The idea of going 'below the frostline' is really something that has more to do with the construction of habitable space...like a building of home. It's really quite amazing how much power can be put forth by water in the freeze-thaw cycle. Foundations that don't extend below the local frostline can, indeed, be heaved by the freezing of groundwater. And then, as the water thaws, whatever has been lifted will settle back, but not always to the same place it was originally. And it really only takes a little bit of vertical movement on a foundation to crack the concrete, the sheetrock walls, the ceilings, etc. It doesn't take much at all. And, as anyone who has been faced with it knows, the costs of repairs to such heaving can be outrageous.
Walkways and patios aren't looked upon with the same serious concern as the house, even though costs of repairs after heaving can be hefty. After all, it's only a walkway, right? Well, perhaps not if it's 'your' walk, and 'your' money.
Normally, bed preparation for a walkway doesn't exceed 12" or so. Dry set walkways should be founded upon material that is free-draining, which really just means that water won't just sit there...it will drain 'through' the material and keep going...to where ever it's going. The dryer the material in the winter, the less likely it will be to heave. Around here, we use a range of materials for the base beneath a dry-set design. Gravel (around here that a native mix of sand and small stone), and crusher-run (crushed stone that contains both fine and coarse aggregate all being angular, which compact better) are the most common materials. They're both free-draining and they can both be compacted very tightly. Sand is sometimes used by some folks, but as the tiny particles of stone that make up sand are mostly rounded in form, they tend to not compact as will as the others (in a sense it can be like trying to compact ball bearings). And there's crushed stone, which has a variety of sizes (usually 1/2" to 3/4" or larger) but no finer particles. It can be compacted well enough, and it's free-draining. But as it has only larger particles, finer material placed on top will eventually filter down into it, allowing the surface to drop.
I like crusher-run myself. A vibratory plate compactor (the size of a lawn mower) can compact it quite well.
For mortar set stonework, bluestone or granite, a cast-in-place concrete base is best. If I were doing it, I'd place some standard concrete reinforcing wire right at mid-depth in that slab to keep it from moving when it cracks, and it will crack. On top of that slab, the mortar setting bed can be installed, and then the stones set in the bed.
As to the best stone...it's a toss-up and really has more to do with your taste than the strength of the stone. Both bluestone and granite are noteworhty in their ability to endure New England winters. Bluestone will be found most often in a 1 1/2" thickness, although thicker is available (for extra cost). I've found the standard thickness to be quite adequate. But note that bluestone, while cut to exacting sizes for length and width, will not be entirely uniform in thickness, so if you're picking it yourself, I'd tend to stay on the thicker side, at least 1 1/2 inches. Same with granite. The length and width of the individual stones will depend on 2 things...your design, and the strength of the installer. The bigger the stone, the heavier and harder to handle. If set correctly, the size should not be a concern for movement. The largest regularly available size in bluestone is 30"x30". I'm guessing that granite is about the same. Sizes change in 6" increments. Your 'pattern' will determine size.
AS to whether mortar is better than dry-set, it really depends on quality of installation. I've seen mortared installations come apart quite readily when the slab and setting beds weren't constructed properly. And I've seen some that will last forever. Same with dry-set. If the base is set on uncompacted soils that are very wet, it doesn't matter what you do on top. It will come apart. I've also seen dry-set installations that will last a very long time.
Dry-set installations can see weeds growing in the joints, as weeds will grow anywhere. They will also need periodic addition of joint materials as the weather and foot traffic can move them. Concrete installations may be more costly.
The 'look' is what you should determine first. Then find a competent contractor to work.
Regarding the movement of irregular bluestone, it could be a year or two, or a week or two before you see movement. As with the information above, bed preparation and compaction will be the keys.Making sure that the stones are completely bedded without air pockets beneath them is also important. Going with a thicker and heavier stone will also help to prevent the stones from moving due to foot traffic. Thickness at the edge is a good thing to look for in a stone, too. The more the edge maintains the thickness of the stone the better.
As for tumbled pavers, I can't say, for I've never used the...and probably won't. They're just not my style.
Q
