The SlabCalc Soil Risk Index

What Does Expansive Soil Do to a Slab?

Expansive clay changes volume with its moisture content: it swells as it wets and shrinks as it dries. That is a fundamentally different loading condition than the one most slab-on-grade design assumes. On stable soil, a slab settles a small, roughly uniform amount under its own weight and stays put. On expansive soil, the ground itself moves underneath the slab, and it rarely moves the same amount everywhere at once.

Heave is the upward movement that happens as clay gains moisture and swells; settlement, in this context, is the downward movement as clay loses moisture and shrinks. A slab does not need much total movement to crack — it needs differential movement, meaning different parts of the same slab moving by different amounts.

Two patterns of differential movement show up most often on expansive soil:

• Edge lift — the slab perimeter gains moisture faster than the interior, often after rain following a dry spell, and swells upward relative to a comparatively stable center. Cracking tends to run inward from the edges.
• Center lift — the perimeter dries and shrinks (common where paving, irrigation changes, or nearby tree roots draw moisture from the edge soil) while the soil under the center of the slab, insulated from the weather by the slab itself, stays wetter and comparatively swollen. The edges effectively settle relative to the center.

A slab acts as a partial vapor barrier: soil directly beneath it evaporates far more slowly than exposed soil at the perimeter. That difference — not the total amount of clay present — is usually what drives edge lift and center lift, and it is why seasonal moisture cycling around the slab perimeter (irrigation, drainage, gutters, nearby vegetation) matters as much as the soil's intrinsic swell potential. Keeping subgrade moisture uniform under and around a slab, rather than trying to keep it simply dry, is the practical goal on expansive soil.

ACI 332 (Code Requirements for Residential Concrete) and PTI DC10.5 (Design of Post-Tensioned Slabs-on-Ground) set out engineering procedures — stiffened foundations, post-tensioning, moisture-conditioned subgrade, edge and interior beam layouts — for building on expansive soil. This page does not attempt to reproduce either document; a geotechnical engineer or structural engineer applies them to a specific site and soil report.

Methodology

Area of interest: For each city, SlabCalc draws an 8 km bounding box centered on the same population centroid used by the SlabCalc Freeze-Thaw Index, and queries every USDA SSURGO map unit polygon that intersects it.

Area weighting: Each soil component's contribution is weighted by the acreage of its map unit that actually falls inside the box, multiplied by the component's percentage of that map unit. Weighting by each map unit's listed component percentages alone, without regard to how much of the map unit falls inside the box, would overweight components that happen to sit in a small sliver of a large map unit at the edge of the area — so this index always weights by intersected acreage first.

LEP (linear extensibility percent): Depth-weighted within each component over the top 100 cm of soil, then area-and-component weighted across the box, using only major components. This is a directly measured SSURGO value, not a SlabCalc estimate.

Dwelling-limitation index: The USDA NRCS engineering interpretation rating for “ENG - Dwellings W/O Basements,” area-and-component weighted the same way, on its native 0 (not limited) to 1 (very limited) scale. This is also a directly reported NRCS value.

Soil Risk Score (SlabCalc composite): The two measures above are real, independently reported USDA data. The formula that blends them into a single 0-100 score, and the band thresholds that label it, are SlabCalc's editorial composition, not a USDA, ACI, or PTI standard:

lepScore   = min(100, lepPercent / 12.0 * 100)
limitScore = dwellingLimitationIndex * 100
soilRiskScore = round(0.55 * lepScore + 0.45 * limitScore)

LEP is capped at a 12% reading for scoring purposes because SSURGO LEP values above 12% are rare and represent the far tail of expansive behavior; the cap and the 0.55/0.45 weighting between the two measures were chosen by SlabCalc to keep either measure from dominating the score on its own, not derived from any published standard.

Other fields: Dominant soil series, drainage class, hydrologic group, water table depth and flood frequency are area-weighted (numeric fields) or the dominant-condition value (categorical fields) across the same box, drawn directly from SSURGO component and map unit aggregate tables.

Source: USDA NRCS Soil Survey Geographic Database (SSURGO), via Soil Data Access (https://sdmdataaccess.sc.egov.usda.gov/), licensed Public domain (USDA NRCS).

Limitations

• Every score on this page is an area average over an 8 km box, not a site assessment. Soil composition varies enormously within a single city, and often within a single lot — a low-scoring city can contain individual parcels with genuinely expansive soil, and a high-scoring city can contain parcels with none.
• LEP is only reported and averaged to 100 cm depth in this index. Jackson, Mississippi is the clearest illustration: its LEP reads 2.08% (low, on its own), because its expansive clay sits below that 100 cm window, which is exactly why the dwelling-limitation index is carried alongside LEP rather than reported on its own.
• This index does not substitute for a geotechnical report. It is a planning-level screening tool built from public survey data, not a site investigation. Any project on soil scoring moderate or above should be evaluated by a geotechnical engineer against the actual soil at the actual building location.