<p>Carbonate sands are biogenic granular materials whose mechanical response is strongly governed by their high crushability, even under relatively low stress levels. This study proposes a critical state-based elastoplastic constitutive model that incorporates a simple stress-dependent breakage function to capture the influence of particle crushing on stress–strain behavior. The formulation features a dynamically evolving critical state line (CSL) that translates with the progression of breakage and a modified yield surface derived from the unified Clay and Sand Model (CASM), in which the shape parameters evolve as crushing occurs. The model includes thirteen physically interpretable parameters, calibrated using conventional oedometer tests, drained and undrained triaxial tests, and particle size distribution analyses. Its performance is assessed against published data for different initial densities, various loading paths (drained, undrained, and constant mean effective stress), and stress levels up to 8 MPa. Results show that the model successfully reproduces key features of carbonate sand behavior, including reduced dilatancy, strain softening, and breakage-induced CSL evolution. Overall, the model provides a compact and effective framework for predicting the mechanical response of uncemented carbonate sands under diverse loading conditions.</p>