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?

 

[BarryEng]

I assume that you are interested in how to protect a pipe that has a minimum cover over the pipe.

What I usually do is to use a relief slab. The slab will be about 150 mm thick with reinforcement about N12-150 EW. The slab will be designed as if the surface loading is over the (horizontal) diameter of the pipe & the slab take the load as if it is supported on both sides of the trench (if the trench was there).

I usually place a 25 mm layer of polystyrene over the pipe width to allow for an indepentant deflection of the concrete slab without any support from the soil.

Do I have the correct interpretation of your problem?

BarryEng

 

[ACtrafficengr]

Barry, IF I read your post correctly, you are suggesting something like this:

ground level ______________________________

slab & EPS ===========

pipe O

I suppose another advantage would be if the pipe ever breaks, you could saw cut the slab w/o risking further damage to the pipe?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[wolfhnd]

In this case we are encasing the pipe to protect the highway in the event that the pipe ruptures. It is also important that the pipe can be easily jacked through the encasement in the event of a failure. The cost of relocation is prohibitive and is the responsibility of the local government, which seems to have only a minimal interest in the highway project. It may be of interest to some of you that this is a high profile project. This highway improvement will see the first implementation of a Diverging Diamond Interchange in the United States.

 

[BarryEng]

To ACtrafficengr
Yes, but I better add some extra thoughts.
On several occasions, we have had a major highway lowered in the same location as a major water main (up to DN 1,400) but this lowering would only be for a year or two, then the normal cover of 750 mm would be restored. So it seemed a very expensive exercise to lower the pipe & then put an extra soil cover back. The slab can be cut up & removed, if required at any time.
I have found that all you have to do is excavate a 150 mm depth in the soil, place N12-150EW in the excavation, then place a 150 mm layer of concrete. If you do the sums it is usally OK for all highway loadings even with a small cover.
Place 25 mm polystyrene on the bottom of the slab immediately over the pipe to allow for any traffic loading deformation to take place there. This is a bit probermatical because polystyrene does have a modulus & some loading will be transmitted thru the material - however, it always seemed to be a good idea to me - I'm not quite happy with long term effects of polyurethane on the environement.
Make the slab slope slightly away from the pipe on each side. I do this just in case there is some settlement in the cover soil. The slope means that any deformation will gradually be less, the closer the traffic will come to the slab over the pipe. I believe these relief slabs have been used as approach slabs on bridge abutments for the same reason.
As you can see, the sums are very simple, formwork is practically nil, very little steel & concrete, with the result a very simple design.
I design the slab as simply supported over a span equal to the diameter of the pipe but this is not strictly correct. Some of the support load (on both ends of the slab) must go vertically down & some of this load must go to the sideways support of the pipe. Remember though, the greatest deflection problem in an empty pipe (once in a blue moon, but still a design condition) is a vertical deflection on top of the pipe & an empty pipe design always assumes a design zone of D (pipe diameter) high & 2.5 D on either side of the pipe. The slightly extra side load will in actual fact, help support the empty pipe from the vertical deflection.
Another method of taking a load, is to place concrete over the top half of the pipe &, in effect, make a pipe arch over the pipe. In this way, the vertical load is taken by arch action to both sides of the pipe BUT as a vertical load with very little influence to the structural action of the pipe. Of course, use a polystyrene layer over the pipe.
BarryEng

 

[rconner]

You have mentioned the existing main "cannot be taken out of service", but it appears that the cost of e.g. a parallel main would be "prohibitive". While I am certainly not saying that what it appears you are thinking about is not possible, this sort of construction might not be real cheap either, I guess particularly depending on a lot of factors involving the existing main and if it is this work bidder who must cost in the risk and ramifications of disturbing the existing main's integrity throughout the process (and I guess additionally for some warrantee period). You have not provided a lot of details, but I was just curious roughly what percentage of the overall road/bridge project would a new "parallel" main amount to?

 

[BarryEng]

To wolfhnd
In some states in Australia, railways will NOT allow a pipe to cross a railway without a carrier pipe. The reason is as you said "any burst will be discharged outside the embankment & also the pipe can be pulled out & replaced without interfering with railway operations".

Electicity authorities are not too happy with water mains either. If the pipe bursts, there is a relatively long time before the pipe can be shut down & any repairs to the power lines can be commenced. There is also the problem of induced current in the pipe with all of the associated safety issues.

When I install a pipe inside a carrier pipe, I use a series of timber skids that are attached to the outside of the pipe using a SS band (housed in a groove in the timber skids). There is sufficient space for the discharge of any water in the event of a burst, but also the pipe can be easily pulled back (when replacing the pipe) without too much trouble.

By the way (for a non road engineer), what is a Diverging Diamond Interchange?

BarryEng

 

[ACtrafficengr]

Don't feel bad, Barry, I had to look it up myself:

http://en.wikipedia.org/wiki/Diverging_diamond_interchange

I'd have liked to be a fly on the wall at that public meeting. And I thought people had conniptions over roundabouts!

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[wolfhnd]

The length of the encasement would be 800ft. The cost for relocation would be $700,000 to a $1,000,000.

The DDI is estimated to cost $7,000,000, it replaced the previous alternative, a conventional diamond with 10 lanes costing about $11,000,000. More than half the cost for each alternative is Right of Way. As is the case with most state DOTs we are nearly bankrupt so budget changes are practically impossible. The cost of the relocation could be a project killer. I have been given 3 weeks to complete the plans so a timely solution is needed. I believe that we have $250,000 alocated for the encasement.

The project was well received by the public, possibly due to the serious congestion and the understanding that Right of Way is limited. The interchange is the only access to an industrial area with numerous warehouses totaling perhaps 100 acres in floor space, there is also a large powerplant and Bayer chemical manufacturing plant. The chemical plant and powerplant are of concern if water is limited.

If you would like additional information on the DDI let me know.

 

[BarryEng]

To wolfhnd
I assume from your thread, that the existing pipe can be exposed & protection placed. Have you considered placing an upside down U of steel as a carrier pipe over your pipe? The steel U can be set up on a concrete footing on both sides of the pipe. This will still satisfy the principle of allowing a sufficent area to allow for the discharge of water from any burst, & also allow for a future replacement without digging up the arrangement again.

You mentioned jacking a steel plate under the pipe. Why do this when you can just place a longitudinal footing on the side of the pipe but at a depth of (say) 300 mm below the pipe invert?

___
/ \
/ 0 \ ____ invert level
/ \ ____ top of footing
| | | |
---- ---- ____ bottom of footing

This will allow the simple construction without interfering with the existing pipe, take any surface loading (depending on the details of the carrier pipe arch), allow a path for discharge water without risking the structural system, & allow future replacement without the necessity to dig up the carrier pipe.

Might still be fairly expensive but will leave the exisiting pipe in operation.

BarryEng

 

[wolfhnd]

Your idea is what I first suggested but the objection was raised that the material under the pipe would erode if only three sides were encased.

 

[BarryEng]

The material would only erode in the event of a burst (which is, after all, a fairly rare event). If the soil is sand, you require a velocity of (say) 0.5 to 0.6 m/s to effectively keep the particles in suspension (the basis of most sewers & drains) & you could make the water path larger to ensure that this does not occur. You are getter a larger & larger structure with every idea, aren't you?

If the carrier arch was deep enough (say 300 to 600 mm below pipe invert) it may not be a problem.

I was also going to mention that another method would be to cast an upside down U of concrete (such as an inverted rectangular culvert section) over the existing pipe with an appropriate clearance for a water path if required. This alternative suffers the same problems that you mentioned with the steel inverted U.

BarryEng

 

[blueoak]

Another question is what is the condition of the pipe. It might be worthwhile to have the pipe inspected prior to all the work. It may be that the pipe should be replaced regardless of the interchange.

What is the original pipe, could you use it as a carrier for an HDPE pipe or something? If your internal pipe fails, it is likely that your existing pipe can hold the pressure and you monitor any leakage between the two. Your main may have extra capacity or increasing head loss might not be an issue for only 800'. Seems unlikely it has enough room but it would be cheap.

 

[blueoak]

Oops, forgot the continued service part.

 

[rconner]

Just curious what size is the existing 800 feet section of main to be "encased", or is it already under some sort of working highway that cannot be open-cut etc. (with very few such details it is some hard for me, probably due to me being more dense than the backfill, to get much of a grasp on this situation and also understand the apparent 4X/million dollar cost of a new short section of parallel properly encased pipe and tie-ins etc.?)

 

[cvg]

You indicate a cost of about a million dollars for 800 foot long waterline. This amounts to about $1,200 per linear foot. Unless I am missing something, this would be the most expensive waterline I have ever seen. Are you sure about this cost? Is there something else causing this high price? What is the diameter of this waterline? Is this considering bore and jack or cut and cover construction?

 

[wolfhnd]

I'm told it is a 20" high presure main. The cost was estimated based on $600 a foot for a pushed pipe. The additional cost comes from trust blocks and addition relocation to make the connections above and below the encased section. I still believe that a concrete encasement over a steel arch is workable provided enough relief valves are placed along it to reduce the pressure if it ruptures. This idea does not seem to appeal to my associates however.

 

[cvg]

$600 for jacked casing and installing the pipe seems reasonable (unless there are some geotechnical issues I am not aware of) But the additional cost you have estimated for connections and thrust blocks seems high to me. Also, it doesn't seem that you would have to bore and jack the entire 800 feet.

 

[wolfhnd]

The project is scheduled to be let in February but the pipe work must be done during the cool season after most of the roadway will be completed. I'm getting desperate here folks please ask anyone you know if a cast in place solution is feasable. An example locatiom would be nice if I meet with objections.

 

[cvg]

I am unclear how you would jack a plate under the existing pipe without disturbing the pipe. This sounds problematic to me. I'm not convinced this option will be cheaper. Why don't you contact a contractor and get a realistic opinion of the cost of your proposed encasement? Suggest you talk to a specialty jacking contractor.