Sewer Force Mains On Top of One Another 1

[tbarkerjr]

Does anybody know of any wastewater standard (I am located in Long Island, NY) that has language regulating the placement of a force main directly above another force main?

 

[RWF7437]

NO

 

[PELS]

Are you talking about parallel with and on top of as opposed to crossing over?

 

[tbarkerjr]

yes i mean on top of and parallel with

 

[PELS]

I have not run across any design standards that expressly prohibit this, but these are some things I would concern myself with:

1. Force mains are normally at a shallow depth. Placing a new one over an existing one would create an unusually shallow depth that may require special treatment.

2. Should allow for enough vertical separation between the two to allow for operation, maintenance, constructability and proper bedding, backfill and compaction.

3. Ability to distinguish surface components of the two mains (valves, pig launchers/receivers, cleanouts, etc.).

4. Potential conflicts with water mains and dry utilities (gas, electric, telecommunications, CATV).

5. Facility mapping.

6. Pipe restraint at shallow depth.

 

[RWF7437]

Why would you want to do this ?

 

[bimr]

Agree with the previous posters, it is not a good idea.

 

[tbarkerjr]

i don't want to do this: it's just a situation where two separate designs are going on at the same time and there si a conflict with this particular issue. they want to put a force main over ours and we didn't want them to do that, but there response was: "what is prohibiting us from doing that."

 

[RWF7437]

"they want to put a force main over ours and we didn't want them to do that, but there response was: "what is prohibiting us from doing that."

Who is they ?
Who are "we" ?
What exists and what is proposed ?
Who owns what ?
Are these force mains public or private ?
Who owns the land(s) through which these force mains pass ?
Are these force mains in easements and are those easements "exclusive:?

 

[bimr]

Through the deflection of the pipe wall, the applied vertical load is carried principally by the soil envelope and to a lesser extent by the flexible (for PVC) buried pipe. If you do not have the soil compacted, you risk crushing your pipe.

The pipe installer needs to obtain sufficient soil stiffness around the pipe to maintain structural integrity. Spacing between pipes in a common trench should be at least 12" apart to permit adequate space for tamping or mechanical compaction. Consideration should be given to using class1 or class 11 embedment materials, which require minimal compaction effort.

You should have enough clearance between the force mains that you can operate a compaction machine between the force mains.

 

[NinetyWt]

Is there not any way to combine the two lines so that there is only one force main needed?

 

[ptmoss]

"what is prohibiting us from doing that"

tbarker, I don't envy your position on this. If you feel that the sewerline being placed above yours (well, your clients..) may in any way compromise your clients pipeline then I personally would fashion a well thought out letter to the other design firm, or contracting firm, and spell out exactly your concerns. If your client has an attorney then I would have them review the letter. Then it should go out certified, return receipt requested, with copies (cerified also) to the owners, developers, etc., of both projects. And perhaps even to any reviewing agencies.

There might be very good reason for what they have to do, and it might be that it can be constructed in a manner that won't impact your clients pipeline, but you must make yourself explicitly clear on this to protect both yourself and your client. It sounds like you got a bit of Attitude from that other firm, and personally I don't have a lot of tolerance for attitude like that where there really doesn't need to be any.

You would also do well to include in your letter what you expect of the others in the way of confirmation that they "did no harm", such as a video inspection of your line. You might also recommend to your client that they perform their own video inspection as a baseline. They might not, probably won't unless required by a reviewing agency, do such an inspection, but I think you should formally put it out there.

 

[ptmoss]

I actually have a similar project where I know another development is going to have to pump their sewer along the same highway right-of-way as my client. My client is above the other development and closer to the manhole. I know generally what the other development will be adding for flow and I'll consult with the other engineering firm when I do a detailed design to get their input. The line my client builds will serve both develoments and he'll leave a manhole and a stub that can be tied into in the future. Since the worst case is that my client might have to increase from a 2" to a 3" forcemain (not a lot of flow from either development, really) he doesn't have a problem.

I also think the state DOT will require something like that anyway - whoever builds first will have to accommodate the other.

 

[tbarkerjr]

Thank you for all your responses. PTMOSS is right on target with his assessment: it doesn't matter what we think/know is right I just needed some standard that stated this in writing. I am an entry level engineer just feeling my way out and someone brought this issue up to me and I wanted to do a little research; I really appreciate all the help and apologize for my lack of knowledge.

 

[civilperson]

What happens at valve locations? Is there a conflict between the lower pipe's valve and the top pipe?

 

[Sundevil99]

Poor engineering is "prohibiting" the other firm from doing it. How do you repair the force main on the lower level without taking the upper line out of service?

Make sure you bring your concerns up to the reviewing agency if you make a good argument I can't see any agency approving their design.

If for some reason they do allow it make sure your valves are laid on their sides so you can bring the valve stem to the surface without conflicting with their pipe. Good luck....

 

[shirsch]

I would make an effort to coordinate the two force mains into one. The POTW will not accept two force mains on top of one another due to lack of access for normal operation and maintenance. Save your client some money and coordinate with the other pump station project. You may even be able to combine into one pump station.

 

[rconner]

You did not originally note the sizes, materials, nor pressures etc. of the mains involved, but nevertheless you got some information in many responses. In any case, while I think this kind of issue may be kind of difficult to handle with absolute "standard" verbiage I’ll add just a little more information. One reason is I believe many experienced Engineers (including those pretty good and dedicated ones who assist drafting minimum consensus etc. standards!) understand that occasionally, due to right-of-way restrictions, common utilidors, other strong desires/necessity of the Owner etc., some pipelines inevitably will be placed rather closely together at some point.
That being said, I am aware e.g. I think fairly well-vetted ANSI/AWWA C600-05, Installation of Ductile- Iron Water Mains and Their Appurtenances does contain the words,

“4.3.1.3 Clearance. When crossing existing pipelines or other structures,
alignment and grade shall be adjusted as necessary, in accordance with the contract
documents, to provide clearance as required by federal, state, or provincial, and local
regulations or as deemed necessary to prevent future damage or contamination of
either structure.” and also in another area,
“ 4.3.2.5 Rock conditions. When excavation of rock is necessary, all rock shall
be removed to provide a clearance below and on each side of all pipe, valves, and
fittings of at least 6 in. (150 mm) for nominal pipe sizes 24 in. (610 mm) or smaller
and 9 in. (230 mm) for nominal pipe sizes 30 in. (762 mm) and larger. When
excavation is completed, a layer of appropriate backfill material (see Sec. 4.2.5) shall
be placed on the bottom of the trench to the appropriate depths, then leveled and
tamped.
4.3.2.5.1 These clearances and bedding procedures shall also be observed for
pieces of concrete or masonry and other debris or subterranean structures, such as
masonry walls, piers, or foundations that may be encountered during excavation.
4.3.2.5.2 This installation procedure shall be followed when gravel formations
containing loose cobbles or boulders greater than approximately 8 in. (200 mm) in
diameter are encountered.
4.3.2.5.3 In all cases, the specified clearances shall be maintained between the
bottom of all pipe and appurtenances and any part, projection, or point of rock,
boulder, or stone of sufficient size and placement that could cause a fulcrum point or
pointload.” (See the underlining I’ve added – whether it was originally intended by the standard developers or not someone could certainly argue that other adjacent or underlying pipelines would be at the very least a sort of “subterranean structure” that mustbe considered!)

Now, I think many also well-meaning authorities and individual Engineers have gone far beyond such absolute minimum standards requirements, looking at many additional issues e.g. some as already discussed on this thread (and others regarding potential vulnerabilities e.g. as talked about in some published papers like

http://scitation.aip.org/getabs/ser...00282040507000046000001&idtype=cvips&gifs=yes

). Large diameter and high pressure pipelines obviously might arguably deserve more attention and respect in this regard and many others. It should be remembered as well that at every change in direction or othr thrust foci of more than one parallel pipelines close together, there will in essence be an accumulation of two thrusts for the essentially one soil mass to deal with, and this should probably be appropriately considered in the thrust restraint design..