A Fall project installing 4" schedule 40 rain gutter downspout runs out to and through the street curb. The A/C unit was next to the house and moving it would have taken more effort than tunneling under. However, I needed something to "drill" through the dirt.
A section of 3/4" iron pipe and a section of scrap from the screw jack build was all it took.
It worked fine for this one-off application. However, if I was to do it again I'd cup the shovel end like a spoon so it could be shoved into the clay and rotated cutting a chunk of dirt free. The straight piece of steel just plunged into the ground without breaking anything free most of the time. The altered design would have cut the effort in half.