A Moment of Science

Sugar bowls and holes

If you've ever had to dig a hole and then fill it back in, you may have noticed a mysterious thing: there was always a little dirt left over. Why?

Shovel and hole on beach

Photo: &y (flickr)

Digging a hole then filling it back in may result in having leftover soil due to "pore space"

If you’ve ever had to dig a hole and then fill it back in, you may have noticed a mysterious thing: there was always a little dirt left over.  This can often be seen when construction crews dig down to get at sewer pipes or electric cables. But how can there be more dirt going in than there was coming out?

The answer is that the amount of soil itself remains the same; what has changed is the arrangement of the individual grains. To see how this happens, think of a sugar bowl. If every crystal of sugar is evenly spaced, the ratio of crystals to empty space will be 60 – 40. That’s true no matter how big the bowl: 40% of the inside will always be empty.

Now put a spoon in and smooth the surface of the sugar: all those evenly-spaced crystals will be packed down into an uneven arrangement, filling the empty spaces. The result is still a 60 – 40 ratio, only now we have a lower level of sugar, with the empty space moved up top.

It’s the same with soil. Rainwater and passing feet press the soil down so that the grains lie closely together with little empty space, called “pore space.” When the shovel pulls the soil up and scatters it in a big pile, the soil grains fall into a pattern closer to the way they were before being packed down. Once the hole is refilled, all you have to do to make it level is get rid of that pore space — which can easily be done with the back of a shovel.

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