Resilient Modulus of Minnesota Road Research Project Subgrade Soil

Abstract

Laboratory remolded subgrade soil samples have been widely used to study subgrade resilient modulus (M_r). But physical conditions, such as moisture content and density, of such specimens may not represent in situ conditions very well. Therefore, AASHTO and the Long-Term Pavement Performance program have recommended that undisturbed, thin-walled tube samples be used to study subgrade resilient behavior. The Minnesota Department of Transportation (MnDOT) is developing mechanistic-empirical pavement design approaches through the Minnesota Road Research project and has realized the importance of M_r in the design approaches. Currently, MnDOT is making an effort to study the M_r of unbound pavement materials through laboratory experiments. Under a research project at MnDOT, several thin-walled tube samples of sub-grade soil were obtained from six different pavement sections at the Minnesota Road Research project. Repeated loading triaxial tests were conducted on the soil specimens to determine the M_r at the MnDOT laboratory. Also, some soil properties, such as resistance, R-value, and plasticity index, were obtained. R-value is an indicative value of performance when soil is placed in the subgrade of a road subjected to traffic. Two constitutive models (the Uzan-Witczak universal model and the devia-tor stress model) were applied to describe the M_r. The objectives of the research were to compare these two well-known constitutive models in describing subgrade soil resilient behavior and to study the effects of material properties on the M_r. From the specimens tested, the experimental results showed that the universal model described the subgrade M_r slightly better than the deviator stress model, and the coefficients in these two constitutive models were found to have correlation to material properties. Also, no well-defined relationships between the R-value and the coefficients in the constitutive models were observed from the results of the tested specimens.

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