The southern Willamette Valley floor, where Eugene sits at roughly 430 feet elevation, is underlain by thick Quaternary alluvial deposits of the Willamette Silt—a wind-deposited unit that can behave like fat clay when saturated. Any shallow foundation design here has to contend with plasticity indices that swing from 12 to over 35 depending on organic content and silt lensing. Our lab runs liquid limit and plastic limit determinations under ASTM D4318, delivering results that feed directly into Unified Soil Classification System (USCS) assignments. For deeper stratigraphy we often pair Atterberg data with grain-size analysis to bracket the silt-clay transition, and when near-surface bearing is marginal we reference findings from footings design to adjust embedment depth in high-PI layers.
A plasticity index above 20 in Willamette Silt signals moderate expansion potential—ignoring it can rack up six figures in slab repairs within five years.
