Excavator

Ok, super half baked, but what the hell.

The digger consists of two sharp steel plates with a slight bend to them - like the head of a shovel only a bit wider. The curve is also a lot shallower. The two halves come together and over lap just as far as that curve. The whole thing is hoisted into the air and dropped, the weight of the mechanism driving it into the ground. The hoist mechanism and the weights works together to push the jaws closed. The whole thing operates on two sliding rails so it can be moved over a bucket and unloaded.