Inverted siphon ? 5

[PELS]

We are designing a 10" diameter gravity sewer that has to cross a major thoroughfare. Unfortunately, an existing 20" diameter water transmission main lies in the direct path of the gravity sewer. We have no flexibility in the grade of the sewer as the downstream end is fixed. One proposal was to use an inverted siphon to dip under the water main. We have no experience with this method.

Is this a common solution to this type of conflict?

Are there any pitfalls to be aware of when using a siphon?

Are there other methods to get by the water main without having to relocate it?

Your thoughts and suggestions are appreciated.

 

[pablo02]

For my information, what code requirements does your local (state?) have regarding water mains and sewer lines crossing, etc..? [I am assuming you met them]...

 

[angusa]

Hello Pels,
I have a much larger diameter pipe to modify with an inverted siphon, a 3'-3" one. To make things worse is encased in 5' by 5' reinforced concrete. But it has to be done. We exlpored all other options.

I wondered what the stage of your project was and how you made out. I'd appreciate any tips you might provide.

Thanks.

 

[PELS]

Angusa:

Two messages above yours, I described how we ended up resolving our conflict......we decided not to construct the syphon and the 20" water T-main was relocated to a higher elevation......the local codes, BTW, require a minimum one foot vertical separation, water over sewer.

You have a 39" pipe encased in 5'x5' concrete block and you need to re-figure it with a "dip" to avoid conflict with another pipe crossing? Is your 39" pipe a gravity flow or pressurized pipe? What diameter is the other pipe crossing it and is it gravity or pressure?

 

[angusa]

Thanks for the reply PELS.
It is a 39" old combined gravity sewer in the heart of Toronto downtown, University Ave. and Queen St. West.
The conflict is with a short (less than 10') crosswalk connection from the new Opera House (being built as we speak) and the subway system. The City record drawings indicated lower invert of the sewer than what was discovered when they excavated for the foundations works. The Opera House and the General Contractor are not willing to do any revisions to the structure design as the construction is in process. They believe it would be easier to construct the depressed sewer as a resolution to the conflict.
The slope of the existing sewer is decent, 0.6%, and when I checked the headlosses throught the system they indicated that there would not be any sewage backing.
The problem with this particular site is that it is at a very busy intersection and the construction will cause substantial disruption of traffic.
Another problem is the confined space, the chambers and the barrels would have to be placed within 8' of clearance between the foundation walls of the Opera House and the subway passage wall.
Yet another problem is that due to a leaky watermain or some other reason, the surrounding soil is very wet.
Nevertheless, the project is a GO for now, mainly as a result of the Subway Commission advocacy. They really want to provide this convenient access from the subway to the Opera House and invite more people to take the subway.
So far the design provides for one 15" barrel designed for peak sanitary flow and one 38" barrel for the overflow.
I can tell you more about it if you are interested.
Thanks.

 

[PELS]

Angusa:

Maybe you could avoid the conflict by adding one manhole upstream of the crossing and drop the flowline of that manhole below the crossing pipe (a drop connection MH)enough to clear the 39" beneath it then lower the 39" to the flowline of the manhole (so that you are far enough below the crossing pipe to avoid it). Then hopefully you have enough grade to catch the existing 39" at some downstream location where another manhloe could facilitate a connection back to the existing line. If you have enough slope in the existing 39" to do this, you can avoid using a syphon. You could place the new 39" at the flattest slope possible (0.00044 for 2 fps velocity).