In upstate NY need to excavate (3), 12"x12" square holes, 30" deep in my backyard to evaluate soil for septic tank drain field via "Perc" (percolation) test. This will determine amount of area needed to lay required quantity of 4" dia perforated pipe per Dept of Health specs. Doing the test is "easy" for an old codger like me, but I'd appreciate some creative, experience-proven, suggestions on digging the holes. Each hole has to be flat walled, non-crumbly, etc per NYS guidelines. I own a 4" wide-bit long-handled cold-chisel and post-hole digger; have lots of time but much fewer $. Wondering if some sort of powertool (akin to a battery-powered drill with long bit) is available for one-person use that would do the bulk of the excavation, with my hand labor finishing off the four vertical sides, etc.?