theoretical physics of dirt roads

Uurgh, I've misplaced my keys... I hate it when that happens -- it makes me feel like I'm going crazy or something! I haven't been anywhere, so their disappearance is all the more frustrating. Haven't I looked everywhere? S-:

Anyway. On a completely unrelated subject, here's a question for physicists et al: What causes the 'washboard effect' on dirt roads? I've heard a bunch of qualitative / anecdotal explanations, but I want to model the effect. The car, I think, is pretty easy to model. Mass, spring, damper, wheel. But how do I model a sandy road surface?

Links that might be useful:

• Animating Sand, Mud, Snow - from Prof. O'Brien, whose area of research is physics-based models for computer graphics

This goes on my personal to-do list of hard modeling problems... the other thing on it is to do a proper fluid-dynamical simulation of a lava lamp, without resorting to tracking blob surfaces or other empirically developed (non-physical) solutions.