Here is my $0.02.
I worked as an engineer in a rock quarry company. So here is what I can tell you about stability.
First, the guys that suggested the material you use have flat sides is exactly right. The flatsided rock locks against each other better. Rounded-sided rock will just roll past each other.
Second, the “dirt” or as we like to call it, fines. Is also a desired trait to your sub-base. This is because the fines settle in-between the larger rock and prevent the rock from moving around. Sand is a very subjective term and many here have already touched on the variation of the term “sand”. One word of caution is to not buy “sand” that is washed, like playground sand. This usually has the fines washed out of it so it does not “lock” into place.
Third, the suggestion to go to a local rock quarry is correct. We used to bag some of our product and sell it to the hardware stores. The price difference was a lot higher for bagged product. You will probably save 50-75% if you go to a local quarry and get a load.
Finally, the construction of your roadbed. Start with a solid sub-bed. Use a gravel or something that has a 1/4" and smaller, along with the fines in it. Lay this down and pack it down. I suggest 3-4" deep if you can. This layer is for the stability of your roadbed. Also, if its packed well, water will not soak in as much and thus cause frost heave. Then lay you track down. Dress up the top to look more like ballast with a small clean rock, with no or very little fines. The top layer is just for show. The sub-bed is where your stability and strength are.
Finally, a suggestion for the type of material for the sub-bed. I prefer limestone, although most think it is a pourous rock, it works very well at repelling water when packed in properly. Some granite material is very pourous, be careful what you use. The less pourous, the better to prevent water soaking into the base.
I lied, one more thing. If your doing a floating roadbed, expect to have to do maintenance. You will have to occationally replace the top layer of rock. Hopefully, if you’ve packed the sub-bed well enough you wont have to mess with it much. Make drainage ditches and make sure to install areas where the water can drain under your roadbed. Dont let it pool up next to the roadbed or you’ll have a washout. Think like the “big guys” they have many culverts and stuff installed to control water. You have to do the same. Good luck.
