In place encasement of water main 2

[wolfhnd]

I'm thinking that a water main can be encased in place with a am aluminum or steel plate jacked under it, a metal pipe arch placed over it, and concrete to hold it all in place. Basically I have a main that cannot be taken out of service while a highway is built over it. The cost of a parallel line is prohibitive and would still require extended down time. Comments please?

 

[LCruiser]

The slab idea is good, but the inverted U with footng seems cheaper and more robust - especially considering you really should design the slab to go beyond the line of influence, tough considering deflection. You could also provide a gravel filter to prevent migration of fines in case of failure. I wouldn't worry about catastrophic failure I don't think - but what kind and rating is the pipe and what is the working pressure? Interesting challenge.

 

[wolfhnd]

I'm going to try and get the cities consultant to post. It may help to get his perspective. I'll check back in the morning to see what great ideas you come up with.

 

[wolfhnd]

This is the email reply I got from the cities consultant.

"What are the soil conditions? Knowing the soil bearing capacity will help the discussion.
What is the normal operating pressure of the water transmission main? I checked w/ the City, operating range is 155 psi to 180 psi (1,070 kpa to 1,250 kpa). This does not include “normal” operational surges. To help quantify these numbers, a rupture in this line will blow water steadily 70 to 80 feet in the air.

Also, FYI, the $750,000 was the quick back of the napkin number for doing a complete relocate of the waterline south of the interchange."

The soil is a silty sand, bearing strength is 1000lbs per sq ft.

One thing I need to add is that relocation of water lines has delayed projects in the past for as much as a year. This is unacceptable for this project because it will conflict with work on a major river crossing if delayed.

I'm hoping that a brilliant solution will help motivate the city and prevent this kind of delay.

 

[LCruiser]

What kind of pipe is it???

 

[wolfhnd]

I believe I was told it was steel.

 

[LCruiser]

CMLC, DIP, ???

 

[wolfhnd]

I'm not sure will have to wait until monday.

 

[rconner]

You might also cost a shorter parallel (hopefully that can be open cut) in effect by-pass installation of new (encased if it is required by your authorities) piping arguably more safe under the interchange, that might be tied (perhaps rather quickly or even hot-tapped? with contemporary 20" restrained piping, whatever valves anyone feels are necessary etc.?) into the existing line a safe distance off the new interchange. After tie-in of the new by-pass maybe the uncased piping section could then be removed, abandoned, filled, left in operation as a redundant, or whatever as powers-that-be judge or desire is necessary etc.

 

[wolfhnd]

It's looking like at this point that if I can't find a encase inplace solution the pipe will have to be pushed. The City will not have the paper work ready in time and the work cannot be done until the cool season in the fall. Most of the highway work will be completed by then. A parallel line was rejected due to the size of the thrust blocks that would result from the required bends.

 

[LCruiser]

Again,

If it's a brittle or high pressure (compared to the rating of the pipe) matters. You may also not be able to excavate the whole pipe to encase it at once without hztl and vertical stabilization. It depends on what it is and what kind of joints it has.

Also, you may not need big thrust blocks if you use small angles - but everything depends on the type and rating of the existing pipe.

 

[wolfhnd]

It is my understanding that this line is a weak ductile iron pipe of 1960 vintage.

 

[LCruiser]

Nobody has actually potholed it?? You were going to "engineer" a solution without really knowing what it is?

 

[wolfhnd]

Exposer and surveying is scheduled which I guess will give us an idea of it's condition. I was too busy today to contact the city and find out exactly what the pipe was but it should be in their records. The solution I was shooting for would not be dependent on the pipe or at least that was the idea. If a three sided structure is place over the pipe without disturbing it the only complex issue would be providing enough venting that during a rupture velocities at the vents did not exceed that needed to suspend fines, say 3 ft per second. That said the suggestion to place a slab over the pipe to prevent failure from new loading may be the way we go. Of cource this does not protect the highway which is a compromise that will meet with some resistance. I think placing an empty encasement in the project along with the slab over the existing pipe may satisfy the city. If we pay for the encasement it may prevent any delays to the project that this utility relocation may have caused. I still prefer a three sided encasement but I was hoping that someone had done this before so it would not be totally untested. I don't think this can be resolved to all parties complete satisfaction.

 

[BarryEng]

To wolfhnd
I was looking for something else & found a reference for your future info.
The US Corp of Engineers have a publication Conduits, Culverts & pipes, EM 1110-2-2902 dated 31Oct97. There is info in Chapter 8 (with sketches) for encasing a steel pipe in a carrier pipe.
Not sure about the method of access but it is a public document & I originally found it on the net (searching the US Army Corps of Engineers web site).

BarryEng

 

[LCruiser]

You may want to radar it right now to see if it's leaking as is. I forget what the old pipe guys call it, but when you uncover it, it may "pic-up-stix"(?) without lateral and vertical support. Let us know when you find out what it is.

That said, a casing for future 10 feet away and a flat slab out to a 1:1 line of influence might be the safest solution.

 

[wolfhnd]

Thanks guys. I was out of the office all day so maybe I can get some information on the pipe tomorrow.

The reason I'm so concerned about this project is that it will get national attention. The DDI is unique and failure to meet the schedule or a dramatic pipe failure would not look good.

 

[rconner]

If there are governing authorities who require encasement of such pipes (e.g. under highways), there is some guidance for casing/carrier pipe installations involving ductile iron pipes in ANSI/AWWA C600. If the governing authority does not have specific requirements for the casing pipes themselves, while I don't have a copy of same I have also heard there is a publication available from the National Utility Contractors Association (NUCA) that has information regarding e.g. steel piping used as encasement.

 

[wolfhnd]

I'm still waiting for my utility engineer to find out what kind of pipe we are dealing with.