August 1997

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R&D Award Winner Is Model of Excellence

Ray Bonaquist, Special Projects and Engineering Division, is the recipient of R&D's Outstanding Technical Accomplishment Award for 1996 for his paper, "A Comprehensive Constitutive Model for Granular Materials in Flexible Pavement Structures." This paper summarizes the results of a 5-year research program conducted in the Geotechnical Laboratory Complex of TFHRC to better understand and model the behavior of granular bases and subgrade soils.

The research program used plasticity theory to produce a comprehensive constitutive (stress-strain) model for these materials. The model that was developed is an extension of the effective stress principle and Mohr-Coulomb failure envelope commonly used in geotechnical engineering. It rationally considers both the resilient and permanent deformation response of granular materials and accounts for the effects of saturation and drainage.

The model represents a significant advancement over current granular material models. It can be used to predict pavement performance, to select layer thicknesses to resist permanent deformations, and to select material properties to ensure adequate performance. To permit easy application of the model in practice, correlations between the model parameters and the internal friction of the granular material were developed. Only conventional triaxial shear strength tests are needed to apply the model.

Permanent deformation is the primary distress mode associated with supporting layers in flexible pavement systems. An approximate method for using the constitutive model to calculate permanent deformations was developed. This approximate solution was used to develop minimum cover thicknesses to limit permanent deformations in supporting layers of flexible pavement systems.
-- Roy Trent.