Bender Element Shear Wave Measurement Based Local Stiffness Characteristics Related to Permanent Deformation Behavior of Geogrid-Stabilized Aggregate Specimens

This paper focuses on a laboratory study directed towards quantifying local stiffening of mechanically stabilized geogrid-aggregate specimens through shear wave velocity measurements using bender element shear wave transducer pairs and, at the same time, measuring permanent strain accumulations in repeated load triaxial testing. Crushed stone aggregate base material samples were prepared at three different gradations conforming to the upper bound, mid-range, and lower bound gradations of the Illinois Department of Transportation's CA06 dense-graded aggregates. All specimens were compacted at the target maximum dry unit weights and optimum moisture contents. Geogrid-aggregate specimens were stabilized with geogrids of two different triangular aperture sizes placed at mid-specimen height. In addition, unbound aggregate control specimens were prepared and tested for comparison. Three pairs of bender elements were installed above mid-specimen height for all the specimens tested with and without geogrids. Repeated load triaxial testing was conducted following AASHTO T307 standard test procedure for resilient modulus. Shear wave velocities and permanent deformation accumulations were measured at the conditioning stage and at each of the 15 applied stress states. The trends of permanent deformation accumulation in test specimens reveal that the engineered mid-range gradation specimen stabilized with the smaller aperture geogrid had the least permanent deformation accumulated when compared to other aggregate gradation and geogrid combinations. This was also in agreement with the largest shear wave velocities measured in the vicinity of the geogrid thus indicating that better geogrid-aggregate interlock is observed for densely packed engineered mid-range gradation specimens stabilized with the smaller aperture geogrid.