neb:MikeAqua:That's a really clever design! I've only seen the handheld one/two-person ones which I imagine wouldn't have a hope in hardfill.
I've used a hired post hole borer to dig holes in compacted hardfill. It was a pendulum style, hydraulically driven machine.
I hired one to dig a couple of hundred holes at our last place. The whole section had been built up close to 800mm with compacted fill. Took a couple of digs per hole, but got it done.