Mixtures of shredded tyres with sand have become accepted geo-materials in the past decade, especially for use as fill materials. Owing to the large size of tyre shreds commonly used in practice, large-scale direct shear testing equipment is specially fabricated to obtain the shear strength parameters of mixtures. The effect of tyre shred-to-sand mixing ratio on the mechanical response of the mixtures is studied by performing direct shear tests on mixtures prepared at different mixing ratios. Out of the four types of samples tested, three (with mixing ratios of 20/80, 25/75 and 35/65 by weight of tyre shreds to sand) are prepared using 50 to 100 mm long tyre shreds and one (25/75 by weight of tyre chips to sand) using tyre chips with nominal size of 9.5 mm. The end-of-test friction angle of all the tyre shred–sand mixtures considered in this research is in the 30–338 range. Results of direct shear testing from the present study and other studies in the literature show that the shear strength of tyre shred–sand mixtures is greater than that of tyre chip–sand mixtures because the elongated tyre shred pieces are able to provide greater reinforcement effects than are possible with equidimensional (cubical) tyre chips. © 2019 Lippincott Williams and Wilkins. All rights reserved.