10-States Standards and TR-16 Standards applied to HDD

[ClarkBart]

Hi:

I'm just curious to hear from people about their experience with applying 10-States or TR-16 Standards to installation of sewer pipes using HDD(horizontal directional drilling). 10-States requires consistent line and grade between manholes. Most HDD work would have trouble meeting that standard especially in adverse conditions (i.e. - soil with cobbles, stones and boulders). Have you had HDD installations that meet this requirement? Do regulatory agencies offer a relaxation of the standards given the nature of the HDD process? I'd be interested in hearing about your experiences. Thanks, BC

 

[rconner]

There are many past threads on this subject e.g.

http://www.eng-tips.com/viewthread.cfm?qid=378395

. I understand this is very difficult for at normal grades.

 

[ClarkBart]

rconner: Thanks for your link. I have seen the past threads and they focus on how likely (or mostly unlikely) it is that HDD can meet the typical requirements of the standards for gravity sewer. I have several HDD contractors insisting that the Standards were developed at a time when the only choice for installation was open excavation. They feel that it is now time to have a conversation about a standard that is developed with "trenchless" installation methods in mind. The contractors, some engineers and regulators feel that there is some ability to move away from the laser straight alignment for line and grade and have constructed and gained approvals for installations that are grossly out of compliance with the standard. The feeling is that, especially with HDPE, if the pipe has a few curves in the alignment horizontal or vertical, there is no impact to long term maintenance. If there happens to be a short section with a belly the typical 8" pipe is not going to have enough reduction in capacity to make any difference to performance or long term maintenance. Personally, I think that there is reason to consider a discussion of tolerances from the laser line and grade; however, I don't think that the standard should be abandoned entirely. What are your thoughts, are the tried and true standards due for a re-examination for new technology and materials?

 

[cvg]

the standards were originally written in the 50's but they were updated in 2014. HDD has been around for quite a long time, so I would have to assume that they considered it but did not feel a need to relax standards in the latest version. As far as slope and alignment goes, it does not give any allowable tolerances, so that is up to the Agency or the Engineer to specify. For horizontal alignment, it appears there could be some wiggle room as I see the term "in general" used in that section. The code may be up to interpretation, but I would caution jumping on the HDD bandwagon for gravity sewers. Maintenance issues are definitely a concern.

33.44 Slope Between Manholes
Sewers shall be laid with uniform slope between manholes.

33.5 Alignment
In general, sewers 24 inches (600 mm) or less shall be laid with straight
alignment between manholes. Straight alignment shall be checked by
either using a laser beam or lamping.

 

[ClarkBart]

I can't say that I am jumping on the HDD gravity sewer band wagon. I believe that there needs to be a broader conversation that results in a guidance document to help contractors understand issues that engineers are concerned about and provides the benefit of others experience to those who may not be familiar with with the process. That kind of conversation may help avoid expensive failures while providing low cost solutions under the right conditions.

Some of the thoughts/concerns that I have had based on recent experiences are:

1. If we move away from laser straight line and grade, what standard should we apply to as-built drawings? The location of the pipe between manholes can no longer be predicted by measuring the location and elevation of the inlet and outlet. When does this become a problem?
2. If we allow bends in the pipe, does mandrel testing become more important to the likelihood that deflections/out of roundness becomes a problem to flow or passing maintenance equipment through it?
3. With significant changes in direction and grade, there become a greater chance that turbulent flow features develop. Will this create a larger release of corrosive and/or odorous gases to the atmosphere in the sewer? Will this increase the likelihood of manhole corrosion and odor complaints?
4. The minimum 8" pipe diameter, should that be revisited? The theory is that the smaller the diameter the more likely that HDD will be successful avoiding obstructions. Most sewer maintenance equipment can fit in much smaller pipe than it used to.

I think that we have enough experience as an industry to come up with specific concerns and guidance. When we discuss generalities, such as "maintenance concerns," if could give the impression that we are defending the standards out of an "institutional momentum" rather than fairly considering other points of view, regardless of whether that is true or not.

 

[rconner]

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

https://www.csu.org/extrx/util_dev_...ater_Line_Extension_and_Service_Standards.pdf

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

http://nepis.epa.gov/Exe/ZyNET.exe/...ckDesc=Results page&MaximumPages=1&ZyEntry=33

) 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.