HDPE pipe elevation tolerance.

[gusongjun]

Hi, guys,

I have some deep underground HDPE pipes, DR 9 and DR 11. I am wondering who know elevation(grade) deviation for these kind of pressured pipes.

Many thanks for your kind consideration. Any reply is greatly appreciated.

Best regards,

 

[coloeng]

What is your concern with the elevation differences? If it is pressurized pipe, you may be concerned with high spots that collect air, or maybe that if the pipe is to shallow it may be within the frost zone of live loading may be a concern, or if it is too deep it may be difficult to excavate and repair if there are maintenance issues.

Without knowing what your concerns are, it is difficult to say what deviations are reasonable.

 

[gusongjun]

Thanks.

These pipes are 8" and 10". As built deviation is around 100 mm. Without any sepecial reason, I think 100 mm deviation is acceptable. Do you have any similar experience?

 

[bimr]

Usually there is no deviation on the burial depth that is specified. The minimum depth is usually specified as the frost line, but the maximum depth is not specified.

 

[rconner]

That is maybe easier said than done with that pipe, perhaps for a number of reasons!

 

[cvg]

100 mm is very sloppy. there is no reason a contractor can't do much better than that. A typical specification we use requires a tolerance of 20 mm for pressure pipe.

 

[gusongjun]

To cvg

Yes, I permitted 50 mm deviation of elevation for contractor. I don't know why they can't install pipe within the tolerance. These pipes are not big or heavy. Gee.

 

[bimr]

Bedding
Where the trench bottom soil can be cut and graded without difficulty, pressure pipe may be installed directly on the prepared trench bottom. For pressure pipe, the trench bottom may undulate, but must support the pipe smoothly and be free of ridges, hollows, and lumps. In other situations, bedding may be prepared from the excavated material if it is rock free and well broken up during excavation. The trench bottom should be relatively smooth and free of rock. When rocks, boulders, or large stones are encountered which may cause point loading on the pipe, they should be removed and the trench bottom padded with 4 to 6 inches of tamped bedding material. Bedding should consist of free-flowing material such as gravel, sand, silty sand, or clayey sand that is free of stones or hard particles larger than one-half inch.

http://www.plasticpipe.org/pdf/chapter07.pdf

The grade deviation should not matter unless you are outside the cold bending radius of the pipe.

One would assume that CVG is referring to the recommendations for location of services. In the UK, there are some recommended depths for water mains. The tolerances at the recommended depths vary from 150-300 mm. The recommended locations will be set by the local utility planning agency.

 

[cvg]

no, my recommendation is on the allowable tolerances for a contractor that is constructing the pipe. Our plans usually show a profile with required elevations and our construction specs require that it be installed to be within plus / minus 20 mm of this. This deviation is usually allowed within a 3 meter distance.

cold bending radius is only one parameter that should be evaluated. there are others including the references to air pockets and frost zones which have already been mentioned.

 

[bimr]

+/- 20 mm is a very tight tolerance, considering that gusongjun has not even said what type of application or burial depth this encompasses.

My assumption is that this is water pipe. He may be talking about sewer pipe, irrigation pipe, force main, or whatever. He has also not said what type of terrain that he working in. He might be working in Times Square or the open plains.

 

[gusongjun]

There is 300 mm bedding sand for those pipes. Bedding sand shall be comptacted to 98% SPMDD. So I think 50 mm tolerance is not tight. But the contractor intalled pipes some where is out of tolerance. The worst place is 78 mm deviation. Gee, do you guys have meet same suitation?

Thanks!

 

[bimr]

gusongjun,

Perhaps you can tell what this installation consists of:

New or existing location?

Application of Piping: water main, water sewage, gas, or whatever?

Industrial or municipal?

Installed in city street or in farmland?

 

[rconner]

I am not going to guess exactly what has happened here. However, in addition to what has already been said and obvious questions (if not here, in other jobs) about flotation of a pipe at least with high groundwater conditions in installation or otherwise, that even full of water let alone as opposed to normally empty is at least a little lighter than even pure water and is admittedly slick at least on the exterior, there can be even other issues perhaps more non-obvious to some with very lightweight pipes.
See e.g. the reported issues raised (so to speak) in the second paragraph at

http://www.dnr.state.oh.us/water/pubs/fs_div/fctsht58/tabid/4145/Default.aspx

. This reference indicates (I expect based on experience) that, “It is important to take special care in the haunch area to prevent the pipe from lifting off the subgrade and disrupting vertical alignment.”
With high levels of compaction also required by many specifications (as it appears here) for any sort of long-term, dependable support of plastic pipes (at least those with low long-term stiffness in and of themselves), it doesn’t explain exactly how this conundrum is to be avoided!

 

[gusongjun]

To bimr

The pipe is new installed in industrial area for waste water. Before installation, dewatering had been done to dry trench.

 

[bimr]

For that application, I see no reason for laying tolerances. One would assume that the Contractor is not going to install the pipe any deeper than he has too, since the extra cost for deeper burial is borne by the contractor.

The minimum depth is usually specified as the frost line, but the maximum depth is not specified.

Having said that, you must also keep the force main the specified distances away from water mains and other buried utilities.

I did find a standard specification online for laying tolerance:

Installation tolerances for the pipe shall not vary more than 50mm (2”) horizontally or 25mm (1”) vertically from the alignment and elevations shown on the Approved Plans.

http://www.sdwas.com/Documents/Vol2/Section2/Division15/StdSpec15064_02-01-08.pdf

However, unless you have that specified in the Contract documents, there is no way that you can force the Contractor to maintain that tolerance.

The only reason that one would want a laying tolerance is to assist in the future line tapping

 

[dicksewerrat]

I would have the contractor dig and replace existing pipe with new pipe at the required elevations. 4 inches on an 8 or 10 inch line is not acceptable.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com