Screen a parcel for expansive/shrink-swell clay risk using NRCS Soil Data Access linear extensibility (lep_r) data before scoping foundation waterproofing work
domain: sdmdataaccess.nrcs.usda.gov · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
POST a SQL query to https://SDMDataAccess.sc.egov.usda.gov/Tabular/post.rest with a JSON body containing 'query' (SQL) and 'format' (e.g., JSON+COLUMNNAME) parameters
Query the chorizon table joined through component/mapunit for the map unit(s) intersecting the parcel, pulling the lep_r (linear extensibility percent) field for horizons within 0-100 cm of the surface
Classify shrink-swell potential using NRCS National Soil Survey Handbook Part 618 breakpoints: low (<3%), moderate (3-<6%), high (6-<9%), very high (>=9%)
Alternatively, resolve the parcel's map unit key(s) first via the Spatial/post.rest spatial query, then join to lep_r in a second tabular query
Document the dominant component's lep_r value and drainage class in the pre-construction risk memo, noting this is a screening tool, not a substitute for a geotechnical report
Known gotchas
lep_r is reported per soil horizon per component; take the max value across horizons in the surface 0-100cm and weight by component percentage — a naive single-row pull can understate risk
SDA enforces a 100,000-record / 32MB JSON serialization limit per query — rarely hit for a single-parcel query but can bite region-wide screens
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