I'm curious what you folks have to say about surfacing gravel roads. I know a lot depends on rock type. Around here we have roughly 3 types of rock used on roads. I am referring here to 1/2" to 1" minus with fines. 1. Hydraulic mine rock. This consists of 6" cobbles down to sand dug from the river bottoms where in some places it lies in enormous quantities. They crush the cobbles but a lot of the rocks are already smaller so they might only get broken in half once to make size. This leads to product with a lot of round edges which in my opinion might not lock together as well. Also I think it consists of a fair amount of river sand which is not all that locking, either, and the way the quartz rock crushes it also tends to lead to larger pieces. Largest quantity is made of this stuff, I guess because it is easiest to just scoop it out of the river? 2. Crushed granite. At least this stuff has all the edges not round. But again in my estimation the way the rock is internally granular causes the fines to be larger and the corners not so sharp. Also vehicles running tend to break off the corners of the rock. Not real common except in a certain radius from the plant that makes it. 3. Crushed blue rock. I don't know what type of rock this is but it is a uniform blue gray. When supplied with fines they are very fine and when compacted with water it almost looks like concrete. Of course to get it, it has to be blasted out of the mountainside and broken up to be crushed so it costs a bit more. Used on high-end projects it seems? They also make a product called class 2 aggregate base which I don't quite understand. It is product #1 above with a lot of brown dirt mixed in which is dug out of a hillside near the river. Instead of white in color this makes it very brown, at least until placed and rained on when it looks more white again. I wonder if they are adding the dirt to make it compact together like #3 above. I would think that if used as a wearing course it would be excessively dusty. Thoughts?
#2 or #3 would be best IMO but are probably the most expensive. #1 might be OK depending on how much sand is scalped off before crushing and the percentage of fractured rock against the percentage of unfractured. Mike
Being in a dry area, mixing some calcium in the top few inches would provide you with a lot of compaction potential over time. It helps the fines to really bind things together.
I spread gravel on driveway's everyday. Here we use crushed limestone and most often use #57 stone 1" to 1 1/2" in size
A lot of the choice will depend on what traffic it will see and what the underlying base is. We mine our own bank run gravel and it is fairly consistent in structure. Mostly 2" minus with very coarse sand mixed in. Rock content is high, but round. We strip to subsoil and grade in 12" or more on major roads. It packs quite well and stands up to semi truck/ trailer traffic. We try to crown/slope at least 6" on single lane for runoff. Problems- if the underlying base won't/can't drain it turns to soup in heavy rain until very well packed. Also tracked vehicles tend to bring big cobbles to the surface if overly dry. Not good for tires. A consistent sharp edged product with more of a binder in it would probably be better but this stuff is free. We have tried screening to 1" minus and top dressing with mixed results.