Fresno County sits atop thick Quaternary alluvial deposits from the Sierra Nevada, where smectite-rich clay layers often exceed 40% fines content at shallow depths. These expansive soils, known locally as 'San Joaquin clay,' demand precise Atterberg limits before any foundation or pavement design. The liquid limit in these deposits typically ranges from 45 to 75 — well into the high-plasticity CH zone per USCS classification — and even modest moisture changes trigger significant shrink-swell cycles. When the water table fluctuates between 15 and 30 feet below grade during the irrigation season, the plasticity index becomes a direct predictor of differential movement risk. For projects near the Fresno-Yosemite Airport or the expanding residential tracts of Clovis Unified School District, we combine ASTM D4318 with grain-size analysis to establish a complete soil profile, because relying solely on blow counts misses the volumetric behavior that cracks slabs and displaces footings across the Central Valley.
A plasticity index above 25 in Fresno alluvium means the soil can double its volume between dry summer and wet winter — a 100% swell potential that standard bearing capacity equations ignore.
