How to protect an old iron main

[Ben and Jenny]

Hello, people who know much more about this topic than I...

We're building a rear extension to our end-of-terrace house, which required moving a public sewer and manhole; in the process we uncovered the old iron water main that feeds our house and the three others in our row. Photo: https://drive.google.com/file/d/10XTLYyj2QsW0L7Li5cBGIlxH5aQCdz1_/view?usp=sharing - the supply direction is from top left, the T is our supply, then it wraps around to run under the rear gardens.

That main is in poor shape; the branch to our house was so bad that my plumber has replaced it with alkathene. However, he tells me it's not possible to safely cut/thread/join on to this kind of pipe in this state, so we can only connect at existing joints. (He thinks the torque from trying to thread it would just split the pipe.)

Unfortunately there's almost certainly no joint between the elbow in the middle right of the photo, and 6m downstream of that, which is under my neighbour's conservatory floor. Rerouting all the way around the extension and conservatory is not an option since said neighbour has fancy patio over most of the area the trench would need to run through.

So it seems I have to re-bury the old main under my new extension - that prospect thrills me, let me tell you. (Although really even if we could find or create a joint under the floor, that wouldn't be much more comforting; the new foundation will touch the existing footings of the conservatory, so there's no gap in between where a joint could be relatively easily accessed.)

I'm probably going to switch from a slab floor to block and beam to make it easier to get back down there if there's a leak in future, but I also want to do everything possible to preserve and protect the old pipe.

We've already had two pinhole leaks in the process of digging this out, which were fixed by my builder using steel epoxy putty (clamped in place with a bit of old lino and jubilee clips to hold it til it fully cures). That solution seems to be holding OK for now.

What would you recommend in terms of protecting the pipe long-term?

My best guess is: expose the whole thing, clean it gently, identify any leaks and patch with steel epoxy, then enclose the whole length inside a 4" or 6" plastic pipe full of pea shingle so any future leakage (hopefully) drains away from the surface of the metal to slow further corrosion. That may be wholly daft though, I'm a biologist not a civil engineer...!

Cheers,

--
Ben

 

[denso13]

What would you recommend in terms of protecting the pipe long-term?

However hard it is, it needs to be replaced. There is no way I'd be building over that.

 

[CBW]

However hard it is, it needs to be replaced. There is no way I'd be building over that.

Yep, I did one once as a job on the side, ended up working until midnight because it fed 3 other houses, and when I repaired it, the main decided to pit somewhere else on the pipe, at least 3 times!!!

 

[Ben and Jenny]

That's reassuring thanks for the advice, though.

We're trying to find someone who thinks they can cut and thread the old main as close to the party line as possible, which at least isolates the problem to the footings of our new wall adjacent to the neighbour's conservatory (and praying the pipe doesn't split all the way back to the existing concrete when they do that). Then we'll wrap the protruding nub of old pipe in umpteen layers of waterproofing (a solid coat of steel epoxy, if all else fails) in the hope the bit under our new wall lasts longer than somewhere else along the run.

We're also going to run a deeply buried alkathene branch from the new section, a good distance down the garden and across to the neighbour's boundary fence (I'll stick a garden tap there for now). There's no way they (or the next house down) are going to dig up their patios until the old main actually fails, but at least this way we've got our section of the future replacement main ready to continue from when that happens.

I've learned during this process that there was a leak years ago from this main, which ended up with the water board excavating both their old patios at the time trying to find it, and eventually discovering it somewhere on what's now my property - I suspect that's why they're so opposed to a repeat performance. (Weirdly, I still can't find the location at which that repair was apparently made!)

I feel like that's the best we can do without more cooperation from the downstream houses - what do you think?

 

[denso13]

We're trying to find someone who thinks they can cut and thread the old main as close to the party line as possible

A normal transition coupling might do which means the pipe doesn't need threading. Depends how rough the pipe is though. https://www.bes.co.uk/25-mm-x-21-mm-27-mm-straight-universal-transition-fittings-polyfast-13545/

the branch to our house was so bad that my plumber has replaced it with alkathene

If you mean the blue MDPE in your picture it should be at least 750mm deep.