I'm not going to attempt to answer all the later questions but I believe performance relating to backup of gravity sewers in good pipe condition and even most straight alignment is primarily a function of the pipe size and adequate, as-constructed slope. Those factors generally result in an adequate average velocity to minimize deposition and keep the sewer reasonably cleansed. As you have reportedly read all the prior threads and linked references, you are aware however that sags, bellies or undulations have been implicated as something that by various mechanisms that can reduce effective or average velocity in sewers, and at some point result in backups, blockages and/or overflows. I would thus be quite cautious in any perception that long-term maintenance of sewers will not be necessary with new piping materials, as may have been inferred by the reference e.g. "especially with HDPE". I happened to notice that in the utility site at
they have recently revised their standard specifications with introductory notes that they, "Heavily revised section for pipe installed at slopes less than 1.04%, and also referred to a, "Completely new chapter that has been added to clarify requirements for HDPE and Trenchless Technology based on our experience and changing technology..." In scrolling to the latter chapter, one finds the requirements under the heading, "Minimum Percent of Grade The minimum grade with the use of butt fusion shall be 6 % for HDPE Wastewater Mains unless the internal beads are removed, in which case the minimum grade may be 3%. The minimum grade with the use of electrofusion couplings shall be 3% for HDPE Wastewater Mains." Assuming the weld beads can be effectively removed without damages or obstructions, this would nevertheless appear to require e.g. a 12 feet drop in invert elevation from manhole to manhole in a 400 feet stretch of gravity line, and that in many areas would appear to get one pretty deep pretty fast headed to the plant! It is perhaps ironic that some new plastic pipe materials that are promoting very flat slopes are those that have specifically linked by our EPA (see table at
) to "Grade" defects? Some technical reasons for this however are also explained in prior threads on these forums. Smaller diameters of particularly plastic pipes are not likely to be laid straighter in the real world.
That being said, I think you ask some good questions. I believe I have seen in different specifications allowable ponding or apparent sags or bellies ranging from as little as only 1/8" all the way up to 10% of the pipe ID (i.e. slightly over 3/4" e.g. for 8" pipe). As someone who has been around practical open cut and HDD pipe work now for several decades (and is generally not in favor of relaxing Engineering standards or guides, meant to protect the public and important watersheds, without good reason), for some often tough dewatering and bedding work going on down deep in the ground, the 1/8" sounded pretty restrictive to me for even open cut! In fairness to Owners who wants a pretty straight ship, I guess the perception probably is that the alignment won't improve with the passage of time. All have a good weekend.
