When I build or repair a mortarless stone wall, I fill the voids between the stones with gravel. This makes the wall more stable by locking the larger stones in place. It will also increase the mass of the wall — a wall built without mortar can be nearly half air space.
If you don’t “dry-grout” like this, nature has a way of gradually filling the voids with soil, roots, vines, mice and snakes. This organic material can often be the downfall of a dry wall. The gravel will also help to prevent the damage caused by frost heaving.
—Stephen M. Kennedy, Orrtanna, PA
Edited by Charles Miller
From Fine Homebuilding #23
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Seems obvious. The best dry stone walls have always been built this way. Shards and cut offs can also be used. The mantra is "avoid the voids." All the small pieces go in the middle.