A Casagrande cup on a lab bench in Irving spins at exactly 120 drops per minute, cutting a groove through remolded soil until the two halves close over half an inch. That routine, governed by ASTM D4318, gives us the liquid limit, one of three numbers that form the Atterberg limits. We run these tests on samples pulled from depths between three and fifteen feet across the city, from the alluvial terraces near the Trinity River to the stiff residual clays underlying Las Colinas. The plastic limit comes from rolling 3 mm threads by hand; the plasticity index is simple subtraction. Together, the three values bracket the moisture range where Irving soils behave plastically. For any earthwork contractor dealing with the expansive Eagle Ford Shale or terrace deposits that lose strength after rain, these limits are not academic, they determine whether a fill lifts pass or fail. Many of our Irving projects combine Atterberg limits with a grain-size analysis to confirm the fines fraction driving the plasticity, especially when the Unified Soil Classification System letter symbol will appear on the geotechnical report cover sheet.
In Irving's Eagle Ford clays, a plasticity index above 25 signals high shrink-swell potential that no foundation engineer can ignore.
