Eugene's position at the southern end of the Willamette Valley presents a specific challenge for rigid pavement design: the underlying geology is dominated by deep alluvial silts and clayey silts deposited by the McKenzie and Willamette rivers. These fine-grained soils are highly susceptible to moisture changes, swelling when wet and shrinking during the dry summers. A concrete pavement placed without a thorough geotechnical analysis of this subgrade will inevitably suffer from pumping, faulting, and uncontrolled cracking. The design of a rigid pavement here must therefore integrate a mechanistic-empirical approach, correlating the concrete slab's flexural strength with the resilient modulus of the locally stabilized subgrade. We correlate CPT data to estimate the modulus of subgrade reaction and verify it with triaxial testing to model long-term deformation under traffic loads.
A rigid pavement in the Willamette Valley is only as durable as the drainage layer beneath it; controlling the subgrade moisture regime is non-negotiable.
