Severe risk About 66% of Montgomery County's soil area is high shrink-swell (expansive) clay — 2.6 times the Illinois average of 25%, and 4.0 times the national average of 17%. That places it #1 of 102 Illinois counties for foundation soil risk.
Share of the county's ~450,508 acres of USDA-mapped soil with linear extensibility ≥ 6% in the top meter (SSURGO).
| High shrink-swell (expansive) clay | 66% |
| Moderately expansive | 22% |
| Low / non-expansive | 12% |
| Foundation risk tier | Severe |
| Rank in Illinois | #1 of 102 counties |
| Higher-risk than | 97% of all U.S. counties |
Expansive clay swells as it takes on water and shrinks as it dries, and that repeated movement is what lifts and drops a foundation unevenly — opening stair-step cracks, racking door and window frames, and, left unmanaged, cracking slabs and footings. Montgomery County's exposure is extreme. In a county this exposed, water management is the highest-leverage thing a Montgomery County homeowner controls: gutters and downspouts that carry roof water well clear of the slab, positive grading away from the house, and — most of all — consistent soil moisture through drought, because it is the wet-to-dry swing that cracks a foundation, not moisture itself.
Montgomery County's shrink-swell risk is concentrated in the Herrick soil series alongside Virden and Oconee — clays the USDA maps as strongly expansive, swelling and shrinking with every wet–dry cycle. Homes built on these series most need the drainage and moisture discipline above; a lot-level soil report (or the county NRCS survey) shows which one sits under a given address.
| County | High-risk soil | |
|---|---|---|
| This county | Montgomery County (#1 of 102) | 66.1% |
| Lower risk → | Jasper County | 59.3% |
For context, the average Illinois county is 25% high-expansive soil and the average U.S. county is 17%.
Foundation repair is one of the most over-sold jobs in home services — quotes for the same house can vary 3×. Before you sign anything, learn how to get honest bids and what a fair price looks like.
How to get repair quotes →Costs follow the same structure everywhere — from a few hundred dollars for a single crack injection to $8,000–$25,000+ for pier stabilization on a settling home. Because expansive clay drives recurring, moisture-linked movement here, correcting drainage first often heads off a far larger repair later. See the full foundation repair cost guide for method-by-method pricing.
Risk metrics are computed from USDA SSURGO soil survey data (linear extensibility of soil components, area-weighted by county). Soil varies lot to lot — this is county-scale context, not a substitute for a site-specific geotechnical or structural assessment.