Frost heave often causes structural damage to a stone wall. Erosion and the gradual deterioration of stones by freezing and shifting are other common problems.
Mortared walls are less subject to erosion because of their footing, but their mortar joints -- and the surface of the stones themselves -- can crack and take in water, then freeze and split. A damaged wall that doesn't receive timely attention poses a safety hazard.
You may be tempted to replace damaged stone with synthetic stone made from epoxy or cement-based materials. Such imitation stone is less expensive. Though the patches may look good at first, they will become conspicuous with further exposure to the elements and ultimately mar the appearance of the wall.
From 6 to 16 hours, depending on the nature of the wall, the extent of damage, and the kind of stone
Small sledgehammer, wooden wedges, tamper, 2x4 beater block, crowbar, marking chalk, cold chisel, spray bottle, pointing trowel, stiff-bristle brush, circular saw and masonry blade
Cutting and handling stone
Mortar, replacement stone (if required), gravel (for erosion repairs), carpet-covered 2x4 lumber
To return a popped stone to its original position, drive wedges between stable stones to take the weight off the popped stone. Work the popped stone out of the wall carefully, without dislodging the other stones. Drive the wedges into the wall slightly further.
Reinsert the popped stone in its cavity and tap it home with a carpet-covered 2x4 and a small sledgehammer. Use a crowbar to take the weight off the wedges and remove them. You may have to work the wedges and the crowbar back out of the wall at the same time.
A collapsed section of wall can often be repaired without total replacement. Inspect the wall and visualize a V-shape section that you'll need to remove. Mark the area on the wall with marking chalk. Don't use spray paint. Then number the stones with chalk so you can replace them to their original positions. Take a photo of the wall to further help you replace the stones in the right order.
Using your snapshot as a guide, rebuild the wall, working from the leads toward the center. Set the stones using the same techniques used to build it. Be sure to replace any small stones used to keep larger ones in place. Fill in the center recess between the wythes with cracked rubble. Replace the capstones.
Remove the damaged stone by chiseling out the mortar around the stone. Angle the chisel in the direction of the damaged stone to avoid damaging others. Then pry out the stone. Using a wider cold chisel, remove as much mortar as possible from the cavity. Brush or blow out the cavity to remove loose mortar and dust (wear eye protection).
If the stone is thin enough, cut it with a circular saw and masonry blade. Then using a mason's hammer, carefully chip the replacement stone until it fits the recess with enough space for the mortar joints. Be careful not to cut too much off the stone; doing so makes the joints wider than those on the rest of the surface and calls attention to the repair.
Mist the cavity and spread mortar on the bottom of the cavity and the top and sides of the replacement stone. Insert the stone and push it into place using the pointing trowel. Pack mortar against all sides of the stone. When the mortar has set, tool it to match the original.