Tolerance for Slope Gravity-flow pipe 1

[S06]

Dear All,

In our project specs. the slope of gravity-flow pipe is "0.5%" from manhole to manhole. However, a tolerance of 6 mm in level is mentioned and 25 mm in alignment. I am searching if this tolerance is mentioned in any standard for a while but I am not able to find. Can any one help me in this regards? Thannks

 

[robyengIT]

slope of 5 % looks high to me. For sewage, usually, 1.5 % slope is common

 

[S06]

"C. Alignment: Tolerances are to be 6 mm in level and 25 mm in line between manholes or access points unless otherwise specified. Where pipe is to be constructed in straight lines between manholes or lampholes, the length will not be accepted if a light at each manhole or lamphole can¬not be seen from adjacent manholes. "

It states this

 

[LittleInch]

Are you sure they don't mean 0.5% ?? 5% is huge for any simple gravity drain - 5m for every 100 metre length??

6mm seems rather tight to me but as you say, this is a project / company specification and they can say what they want.

If you think it is unreasonable or unable to practically achieve it then apply for a dispensation or deviation and state some other number which is reasonable / practical to achieve.

6mm on its own means very little. It should be also xmm in y mm, so maybe 6mm per 1000mm or 5000mm or something like that.

If the gravity line is buried and quite long, 6mm seems very small to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[robyengIT]

quotes :S06 (Mechanical)(OP)16 Jun 19 10:52
"C. Alignment: Tolerances are to be 6 mm in level and 25 mm in line between manholes or access points unless otherwise specified. Where pipe is to be constructed in straight lines between manholes or lampholes, the length will not be accepted if a light at each manhole or lamphole can¬not be seen from adjacent manholes. " quotes

I cannot understand. If you have a pipe ID 6 mm and 100 meters long with laying tolerance +- 2.9 mm, you can see the light (center to center) but in practice it is almost impossible. That means, as far as I understand, that for bigger diameter and said tolerance the light can always be seen !! In a similar way the tolerance in alignment
LittleInch is correct and furthermore I add that tolerance must be +- (%)

 

[amlinerrichard]

I understand this to mean the pipe can not have more than 6 mm off the .5% slope or 25 mm off the centerline from side to side of the pipe measured from a laser beam from one end to the other.

 

[BridgeSmith]

For a sewer line, the distance between manholes has limits, so the 6mm absolute value could be a useful metric.

 

[robyengIT]

now the OP became 0.5 % (magic wand ?) But for sewage (if it's for sewage, we don't know) 0.5 % is too little because not self-cleaning : all debris will rest

 

[EireChch]

0.67% is self cleaning in the UK. I also agree with amlinerrichard, you have specified a level, if it is within 6mm that is ok. However, the minimum slope should be 0.5%. If a pipe is installed with a grade flatter than 0.5%, it is rejected and needs to be replaced as it will be a maintenance issue.

 

[BridgeSmith]

The little bit of info I've seen on the subject indicated that for gravity flow sanitary sewers, the ideal slope was 1%, with a tolerance of minus 0.5% or plus 1%, giving an acceptable range of 0.5% to 2%. Any less and it won't flow; any more and the liquids flow past the solids and leave them to build up in the pipe. That's just what I read. Hopefully, someone with knowledge of the design parameters can confirm or correct my understanding.

 

[robyengIT]

HotRod10 : mainly you are correct. Values depend from pipe material : for concrete pipe (material I know quite well) the usual slope accepted is 1.5 % (as already said in my 1st reply). Of course with smoother material (i.e. PVC) can be smaller, but 0.5 % slope is a bit small for any material

 

[BridgeSmith]

That makes sense, robyengIT. I should have been clearer in what I meant be 'tolerance'. As I understood it, the 0.5% was generally not a value for design, but as an absolute minimum for installation (of PVC), i.e. acceptable for short distances as an installation tolerance limit.

 

[charliealphabravo]

It doesn't seem like OP's specs are particularly worded to specify what happens to the pipe between the manholes as long as the average pipe slope between manholes, horizontal alignment at the manhole, and pipe invert at the manhole meets the tolerances.

Also it is helpful to know the application when setting tolerances. Is it sanitary/storm sewer or water conveyance? If there are no solids then water conveyance systems have used slopes as low as 0.004 for centuries. Large pipes tolerate larger deviations than small pipe. Rough pipe is less tolerant than smooth pipe.

Most foundation are designed on 2" increments. I'd think a 1-inch alignment tolerance for buried pipe is a bit tight. Maybe the same is true of the 1/4-inch level requirement.

 

[amlinerrichard]

It would be nice to know the diameter of the pipe and what is going through the pipe.

 

[S06]

These are PVC sanitary drain pipes which will be buried under the road.

The tolerance for installation is of 6 mm in invert level and 25 mm is in horizontal alignment.


For example in 30m Manhole to Manhole pipe line, 0.5% of slope is to be maintained plus there is 6 mm tolerance in invert level of the pipe while 25 mm is in horizontal alignment.

The dia of pipe vary from ID 160 mm to 400 mm.

 

[robyengIT]

the ref Standard in Europe is EN 1610 : parag 8.5.2 says that the pipe should be laid with tolerance as per design/project (but it doesn't specify any number)

 

[charliealphabravo]

I'm gonna say that OPs tolerances are not realistic for buried 16" PVC unless you are only specifying the invert and horizontal alignment at the manhole. Having said that I'm finding DOT specs saying basically the same thing: tolerances on horizontal and vertical alignment at any point of 1/4" or 1% of inside pipe diameter whichever is largest.

 

[GeoEnvGuy]

There is standard specs in most jurisdictions which state tolerance for construction usually to say the contractor can construct to this tolerance of the design line.

My reference would be the OPSS Ontario provincial standard specifications.

Also don't get to hung up on the tolerance for your situation. The as-built condition will likely be positive flow. Then following manhole settlement or heave condition will hopefully be positive for a 0.5% grade.

 

[cvg]

if this is a private drain/sewer than you can set any tolerance you want and should make sure you have adequate slope without low spots and laid in a relatively straight line or along a mild curve as designed. However ... if the sewer is owned and maintained by a public agency (which most sewers are) than you must conform to their specifications.