Subgrade Resilient Modulus Correction for Saturation Effects

Mechanistic design methods for flexible pavement require the specification of the resilient modulus Mr for each soil subgrade encountered in the pavement project. The resilient modulus is usually measured in the laboratory under conditions of maximum dry density and optimum water content to reflect the conditions under which subgrades are usually prepared. To account for the seasonal variation of subgrade moisture content in the design process, a single value of Mr known as the “effective roadbed soil resilient modulus” is often used in design. A rational procedure for the determination of the seasonally adjusted resilient modulus is not described, although the seasonal variation of subgrade water content can be estimated with a reasonable level of confidence. A series of resilient modulus tests was designed to investigate the variation in Mr due to postcompaction increases in water content. Triplicate specimens were prepared for 11 soils throughout Tennessee, with each specimen prepared at target values of optimum water content and maximum dry density. One specimen was tested at optimum, and the other two were tested at increasing levels of saturation. All soils exhibited a decrease in resilient modulus with an increase in saturation, but the magnitude of the decrease in Mr was found to depend on the soil type. The soils with the highest Mr for optimum conditions were found to experience the greatest decrease with saturation. A method for correcting the resilient modulus for increasing degree of saturation is proposed. This method supports the procedure described by AASHTO, but an estimate of the monthly or seasonal variation in water content or degree of saturation is required.