I have clay now at my home in Los Angeles. This is a city lot that just happened to land in a bunch of what we call adobe.
Sunset Magazine is big out here because of its California focus. It has good sections on soil makeup/composition. The information is available elsewhere including the web.
But essentially, clay is very fine particles. Loam has a mixture of these fine particles along with organic materials of varying sizes. (I am over-simplifying here quite a bit.)
The 'fine' nature of those clay particles give it good and bad aspects. It holds nutrients, and it holds water, and it lacks oxygen. That first aspect is typically good for plants. The second aspect is typically bad for plants. Plants do need the water, but they will suffocate without the oxygen.
After years and years of trying to find a silver bullet for clay, I have concluded that there isn't one. For plants you need to kind of get the clay over into the 'loam' category. This means you have to add organic material. You have to add non-organic material of larger particle-size. Then you have to let all that stuff mix together. Then you have to let the organic stuff decompose over a period of time. Then you have to make sure that all of the chemical makeup is then OK.
Etc.
My personal experience leads me to believe that when you have clay, you have a never-ending condition that requires all of these activities of adding materials. It probably isn't REALLY never-ending. But it probably just takes a very very long time. Year 1 you add organics and larger particles. Year 5 you have to do it again because the 'clay' condition is back.
My other observation is that you are only able to physically add material to the topmost layers of clay. This causes problems itself. Even if you have a nice top layer of loam, because of years and years of adding material, there will be a layer of clay underneath which traps the flow of your water.
What I do now is try to find plants that are clay happy. Instead of trying to fool mother nature I just try to find plants that are happy in the conditions that I have. Over the long, long-run, this itself will change the soil a bit by adding roots of organic composition, and top layers of trees which eventually die and become decomposed into the new organic soil.
This is LONG, long-term!
Your question was actually about having clay under a road base. Can't help there. The only thing I'd like to build with my soil is an adobe house! /forums/images/graemlins/smile.gif But I see in this thread that folks have learned to work with it and have learned to appreciate its qualities.
Good luck.
