This study investigates the mechanical behavior of gravelly soil under various confining pressures using large-size triaxial cyclic tests and a novel constitutive model. Key properties analyzed include stress-dependent dilatation, nonlinear strength, cumulative plastic strain, cyclic hysteresis, hardening, and particle breakage. Experimental results show that confining pressure significantly affects volume deformation, strength, and failure modes. Specifically, volume deformation shifts from dilatation to contraction with increasing pressure, and failure modes transition from drum-shaped to compressive shear. The developed model integrates stress-dilatancy equations, plastic flow directions, and plastic moduli within the critical state soil mechanics framework, effectively capturing cyclic loading and unloading behaviors. A particle breakage index and a differential equation for void ratio evolution are included to reflect relative density changes. The material constants of this constitutive model are derived from large-size triaxial cyclic tests. The model's material constants are derived from large-size triaxial cyclic tests. Comparison with experimental data confirms the model's accuracy and potential applications in stress path analysis and complex engineering projects, demonstrating its adaptability to varying mechanical stress conditions.