Maximum Slope 8" sanitary sewer 1

[sewerdesigner]

We have a project that essentially places 20 homes on a hill side. The private roads accessing these homes will be roughly 10-11%

We will be starting design of the gravity sewer system. The gravity sewer will be located close to the road, i.e. 10% grade at the surface. By code, we have to use a minimum size (by code) of 8" sewer which provides plenty of capacity for this limited number of homes.

My question is, at what grades do we have to be concerned about excessive velocities and water leaving solids, etc... We can step the design down the hill with several drop manholes, but we'd like to avoid that if we can.

Comments, suggestions?

 

[Maury]

I calculate a flow velocity of 11.6 fps for an 8" pipe at 11% slope. I don't see any problem with that. The Ten State Standards requires protection against pipe displacement if flow velocities exceed 15 fps and anchoring of the pipes when the slope is 20% or greater. I've never seen a case of solids being deposited because a pipe was to steep. Service lines are very often much steeper than the 10-11% you're talking about.

 

[sewerdesigner]

Thanks Maury.... That is what I suspected but I wanted to get another opinion.

I've heard about the water leaving the solids with pipes laid at a steep grade from several people over the last 12 years, but I've never heard another engineer express this concern. Has anybody had a problem with this? Maybe they are talking about long runs at a 45 degree angle or something?

 

[PEDARRIN2]

From the perspective of the sewer piping on the inside of the building, I have heard the same thing, but I have also heard that there is no real scientific basis for it. I personally have not seen it happen.

There is a problem of the the water flowing too slow but I do not think it can flow too fast to carry the solids. The same gravitational force acts on the solids as well as the liquids.

I am also interested to see what other people have experienced and the basis for it.

 

[JRW261]

I hardly consider myself an expert on sewer line installation. However I did study sewer treatment and transmission manuals and do remember that there can be too much grade.. I dont remember much other than that. With that in mind, ive worked in sewer televising and trenchless lining and in everything I have been witness to, many lines are unargueably beyond that number, what ever it was. None show problems other than the possible errosion of the clay pipe.

I want to say that we have yet to encounter pipe errosion in plastic pipe but for some reason I want to say I remember a storm line that had errosion problems due to grade and gravel. I dont think that will be a problem. If you are truely worried about grade issues and are still unable to find preferred grade numbers, just have a series of "drops" at each man-hole so that your grade is at an acceptable known value. The problem with this is that it is generally quite abit more expensive.

 

[semo]

I have only seen a few systems with steep grades (15% or better) and also have not seen problems with steepness of grade. I also have heard of water running away from the solids. Just never seen it happen.

My thoughts: Sewage is typically composed of 99.9% water so if the water does leave the solids in one flush, the next flush or clothes washer draining lets the water wash the solids ahead of it.

I think the problem with too steep of grades is more like JRW says, erosion. Also must keep the collection system in the ground and not sliding down the hill. The water does have a force associated with it as it travels downhill and encounters increased friction or turns at the manholes. I think that is why 10 states requires anchoring of the pipe.

Just my thoughts on it.

 

[KRSServices]

Sewerdesigner,

Yes, there can be too much grade on the sewer, particularly in low flow conditions. The example you provide is one such case.

You stated guidelines (or regulations) specify an 8" pipe, but with 20 homes, the flow, at least for the interim is going to be very low in terms of capacity. If the grade is too steep, what will occur will be a buildup of solids (over time) to the point where a blockage will manifest. I have seen this occur in areas where the grade is not uniform, such as a drop in the manhole or a dip in the pipe. The solids are usually in the form of non-disolvable debris such as paper, grease, rags, etc. Since there should be scour, at higher volumes of flow, there is usually not a problem, but because there is not enough volume of water to provide for continuous flow (it will be more of a sporadic or mini slug flow) the conditions will be conducive for the buildup of devbris. Accordingly, the municipality will then have to add that section of pipe to it's list of problematic sewers for jet rodding and/or flushing.

To prevent this, the grades can be lessened, but at a cost to the developer. This problem should be brought to the attention of the approving authority and they may want to look into it. If no development will ever occur past your pipe, they may elect reduce the pipe size. If not, they may be agreeable to the short term maintenance costs.

I don't mean to chastise, but you, as the designer should already know this. These considerations are integral to the design process and therefore should not be in question.

KRS Services

http://www.krs-services.com

 

[PEDARRIN2]

You have said you have seen this so that cannot be debated but you said you saw it whent the grade was not uniform such as a drop in a manhole or pipe having a dip.

I cannot address the former, but the latter is not a design problem but a construction problem. If the grade was uniform, then there probably would not be a problem or it would be less.

You did make a good point about the size of the pipe. Working on the inside (plumbing), I have seen a lot of blockages because designers put a 4" pipe downstream of a single toilet on a long run just because they want to. Because of the scouring or lack of it, there are problems.

The code dictates a 8" pipe. While codes are usually not negotiable, but because the engineer has the stamp and assumes responsibility, he should be able, if the situation dictates it, override the code and use a smaller pipe.