Stockpiling of scrap tires in Macao brings out environmental and economical problems which may be resolved by using them as lightweight backfill material. The objective of this study is to investigate the mechanical behavior of tire chip-sand mixture, as a potential backfill material, based on laboratory testing. The tire chips are pieces of scrap tires cut into about 3 cm with specific gravity of 1.17-1.25. The stress-strain-strength and volumetric behavior and the strength parameters of the tire chip-sand mixture were determined in consolidated-undrained and drained triaxial tests. The variation of mechanical behavior with respect to the chip/mix ratio suggested the optimum mixing ratio to be about 40%. The test results were also used to determine the modeling parameters and to compare with the numerical simulations using the hyperbolic model. Recommendations for modeling the behavior of tire chip-sand mixture are provided. Copyright ASCE 2006.