Backcalculated and Laboratory-Measured Resilient Modulus Values

The resilient modulus (M_R) of roadbed soils is a necessary parameter in pavement design. The M_R of roadbed soils is dependent on the soil type, water content, dry density, particle gradation, Atterberg limits, and stress states. Several procedures can be used for the determination of the M_R: laboratory testing, backcalculation with nondestructive deflection testing (NDT) data, and correlations to other soil parameters such as the California bearing ratio, density, and water content. The first procedure is time-consuming and expensive and requires substantial resources to cover the various roadbed soils under the road network. The second is relatively inexpensive and fast and can be designed to cover representative soils under the pavement network. The third procedure is approximate and may be used for the Level 2 design of the Mechanistic–Empirical Pavement Design Guide. The Michigan Department of Transportation (MDOT) sponsored a research project for the development of a systematic procedure to determine the M_R values for all soil types encountered under the pavement network in the state of Michigan. The procedure includes laboratory testing of representative disturbed and undisturbed soil samples, NDT with the MDOT falling weight deflectometer, and developing correlations between the M_R values obtained from the two tests and between the M_R values and other soil parameters that can be obtained using simple tests. The developed correlation equations between the M_R measured values and the other soil parameters are presented elsewhere, whereas the correlations between the laboratory-obtained and the backcalculated M_R values are discussed in this paper.