Why do you use an open-graded stone? Won't the surrounding soils ravel into the voids?

Open-graded stone compacts quickly, and can be used even where some groundwater infiltrates into the drilled hole.  For movement of fines to occur, there has to be groundwater flow, and unless you somehow pump water from the bottom of the TerraPier, there is simply no significant flow.  As an indication, traditional stone columns have used open-graded stone for decades, and migration of fines has never been an issue.