What Shelby County, IL's soil rates for a septic system
Very limited
Soil across 95.5% of Shelby County, IL rates "Very limited" in USDA's septic-absorption-field interpretation. Practically, that rules out a simple gravity system for most of the county - 95.5% of it sits in USDA's most-constrained bucket. Shelby County, IL is 758.508 square miles with a population of 20,542.
What Shelby County's reading doesn't cover
USDA's survey covers the whole of Shelby County, IL at once, not any single address. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Shelby County will actually do.
What a system would cost here
With 95.5% of Shelby County rating "Very limited", a standard gravity system is not a safe assumption here. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.
How Shelby County, IL compares
Nationally, Shelby County, IL is the 600-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 29 of 101.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems