In place encasement of water main
In place encasement of water main
(OP)
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?





RE: In place encasement of water main
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
RE: In place encasement of water main
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
RE: In place encasement of water main
RE: In place encasement of water main
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
RE: In place encasement of water main
RE: In place encasement of water main
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
RE: In place encasement of water main
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
RE: In place encasement of water main
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.
RE: In place encasement of water main
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
RE: In place encasement of water main
RE: In place encasement of water main
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
RE: In place encasement of water main
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.
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
"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.
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
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.
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
RE: In place encasement of water main
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
RE: In place encasement of water main
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.
RE: In place encasement of water main
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.
RE: In place encasement of water main
RE: In place encasement of water main