Constructability & longevity of low slope pipes
Constructability & longevity of low slope pipes
(OP)
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?
Any experienced contractors or engineers have an opinion on the minimum constructable pipe slope for most contractors in comparison to the "calculated" minimum slopes?





RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
RE: Constructability & longevity of low slope pipes
RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Constructability & longevity of low slope pipes
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.
RE: Constructability & longevity of low slope pipes
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Constructability & longevity of low slope pipes
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:/