Development of testing methods to determine interaction of geogrid-reinforced granular material for mechanistic pavement analysis

A new method of examining soil stiffness based on the propagation of elastic waves is proposed and compared to traditional resilient modulus tests. A laboratory testing program is undertaken to study the effect of changing bulk stress, strain level, and void ratio on the velocity of elastic waves. Using a proposed formulation, low-strain (~0.000001 mm/mm) moduli calculated with seismic methods are converted to higher strain (~0.0003 mm/mm) resilient moduli. Results of this study indicate that resilient moduli are approximately 29% that of the seismic moduli based on stress and strain. A simplified seismic testing scheme that can be used on the soil surface was developed and provides an efficient method to compare seismic and resilient moduli. The new proposed methodology allows for the characterization of materials containing large grains (>25 mm) (e.g., breaker run, pit run sand and gravel) that cannot be easily tested with the current resilient modulus methodology. Soil modulus and particle rotation were monitored using micro-electronic-mechanical-systems to determine the aggregate-geogrid interaction in base course materials. Velocity results indicate that the geogrid stiffens soil near the geogrid by a minimum factor of 1.3 (geogrid placed at a depth of 75 mm from the surface) to a maximum of 2.6 (geogrid at 100 mm depth). Rotation tests show a "zone of influence" no more than 50 mm on both sides of the geogrid reinforcement; however, the "zone of influence" depends on the position of the geogrid. Geogrid at 100 mm depth seems to be the most effective. Comparisons made with available field geogrid reinforcement cases support these findings.