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I'm looking to extend an existing water line. The only problem being that I need it to go under an existing retaining wall. I've included a quick side sketch just to get an idea of what I mean.

Side view of wall

The red is what I need to add. The wall is really old and built from river rock and natural stones. As well, it's a little over 5 ft tall from the lower level to the upper. I'm not sure how deep the wall was buried when built, but I'm guessing I've got 6-7 ft (?) to dig on the back side to add the new line through.

Going around, through, or over is not an option here. Likewise, there is an outbuilding on the upper level mostly blocking access from getting any sort of tractor to the location (we do have access to a backhoe - just not sure it'd work here without possibly tearing up the wall)

My only guess was maybe attempting it with a post hole digger and a lot of shovel work, but I'm hoping for some better ideas as we only just a straight shot down. Fittings and such at the bottom can be completed after digging out under the wall.

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Oooo! That's a challenging problem.

An idea that springs to mind is try renting a pneumatic gopher: variously called a hole hog, underground pneumatic piercing tool, air hog, etc.

There is a (crummy) Wikipedia article about them.

Those bore in a straight line, so I expect starting it some distance back from the wall and point it downward at about 45° to miss the bottom of the wall. You'd then have to dig a hole to find the borer.

enter image description here

Here I show digging a big hole at the left for device access. Bore a hole for the new connecting pipe (magenta). Dig a hole at the lower right to intercept the bore and provide access to connect to the lower line. (I should have erased your red line.)

  • A good thought. Same caveats that handyman pointed out in his answer, though. Mess up the drainage and it could be bad for the life of the wall. – DonBoitnott Oct 23 '17 at 14:26
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You could try a post hole borer if you can find one and 'drill' a 6" (150mm) hole down to the bottom and meet the hand dug hole under the wall.

One caveat though; digging behind the retaining wall, one of two things could happen, if the wall was properly backfilled with a decent granular material this could 'leak' into your new dig, or alternatively, any old rubbish could have been thrown in there and hitting a brick etc. is going to severely dent your digging efforts with postholers etc.

Google: 'post hole borers' or 'post hole augers' to learn more... Good luck!

  • Yeah, that's my first guess too. I have one and plan on attempting to use it if I can't come up with something better. -- I will say that most likely it wasn't "properly" backfilled, but you're right that there might be something back there. The wall is coming up on being nearly a hundred years old now. – TFK Aug 22 '17 at 18:38
  • It's never going to be easy to get down that far. A manual dig with a shovel would require quite a big hole to give you enough 'elbow' room once you get below 3 feet (1m). – handyman Aug 22 '17 at 18:45
  • Another thing occurred to me, I wonder if the soil would take a 'punch'? I'm thinking about using a scaffold pole. Push/hammer it in, twist it and pull it out, Knock out the soil and repeat. (Imagine pushing a straw into a strawberry...) Huge Disclaimer: this would be highly experimental, and I've never tried it :-) (I'm thinking about how they take soil samples etc...) – handyman Aug 22 '17 at 18:49
  • You're right, hand digging it would take ages with the size of the hole I'd need and making it safe to even dig around. The straw idea isn't bad either. It'd take a bit of time, but I can see that working as long as it kept out of rock. The dirt is almost like clay from what I've seen so far digging in the area. – TFK Aug 22 '17 at 18:53
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    Don't forget -- if you have a requirement to keep the line a certain distance below grade for frost protection or the like, you must keep it at least that far from the wall also. – A. I. Breveleri Sep 21 '17 at 19:54

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