I just caught an episode of Modern Marvels called "Dirt" that was interesting. One topic focused on dirt construction and provided a good overview of several techniques like adobe, tamped walls and construction using old tires, tamped dirt and aluminum cans for fillers. I watched it on Hulu.
Of course I was disappointed to learn the Chicago Knockers have had to wrestle in cake batter rather than real dirt! Sort of shatters my image of a fave sports team.