|M.Sc Student||Nigem Piere|
|Subject||Investigation of Granular Pavement Materials Behavior under|
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Emeritus Jacob Uzan|
|Dr. Arieh Sidess|
|Full Thesis text|
Flexible pavements include granular layers paved above the subgrade and beneath the asphaltic layers. Granular layers serve as a support panel for the asphaltic layers and distribute, thus reducing, the traffic loads not absorbed by the asphaltic layers to the subgrade. Also, as the base for the asphaltic layers, granular layers have a crucial impact on the asphalt cracking development.
In general, granular material’s behavior under cyclic loading is divided into two components: elastic recoverable component and plastic irrecoverable component. In general, the elastic behavior is represented by the resilient modulus assuming the material is isotropic with constant Poisson’s ratio. However, loading tests for these materials suggest that for medium and high shear stress levels their behavior is anisotropic. Permanent deformation is usually represented via phenomenological models which don’t always represent granular materials behavior under real pavement conditions. This is illustrated via incompatible measured and model calculated (predicted) results.
The objective of this research is to develop a constitutive model for granular materials under cyclic loading. Elastic behavior is represented via nonlinear and anisotropic resilient modulus. Plastic behavior is described using a mechanistic-empirical model associating permanent strain with number of load repetitions, applied stress levels and loading history.
For achieving research objectives cylindrical specimens composed from base granular material were compacted and instrumented with vertical and horizontal transducers enabling vertical and horizontal deformation measurement under cyclic loading. Specimens were tested according to two loading tests: (a) for modeling anisotropic elastic behavior specimens were loaded under varying cyclic multiaxial loading sequences, with every sequence including different initial multiaxial static load followed by cyclic vertical and horizontal stresses with varying stress path each composed from 5 trapezoid cyclic loading (b) for modeling plastic behavior specimens were loaded under multiple load sequences with varying vertical deviatoric stresses while maintaining the horizontal confinement pressure constant. Each sequence included 10,000 haversine shaped load repetitions.
Analyzing test results and characterizing the elastic and plastic behavior using mechanistic-empirical models yielded the following results: (1) granular material’s elastic behavior is anisotropic (2) vertical and horizontal resilient modulus are nonlinearly stress dependent, increasing when amplifying confining pressure (3) vertical resilient modulus is indifference to shear stress, while horizontal resilient modulus decreases when increasing shear stress (4) vertical Poisson’s ratio value decreases when increasing the confining pressure and increases when increasing the shear stress (5) anisotropy level decreases (tending towards isotropy) when increasing the confining pressure and decreasing the shear stress (6) permanent deformation accumulation rate decays with increasing loading cycles (7) increasing confining pressure decreases permanent deformation due to hardening effect, while increasing shear stress increases permanent deformation due to softening effect (8) plastic deformation increases when the applied stress level is closer to the failure envelope.
The contribution of this research is a more rational and reliable characterization of granular material for elastic and plastic behavior. This characterization is essential for reliable and accurate behavior and performance predictions for pavements (where granular layers serve a crucial role) under cyclic loading.