Triaxial and oedometer tests were used to demonstrate that a critical state framework can be applied to sand–rubber mixtures of similar soil grain and rubber sizes. It described well the behavior of a crushable sand and a quartz sand with either rubber fibers or granules of a variety of quantities, from small to large strains. Together with additional oedometer tests on soils of a wider variety of gradings, the work enabled the influences of sand particle type, grading, and rubber shape to be established. The sand particle type, specifically whether the grains were weak or strong, was found to be a key factor. It affected the yield in compression, even when large quantities of rubber were added. It controlled the critical state stress ratio, except for those mixtures with the highest content of rubber fibers, as well as the stress strain behavior. Sand particle type also determined the critical state line (CSL) location in the volumetric plane for lower rubber contents, but at higher rubber contents the behavior tended to converge for the two sand types. The grading and rubber type were not found to affect the compression or swelling indices significantly, which were mainly controlled by rubber content. Gradings that had nonconvergent compression paths without added rubber tended to retain this feature with rubber. The addition of both types of rubber led to higher volumetric compression in isotropic or one-dimensional compression but reduced volumetric strain during shear, altering the shapes of the state boundary surfaces
