Atterberg Limits Testing in Lexington: Plasticity, Moisture, and Soil Classification

Standard Atterberg limits determination — including liquid limit by multipoint Casagrande cup, plastic limit by thread-rolling, and plasticity index — typically falls between US$50 and US$90 per sample. The exact cost depends on sample condition, whether the material requires wet preparation (common in organic silts from the Inner Bluegrass), and whether the test is bundled with a full particle-size analysis. Rush turnaround may carry a surcharge.

The liquid limit is the moisture content at which a soil transitions from a plastic to a liquid state, determined by the Casagrande cup method at 25 blows. The plastic limit is the moisture content at which the soil begins to crumble when rolled into 3.2 mm threads. The difference between the two — the plasticity index — defines the range of moisture over which the soil behaves plastically. A PI below 10 indicates low plasticity silts; above 30 suggests highly plastic clays common in Lexington's weathered shale residuum.

Kentucky's residual soils, derived from limestone and shale weathering, exhibit significant shrink-swell behavior that directly affects foundation performance. The plasticity index quantifies this potential: soils with PI above 25 typically require moisture-controlled backfill, deepened footings, or lime stabilization. The Kentucky Building Code (IBC-based) requires soil index properties for all structures in expansive soil zones, and Lexington's karst geology places much of Fayette County in this category.

ASTM D4318 is the primary standard for determining liquid limit, plastic limit, and plasticity index. Soil classification using these results follows ASTM D2487 (Unified Soil Classification System), which employs the Casagrande plasticity chart to assign group symbols. For Kentucky Transportation Cabinet projects, the companion AASHTO T 89 and T 90 methods may also apply, though they are technically equivalent to the ASTM procedures.

For a typical Lexington building site, we recommend a minimum of three representative samples from each distinct stratum encountered. The residual soils overlying the Lexington Limestone can transition from CL to CH within short vertical distances, and a single determination can be misleading. For linear projects such as roadway subgrades, spacing samples every 500 feet along the alignment, with additional samples at obvious soil color or texture changes, provides defensible classification coverage.