Use of shear waves to quantify local stiffness increase in geogrid-aggregate applications subjected to anisotropic loading

Unbound aggregate base and subbase layers in flexible pavements distribute the wheel load to protect the underlying subgrade. During pavement construction and subsequent trafficking, these granular layers are primarily loaded in the vertical direction to cause a direction-dependent stiffening different in magnitude than in the lateral direction. The geogrid application in the unbound aggregate layer is also known to increase the stiffness in the vicinity of the geogrid and provide lateral restraint. This paper combines these two important aspects and presents results from a laboratory study, which considered the application of shear wave transducers to aggregate specimens, with and without geogrid installed, when anisotropic loading was applied for the characterization of load-magnitude and load-direction related stiffness enhancement near the geogrid. Advanced triaxial test equipment, known as UI-FastCell, was used to apply the static confining pressures and dynamic deviator stresses to the triaxial specimen in both vertical and horizontal directions. A pair of bender elements adaptable to UI-FastCell as shear wave transducers was embedded into the triaxial specimens. According to the anisotropic modulus test protocol developed from the International Center for Aggregate Research (ICAR) studies, consisting of 30 different stress states applied, resilient modulus tests were performed on aggregate samples with and without geogrid installed in mid-specimen. After 25 repetitions of cyclic loading applied at each stress state, the shear waves were recorded. The test results revealed that under the anisotropic loading conditions, there was no significant difference in the values of anisotropic resilient moduli between geogrid reinforced and unreinforced specimens. In contrast, the shear moduli obtained from geogrid installed specimens were clearly greater than the shear moduli obtained from the no geogrid specimens. This paper demonstrates that local stiffness enhancement of aggregate material by geogrid can be adequately quantified under anisotropic loading by the use of shear waves.