The wet winters and silty alluvial soils of the southern Willamette Valley create a demanding environment for rigid pavement design in Eugene. With annual precipitation exceeding 46 inches and a shallow groundwater table across much of the valley floor, subgrade moisture content fluctuates dramatically between November and April. These seasonal swings reduce the modulus of subgrade reaction (k-value) and increase the risk of pumping at transverse joints and slab corners. Rather than applying a generic Portland-cement-concrete cross-section, our approach ties slab thickness, dowel diameter, and base course gradation directly to site-specific CBR road testing and consolidated-undrained triaxial shear data. The goal is a jointed plain concrete pavement that maintains load transfer efficiency through 30-plus years of service, even as the underlying silt transitions from moist to near-saturated conditions multiple times each winter. For projects near the McKenzie River or Amazon Creek corridors, where granular borrow is scarce, we also evaluate cement-treated subgrade alternatives that reduce the required base thickness without importing costly aggregate from the Coburg Hills quarries.
Eugene’s Willamette Silt subgrades lose over 40% of their resilient modulus between August and January. Rigid pavement design that ignores seasonal moisture cycling will fail at the joints within five years.
