Testing and modeling of subgrade resilient moduli for Tennessee soils

A flexible pavement generally made up of three basic layer which are, the asphaltic concrete, the granular base and the subgrade soil. The subgrade layer has been found to contribute significantly to the total deformation of a pavement subjected to traffic wheel load. The resilient deformation of the subgrade layer has been found to a good indicator of the pavement performance. In order to accurately predicted the behavior of the in-service subgrade, the resilient modulus, a parameter obtained by the ratio of the deviator stress to the axial strain, has been accepted to indicate a soil's resilience response under repeated loading.

A laboratory testing program was implemented to determined the resilient moduli of widely used subgrade soils in the state of Tennessee. The tests were conducted according to the Tennessee Department of Transportation 207.04 specifications and the AASHTO T274 test method requirements. Standard laboratory tests were conducted to obtain the index properties of the soils. Using an X - Y plotter as data aquisition device, and cyclic triaxial test setup, repeated load tests were performed over a range of deviator stress to determine the resilient moduli of the selected soils.To characterize the resilient response of the soils, a plot of the resilient modulus versus deviator stress for each test specimen was obtained.

Modified hyperbolic equations were used to model the non-linear resilient modulus versus deviator stress response. Hyperbolic equations were also obtained for the stress-strain relationship for the unconfined compression test. Using the hyperbolic parameters from the repeated load tests as the dependent variables and the index properties and stiffness from the unconfined compression test as independent variables, regression equation were obtained for prediction of the measured curves. The breakpoint resilient modulus, E_ri, at 6 psi deviator stress, was also predicted for each measured curve selected since this indicator of resilient modulus is often in pavement design.