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.
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.
Best Answer
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.
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.)