>How do I build such a drill?
I would have to come poke around your garage and see what you have.
You need a vertical frame. Build it from 1" black steel pipe. Put mounts on your truck bumper so you can stand up the 10' rails at the job site.
What will we use for power? Get a 2 cycle lawn mower engine (that runs). Cheap and powerful.
Put on a square horse trailer hitch box (the female end) in line and perfectly centered on the shaft in place of a blade. This is your chuck.
Get several sections of 1" rebar, say 3' (to start the hole), 5', 7.7', and 10' long.
Weld male trailer hitch ends onto them.
Grind deep spiral grooves into the bottoms.
Clean up the spirals on the rebar to auger the dust better.
Mount guides on the engine so it follows the frame rails and some that you can grab to raise and lower the engine. You might even make an easy way to apply your body weight as downdraft on the engine platform.
Hitch the short rebar auger in.
Start your engine. Drill away. Stop engine. Pull out, switch to the longer auger. Repeat until hole is 100" deep or whatever you want.
You can improve the design by putting a titanium bit on the end, using a real rock auger, adding a rack and pinion crank or a rope and pulley, a lock to hold the engine up,... You could use a reinforced mud mixer stirrer as an auger for the first 6" inches to make that funnel.
Being that you are a one man band, I doubt you'll be complaining to OSHA... unless, did you complain about all that sledgehammering work?
