Recycling or reuse of scrap tires in civil engineering projects has gained significant research attention in recent years. Experimental investigation reports suggest that inclusion of tire chips in sand increases the shear strength and decreases the dilatancy of sand implying the promising and environmentally sustainable use of sand–tire chip (STCh) mixture in civil engineering projects. However, only a few constitutive models have been developed to predict the monotonic behavior of STCh mixtures. In this paper, a semiempirical constitutive model has been developed using the critical state framework, which takes into account the constant stress ratio and state parameter (ψ) to capture the key features exhibited by the STCh mixture during shear loading. The model has been validated with extensive laboratory investigation results. It has been shown that the model can effectively simulate the monotonic behavior of the STCh mixture.