In the Central Basin, the rock decides how deep the wheel goes
Nashville sits in a limestone lowland eroded out of the surrounding Highland Rim, and on much of Davidson County's residential ground the soil runs out before a grinder does. The dominant upland series is Talbott, whose USDA-NRCS Official Series Description is blunt: thickness of solum and depth to bedrock ranges from 20 to 40 inches, over residuum weathered from limestone, sometimes in karst topography. Davidson County also has mapped Talbott–Rock Outcrop complexes, where exposed bedrock is a named map component.
TN Tree Preservation, a Nashville operator, publishes 6 to 8 inches below grade as standard, and on ordinary Talbott ground that sits well inside the profile. On a rock-outcrop phase it may not: the wheel can reach stone while root wood is still in the hole. That matters most when the grind is making room for a replacement planting, because twelve inches is not promised county-wide here.
Two more things follow from the same rock: Talbott and Mimosa are fine-textured and slowly permeable, so a wet week packs clay into the wheel, and in karst terrain — which Tennessee's permanent stormwater manual gives its own design appendix — a chip-backfilled cavity can settle.