Nondestructive evaluation of pavement performance by methods such as the Falling Weight Deflectometer (FWD) are widely used and accepted. Backcalculation and prediction of pavement layer moduli can be effective for both the evaluation of pavement condition and overlay design. However, variations in pavement temperature and subgrade moisture content result in variability in the backcalculated moduli.

In an attempt to evaluate the overall effect of this variability on design, pavement sections in four different regions of the State of Tennessee were instrumented and tested. The in-situ temperature and moisture content of the pavement and subgrade materials were continuously monitored for seasonal changes. Each site was also equipped with a weather station to record precipitation, air temperature, and solar radiation. FWD testing was conducted monthly at each site and the moduli for the pavement and subgrade layers were backcalculated using AASHTO recommended procedures. The seasonal variation in subgrade modulus was compared with measured changes in subgrade temperature and moisture content.

The observed variability in seasonal moduli estimates was used to support a generalized model for estimating seasonal changes in subgrade resilient modulus. A proposed design method that incorporates this generalized model with a relative damage or weighting factor approach is presented. This proposed method will enable the pavement designer to establish a reasonable design subgrade modulus based on limited test data.