This paper deals with the interaction between the geogrid and the tire chip-sand mixture including the determination of the index properties of the backfill materials, the shear strength parameters, the interaction coefficients, and the efficiency of geogrid reinforcements in tire chip-sand backfills. Numerous experiments including index tests, compaction tests, pullout tests, and large-scale direct shear tests were conducted. Saint-Gobain (geogrid A) and Polyfelt (geogrid B) were selected as reinforcing materials. Tire chip-sand mixtures with mixing ratios of 0:100, 30:70, 40:60, and 50:50 by weight were used as fill materials. The test results revealed that the dry unit weight of tire chip-sand mixtures depended more on the sand content, and less on the water content. The mixture at the mixing ratio of 30:70 by weight or 50:50 by volume was found to be the most suitable fill material compared to other mixing ratios. The pullout resistance and the pullout interaction coefficients of geogrid A were slightly higher than those of geogrid B. In contrast, in the direct shear resistance, the direct shear interaction coefficients, and the efficiency values of geogrid B were slightly higher than those of geogrid A. Since geogrid B has the needed uniaxial reinforcement properties and its sufficient interaction characteristics with tire chip-sand mixture, the geogrid B was utilized in this study. The interaction coefficients between the tire chip-sand backfill with 30:70 mixing ratio by weight were found to be 0.71 in pullout mode and 0.92 in direct shear mode for geogrid B. Crown Copyright © 2009.