<p>This paper presents a method for calculating the lateral load capacity of flexible piles based on the state-dependent Mohr-Coulomb (SDMC) model combined with cylindrical cavity expansion theory. The soil surrounding the pile is idealized as a cylindrical cavity, and the stress-strain behavior of the soil during monotonic lateral loading of the pile is described using the cavity expansion model. By incorporating the stress equilibrium equation, displacement compatibility conditions, boundary conditions, and the constitutive model of the soil, a system of governing equations consisting of partial differential equations (PDEs) is derived. These equations are solved to obtain monotonic p-y curves that capture elastic, elastoplastic, and critical processes. Subsequently, the deflection equilibrium differential equation is introduced to transfer the lateral load from the pile top to the tip, and the resulting system of equations is solved numerically using the finite-difference method. The accuracy of the cylindrical cavity expansion theory integrated with the SDMC model is verified through finite element method (FEM) simulations. Furthermore, the validity and capacity of the proposed approach in predicting the monotonic load-displacement response of piles are demonstrated by comparing the results with existing centrifuge test data. The finding confirm that the proposed method effectively captures key phenomena observed in pile load test.</p>