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Community and Q&A

Duplicate Main Water Supply Line

johngfc | Posted in General Questions on

I’m wondering about burying a second duplicate (backup) water main in a new construction. The mains water supply pipe is at the road, about 200′ from the house and the house supply will be buried 6′ deep. I expect we’ll use PEX and the material cost of the of pipe is trivial. I assume the there little additional cost to putting it in the same trench. I’m wonder whether conditions that are likely to cause the first pipe to fail will also affect the second – i.e., would this be a reliable replacement? There will be a PRV, backflow preventer, and meter installed at the road. If you recommend the duplicate pipe, would you also recommend running it through the foundation (penetrating the insulation and another potential leak, etc.) or just plug the duplicate pipe off near to the foundation to be dug up if it’s ever needed? Thanks!

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  1. Expert Member
    Akos | | #1

    I would be looking to run power and irrigation out through the trench. Maybe a length of corrugated pipe (ie Big O) wouldn't hurt where you can fish services out as needed. If pitched away from the house and capped, it should be pretty safe to bring into the basement

    Underground water lines mostly fail if they are not deep enough and burst. Make sure the PEX is not spliced, fittings are the weak point and metal fittings can corrode underground.

  2. user-5946022 | | #2

    Bed the bottom of the trench with stone, then run 2 or 3" pvc as a sleeve, and thread your pex through that.
    The bedding will reduce the chances of the pvc sleeve collapsing, and the pvc sleeve will protect the water line, and allow you or someone in the future to pull a waterline if needed. At each end of the pvc sleeve, I would close off the opening with stainless steel wool, and spray foam it to prevent mice from getting in and chewing the pex (they like pex).

  3. Expert Member
    DCcontrarian | | #3

    Here in DC water line replacement is very common. There are three materials that account for most water lines, and the reason for replacement varies:

    Lead water lines still are not infrequent, they are replaced because they tend to poison the inhabitants.

    Galvanized steel pipe lines are replaced because they corrode over time in the soil.

    Copper lines are replaced because they are too small. For residential use code now requires at least a size larger than when most of the lines were installed. The city is also moving toward requiring all new construction to have fire sprinklers, which means either having separate water service for the sprinklers or a minimum 1-1/4" shared line.

    If a duplicate line had been installed when any of those lines were new, it would not be legal to hook it up today.

  4. Expert Member

    +1 on providing a sleeve and pull rope for any future changes.

  5. johngfc | | #5

    Thanks! A sleeve and pull rope sounds good. Our power and phone like run in another direction so water would be the only utility in the trench or sleeve. In our current house we recently replaced a galvanized pipe main with PEX. We didn't protect the new line in our crawl space from mice - comment #2 is making me rethink that.

  6. Expert Member
    BILL WICHERS | | #6

    I would use HDPE here instead of PEX. HDPE is what most of the new water mains are made from, and it's tough stuff. You can fusion splice it, so no fittings (fusion splices are thermal welds made with a special machine), and it will last pretty much forever underground. If you want it to be more durable, find out whatever wall thickness the water co likes to use and go a size or two thicker wall. I've never seen this kind of line fail unless some digs through it with a backhoe. At work I use this material all the time and spec it for my underground crews to install with directional drilling rigs. There is about a gazillion miles of the stuff installed out there in the utility world for power can telecom conduits, water mains, gas mains, even sewer lines. Make sure the stuff you get is NSF rated for potable water, which will usually mean a black pipe with a blue stripe, but double check the markings to be sure.

    I would not install the pipe in a sleeve. The sleeve is likely to fill with silt over time and be a problem, and will complicate the installation of any splices or fittings. Putting in some pea gravel or sand under and over the pipe at the bottom of the trence isn't a bad idea, but isn't really necassary with HDPE pipe. If you share the trence with other utilities, be aware that code often requires certain amounts of seperation between services, and also be aware that natural gas lines CANNOT enter a house below grade -- they have to sweep up out of the ground first.


  7. walta100 | | #7

    Seems silly to me the likelihood of the first pipe failing while you own the house is almost zero and the chances of the current owner remembering about the second pipe is another zero.

    Wasting materials is very ungreen thing to do.

    If you have money to throw away, please find a good charity.


    1. Expert Member
      BILL WICHERS | | #8

      I agree. In my 25+ years of utility experience, The ONLY time I've seen underground lines fail has been when some other contractor whacks them with their excavator rig, and that will take out any "spare" lines in the same trench at the same time.

      The only lines that fail from age tend to be really old, as in 100 years old or so. HDPE lines can be expected to last for centuries underground, since UV light is about the only thing that causes them to break down, and they're protected from that underground.

      For some perspective here, if you're in a city (like Detroit), where Verizon is not also the landline phone company, chances are reasonably good that some of your fancy whiz-bang 5G cell phone service is going through fiber optic cables that are run through .... hollowed out wooden log conduits leftover from the Western Union *teleGRAPH* network, which Verizon now owns through a long chain of acquisitions. That hollowed out log conduit system is something of a pain to work with, but it's still there, and it dates back to the mid/late 1800's. We even had some old hollowed out log water mains (!!) that got found in another smaller city here, and they were still working too prior to being replaced.

      You really don't have to worry about underground pipes made of HDPE "wearing out" in any kind of reasonable timeframe.


      1. johngfc | | #11

        Bill -

        Thanks for the advice on HDPE - that I can do. As for failed mains, I think 5 of 7 houses on my road has a galvanized water main that's failed within the past 10 years. I think all these houses were built in the 1950-1960 era and a brief internet search led me to believe a 40-60 year life for galvanized water mains is typical. I'm old enough to be confident a good HDPE pipe will outlive me!

        1. Expert Member
          DCcontrarian | | #14

          Note that if all of those houses had had a backup water main installed at the time of construction, the backup would have corroded away at about the same time as the primary. It's contact with the soil that corrodes the pipe.

  8. nynick | | #9

    Yeah, the whole idea seems like overkill.

    1. Expert Member
      MALCOLM TAYLOR | | #10

      Maybe doing it the whole length of the run is? I always sleeve any service going under paved areas or patios. On my own house it paid off. The original waterline was polybutylene, and had to be replaced after 25 years. Without the sleeve I would have lost my whole front entry slab, and would have had to do a lot of intrusive excavation through the perimeter drainage.

      1. johngfc | | #12

        Malcolm -
        I asked the question for precisely that reason: Compared to the cost of house construction or even just digging a 6' deep, 200 ft trench, the cost of a sleeve or pipe is basically irrelevant. A house down the street from me contracted a directional driller in replacing their main, which cost them more than $3,000 for a short (~50) line. I think their total replacement cost was over $6,000 plus expenses to clean up their flooded house. We hired an old, well-seasoned local plumber with a home-made drill who did ours for $1,000 including putting in a new freeze-proof tap. I still consider that a fantastic bargain.

        1. Expert Member
          BILL WICHERS | | #13

          Directional drilling is very expensive for short projects because so much of the project ends up going towards just bringing the equipment out to the site, cleaning up, and coming back to the service yard. For mid-size projects (hundreds of feet), prices are usually around $20/ft or so. For larger projects, it's not uncommon to get down around $12-15/foot. Obviously some of that is going to depend on what type of pipe you're running though, larger pipe costing more.

          It's not necassarily a bad idea to sleeve under driveways, especially with shallow pipes. I have used rigid steel conduit for that in the past, which is similar to galvanized water pipe in terms of having a thick wall. For deeper runs, schedule 40 PVC pipe or conduit can be used. For my commercial projects, I run the HDPE pipe directly in the ground, since it's extremely durable and doesn't really need anything extra. The only time I sleeve things is where it's required (such as when going under railroad tracks), or in a few unusual cases where there are special requirements for a site.

          If you want to do it break the bank defense department style, elevate the pipe on cradles and fill the bottom of the trench with concrete so that you encapsulate the pipe in concrete. I don't really recommend doing that, but it does make for a very durable installation :-D


        2. Expert Member
          MALCOLM TAYLOR | | #15


          I used 3" PVC sewer pipe and brought it right up through the main floor slab. Use sweeps for corners, bed it in sand, and cut a foam plug to block each end.

          1. Expert Member
            BILL WICHERS | | #16

            If you want to do a realy nice job of sealing where multiple pipes/conduits exit a sleeve, you can use a 'duct plug' similar to this:

            Those things work great, and they are removeable. When you tighten the bolts, the two plates compress the inner rubber seal which expands outward, sealing everything.

            I've often just injected some canned foam into the end of the sleeve though which also works, although it's harder to remove, and isn't always long-term reliable, either.


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