What did I do wrong with my flag stone path install?

[JasperFrank]

I dug down 7 inches, put in a base of 6 inches 3/4 minus, then 1 inch of sharp sand and then compacted with a vibration compactor.
Then laid out the flag stones. And ran the compactor again with cardboard to protect the stone. But these there not very thick. At best, 2 inches thick, but most 1 to 1-1/2. We filled the gaps with what locally is called "fractured granite," that was supposed to harden over time, and then set everything in place...... according to the landscape place we buy stuff from.
It has not worked out. The winters just pop all the flags up with hoarfrost. We have no issues with 2 inch thick pavers or brick, which was used in other areas with the same base.
I really don't want to pull all these out and reset them. I've bought a whole lot of Ez-Sand.
But right now I really don't have a clue as to if this will work.
Any and all suggestions, are welcome. Even ones that say I'm an idiot.

 

[Torvy]

You forgot step 1.

Move south until you don't have to deal with frost heaves.

My less than expert opinion is that you need a deeper sand base to "absorb" the movement.

 

[Captain Dirty]

. . . The winters just pop all the flags up with hoarfrost. We have no issues with 2 inch thick pavers or brick, which was used in other areas with the same base.

Given that you say no problem with 2" thick pavers or brick in other areas, I would look closely for differences. The flagstone is as little as 1" thick. Cold from a heavy frost may penetrate the stone and freeze water in the "fractured granite" or "sharp sand". With less mass in the thin pavers, less force would be required to heave them.

The best way to prevent heaving is to keep the substrate "dry"; perhaps a coarser aggregate under the flags would allow water to flow away. Compacted sand may have so few voids that it drains poorly. I am unfamiliar with "fractured granite" which sounds like it could vary from boulder rubble to stone dust. Stone dust itself can vary from flour-like dust to coarse sand. Stone dust has a reputation of absorbing moisture readily but drying very slowly. In the small particles the fines fill tiny voids resulting in an almost concrete like surface that can be impenetrable to water. Coarser stone dust tends to get and stay wet, almost like a sponge.

 

[Rockbadchild]

Your base depth is solid, the problem is the thin, irregular flagstone combined with a soft top layer that holds moisture. That works fine in warmer climate but not with deep frost. Consider using polymeric sand for joints or mortar, replace sand and granite fill with quarry fines or "stone dust" and improve your drainage then you should be set.

 

[LXT]

All good advice above - with thin flagstone (in PA with the freaky weather) I/we would put the stone in a mortar bed - make it 4-5 inch thick and work it to level that you want, the mortar will dry & draw the moisture out of the flagstone.

this will greatly help with frost heave but not completely eliminate it if you experience a lot of wet weather & very cold conditions, that's the maintenance drawbacks with flagstone that's thin. hope this helps

 

[EddieWalker]

For soil to move, it needs moisture and freezing temps. The moisture in the ground expands when it freezes. The more moisture, and the deeper it goes, the more it will heave.

I think the problem is in your sand. It's holding moisture. I'm guessing that your rock bas and surrounding soil are holding the water in the sand. When it gets really cold, that moisture lifts up your flagstones.

Get rid of the sand and build up your rock base.

On commercial jobs, it's common to pour concrete under flagstones. This eliminates all the heaving issues that happen in winter.

And yes, it does get cold here in Texas in Winter. My personal record is -14F. But single digit temps happen just about every winter here.

 

[JasperFrank]

So "basically" - pun intended- you're all say'en I need to pull up, scrap the sand, and re-bed the flagstones in 1 inch wet cement. All 80 feet by 3 1/2 feet. Do I then use grout for the gaps, or will poly-sand work? And would annual sealing be recommended?

 

[IndyIan]

I guess you could try to improve the drainage around your path so the base isn't saturated with water when it freezes. How deep does your ground freeze? If its not too deep, and you have a place to drain water away, you could try a french drain down below the frost line on one side of your path to keep the soil and base under your path unsaturated and then it shouldn't heave when it freezes.

Frost Action – Pavement Interactive

Free online knowledge for the paving industry

pavementinteractive.org

 

[EddieWalker]

Since you already have a base of gravel, I would remove the sand and build up the gravel. I'm not sure what one inch of concrete would do under the flagstones. It might work better, but it's expensive.

 

[jyoutz]

I think the sand is your issue. Everything I’ve set on a sand base moves because sand really doesn’t pack well. Use crusher fines instead of sand. It’s just as easy to work with, but it packs and locks together much better than sand. Polymeric sand is good for filling gaps between bricks, but doesn’t really provide a solid base like crusher fines. Sand particles are round and never lock together; crusher fines are irregular and angular and they will pack solid.