Can You Dig It?

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I remember telling my brother, Bob, 90% of building is digging. Now I'm starting to think that's low.
Not going to hire out the dig. It's too complicated and steep. Besides, I spent most of the budget on the materials, so hand digging will slow the project some while I save up more capital.
What looks like holes left by giant square gophers is the footings for the piers. They were supposed to be 18" cube (18 wide square, 18 deep) but with the slope I think going bigger is cheap insurance. There are 5 piers, each 24" cube.
Here's what I don't get. Each hole was 8 cubic feet of soil removed, so you'd think it would be 1 1/2 wheelbarrow loads. (the wheelbarrow is 6 cubic feet level full, and I was piling it up higher) but it was 4 loads. The dirt is getting re-distributed to flatten out spots around the lot...To add to the mystery, I know from experience that if you take dirt out of a hole then put it all back in, it doesn't completely fill the hole.
If all the soil remains cooperative, I can get 10-12 feet dug per day. Not counting the piers, it's 80' total, plus some for the utilities trenches.