You could almost use the same formula that you would use for dirt. Look into motocross calculations for dirt. Also, another place you could look is golf calculations, because I believe they also use the sand traps. The worst you would have to do is actually compare the densities of dirt compared to sand if you could get the dirt calculations. You could weigh some dirt and weigh some sand and compact it. When you're hitting into the sand, it's like a more compressed dirt. Also, think of what sand is made of. It's going to be silicon and oxygen in those proportions of that density.