When attempting to dig heavy (or partly frozen) soil, some technology (beyond that of the spade) would be useful. This assumes that you don’t want the expense and effort of hiring a fullscale rotovator.
Today’s invention is a fork with three or more conical, screwed tangs. These are chain-driven by a motor in the shaft, so that they drill into the earth.
When they are fully embedded, the user leans on the shaft and the soil yields, allowing it to be dug up by more conventional means.