<p>Sandy pebble soil particles, shaped by their generational environment, exhibit high strength, large permeability coefficients, and significant size variability. These characteristics can lead to soil disturbance during construction, affecting its deformation and strength properties. This paper investigates the impact of coarse fraction content on the initial modulus and peak strength of sandy pebble soil through large-scale triaxial tests conducted under varying confining pressures and distinct coarse fraction contents. Based on the experimental findings, a modified Duncan-Chang (D-C) model is presented by incorporating coarse fraction content as a disturbance factor within the framework of perturbation theory. A specific disturbance function is derived to quantify the effect of particle gradation on mechanical parameters. Results indicate that while peak stress and initial modulus increase with rising confining pressure, they decrease as coarse fraction content increases when confining pressure is kept constant. The impact of coarse fraction content on peak stress is shown to be more significant than that of confining pressure. Comparisons among laboratory tests, the modified D-C model, and the original D-C model demonstrate that the modified model achieves higher consistency with experimental results. Furthermore, numerical simulations of tunnel excavation implemented in Abaqus verify that the modified model predicts surface settlement and surrounding rock deformation with greater accuracy than the traditional model. Finally, quantitative relationships between shear strength parameters (internal friction angle and cohesion) and coarse fraction content are established.</p>