Constructability & longevity of low slope pipes

[CrimsonEngineer]

My question is about minimum pipe slopes for gravity sanitary sewers (or storm). I know what the jurisdictional requirements are and that slope should produce a minimum flushing velocity in pipes; However, I have doubts about the constructability & longterm performance of designing pipes at less than 0.5%. I have seen a few video inspections of low slope pipes that have dips and are half full of standing water. Ignoring the possibility of settelment; I have doubts about the ability of contractors to install a pipe at less than 0.5% without dips.

Any experienced contractors or engineers have an opinion on the minimum constructable pipe slope for most contractors in comparison to the "calculated" minimum slopes?

 

[cvg]

you are correct to be skeptical

Contractors can do it, but it requires the inspector to hold their feet to the fire. Water testing is the easiest way of checking and is quite efficient at locating the low spots. Preferably it should be done frequently during construction. Recommend doing it between each successive manhole at the very least.

 

[TerryScan]

While flatter slopes may be difficult to construct, they are most practical in my locale (coastal plain). Because the existing terrain is so flat, sewer depths would get too deep if one designed with steeper slopes and storm sewer systems would end up below the outfall channel.

Local contractors are used to the slopes we deal with and most are capable of constructing the pipes to sepecified grades (I try to keep storm above 0.10%, we typically do sanitary at minimum). The more common construction issues we have are typically related to proper bedding, backfill, and compaction. When these are deficient, pipes kink at joints, joints seperate and leak, and pipe structure is compromised. After a recent sewer project near my home, the road has "inverted speed bumps" at every lateral within months of completion.

 

[bimr]

The minimum practical slope for construction is assumed by several industry sources as being 0.08%. For a sewer with manholes 400 feet apart, that equates to about 4-inches

 

[gbam]

Bedding will be a major factor in mild sloped pipes. We typically specify a slurry bedding and "hope" that the inspectors review as cvg stated hold the contractor's feet to the fire.

 

[cvg]

slurry bedding does not guarantee it gets done right. slurry bedding can cause the pipe to float and the contractor needs to be fully aware of that fact. water in the trench also can cause settlement.

recent project did have slurry bedding up to springline. standing water was up to 18 inches deep in sections of the drain. it had an extreme combination of both low spots and high spots, which combined to create a large ponding area. Apparently they had the perfect storm of flooding in the trench, flotation caused by the slurry and poor quality control and inspection. flushing velocity was adequate for design flow, but not for nuisance flows, therefore sediment was building up. the pipe had to be removed and replaced at great cost to the contractor.

 

[gbam]

I have never encountered that situation with a slurry bedding or I s never informed of it. But in regard to the original issue, it is up to the inspectors to ensure correct installation.

 

[CrimsonEngineer]

Thanks for all your responses.

I have never worked with a slurry bedding so I will have to do some more research on that. The thought of 4" of fall over 400' seems like it would require an inordinate amount of precision/time to keep a consistent slope and avoid lowpoints.
However I might be overthinking this. I guess when designing for open channel flow, if the pipe has some dips it will still likely have capacity with a little bit of headwater.

 

[rconner]

What size and type(s) of pipes are involved?

 

[jgailla]

Slopes of 0.4% for 8" gravity sanitary PVC pipes and 0.25% for 12" to 15" pipes are routinely designed and constructed in Florida and south Georgia.
I require all pipes to be TV'd after construction. The camera will pick up any low spots since the pipe is flushed before TVing. Any dip greater than 1/2" is required to be dug up and fixed. Since this is highlighted in the specs (and stated in the the preconstruction conference) the contractor generally will take great care to get the slope correct.
I have only had one contractor have trouble with this, and that contractor had no experience and had no business being allowed on the job. In that case, there were a couple of 1" dips and the contractor ended up paying (or being assessed, since they went out of business) a pretty substantial penalty.
I use down to 0.1% for concrete storm pipes. In this case, the pipes are usually below the groundwater (or detention pond) level and are full of water. The profile may meander a bit, but they usually get the upstream and downstream inverts at the inlets/outlets pretty close. As long as the joints are tight there should not be a problem. I require (as do all the municipalities in my area and the state DOT) that the joints be wrapped in filter fabric for soil intrusion. I also started requiring TV tests for storm, following the practice of a local county. A contractor had to line several pipes on one job due to excessive cracking. On the next job with the same contractor, no remedial work was necessary, so I assume the contractor learned his lesson.

 

[beej67]

In South Florida, they don't really care what the slopes are for storm drainage pipes, since all flow is head driven anyway. I've seen quite a few surveys with storm pipes laid in at inverse slopes, but were working as intended, since everything is essentially an equalizer pipe down there. They do have the benefit of sandy soils that aren't as likely to silt up their pipes though.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

south florida has significant local issues affecting the design. Most areas do not have these same issues.

flat or reverse slopes work if there is adequate flows to periodically and frequently flush the pipe. if you get sediment deposits that are allowed to deposit in low spots and then dry out, you can get significant reduction in cross sectional area. Dried out deposits of silty/clay can be difficult to remove unless you have significant flushing flows. These areas may require future maintenance. How frequently are flushing flows required? who knows. Design and construction to maintain positive slope with adequate flushing velocities under most flows is the best approach.

 

[beej67]

south florida has significant local issues affecting the design. Most areas do not have these same issues.

You said it buddy.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[rconner]

If references would be helpful, I believe minimum gravity sewer pipeline slopes are stated in ASCE MOP No. 37 (WPCF MOP No. 37), “Design of Sanitary and Storm Sewers”, ASCE MOP No. 60, “Gravity Sanitary Sewer Design and Construction”, and also the “Ten States Standards”. I believe all of these references are fairly consistent in minimum requirements, that of course vary depending on pipe size (to develop a flow velocity that among other positive benefits is hoped to at least be somewhat “cleansing” -- e.g. see the table in Sec. 33-4 Slope for the latter that I have noticed can now be viewed online at

http://10statesstandards.com/wastewaterstandards.html).

As others have indicated minimum slope requirements are in effect often “at war” with construction cost/profit? and other issues, so I guess it is not surprising that lesser (than these minimum)slopes have sometimes been employed. Some sort of defensible standard of care should perhaps be employed with very low or undulating slopes, however, as issues even beyond basic hydraulics or initial “water in water out” could conceivably be involved (see my reply on concurrent thread

http://eng-tips.com/viewthread.cfm?qid=298065&page=1

, advising of another ASCE/EPA reference of other perhaps non-obvious implications of ponding or sag-type indications in sewers, including increased FOG build-up when oleophilic contact materials are employed etc.) As indicated in many references including the former publications, low slope can in some situations amenable to its formation also result in increased generation of objectionable sewer gases like hydrogen sulphide, and some more recent papers have indicated that even more slope and velocity may be helpful in even larger sewers, e.g. if one wants to develop sufficient interface shear to e.g. move some larger “grit” or detritus.