Sharp augers shave caliche quite well. I drilled a bunch of holes with the aforementioned auger right after a wet spring and after I got about 2' down, I hit vanilla cake batter. It was crazy, had to pull the auger out and run it up to full speed to sling the stuff off. When the clumps and piles dried it was like mortar mix again. The problem with using clamshell diggers in caliche is like Popper said, if it's too dry. Not enough mass, they bounce right off. Break it up with the bar and dig it out with clamshell. If you're not in a hurry, start the hole down to about 6", fill it with water a few times, go back to it in a few days, dig another foot or so, and repeat.