October 4, 2012
Despite Paul Newman’s charisma, it’s tough to miss Hollywood’s swell sleight of hand.
By Bill Langer
My wife, Pam, and I recently watched the 1967 movie, Cool Hand Luke, starring Paul Newman. There is a scene where Luke is shown a piece of ground and is told to get his dirt out of Boss Kean’s ditch. Luke was then made to dig and fill in the same grave-shaped hole time and again. I noticed that after refilling the hole, there was no dirt left over. All the dirt that Luke dug out of the hole fit back into it.
As a geologist who has mapped many a sand and gravel deposit, I have dug hundreds of test holes. As I recall, every time I filled in one of those holes, I had some dirt left over.
There is a good reason for this. Nature neatly placed the soil in the ground with very little space between soil particles. While digging the hole, I physically broke up the soil into particles and clods of various sizes. That created air pockets that were not present in natural soil, and resulted in an effective increase in the soil’s void volume and a decrease in density.
The engineering term for this phenomenon is “swelling.” Whenever I dug a hole, the sand and gravel taken out of the hole “swelled.”
Swelling also happens on a larger scale when an aggregate operator excavates sand and gravel or drills and blasts crushed stone. The same weight of sand and gravel or blasted rock in the truck or stockpile occupies more volume (loose cubic yards) than it does in the ground (in-situ cubic yards).
It is important to know how much aggregate will swell when it is excavated in order to determine how much space is needed to accommodate the excavated sand and gravel or blasted rock, and the subsequent hauling and stockpiling requirements. The amount of swell depends on the kind of material, the way it was loosened (excavating, blasting, ripping, and so forth), and the size of the particles after the material has been loosened. One of the biggest factors for crushed rock is how it is shot. If the shot pattern is too large, there will be more oversize material, which generates many voids, reduces the load volume, and increases stockpile requirements.
Aggregate operations can calculate the swell factor of their product as follows:
If you are opening a new quarry, have no first-hand knowledge of your rock properties, and need to estimate how much your aggregate will swell when excavated, swell factors for common materials can be found in engineering literature and on the Internet. Cast around a bit. What you will probably find in most sources is that sand and gravel will have a swell factor somewhere between 10 and 30 percent; blasted, hard igneous and metamorphic rocks will be between 65 and 80 percent; and blasted, softer rock will be between 50 and 70 percent. The dirt Luke was digging in (similar to overburden) will range between 40 and 50 percent.
When I tried to explain all this to Pam, she replied that I was insensitive to Luke’s suffering. She said the scene turned her stomach, and I should not trivialize it with geologic nonsense. Then she hit me with her best shot, quoting the most famous line of that movie (or perhaps any movie).
“What we have here…is failure to communicate.”
Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at