The Juarez-Badillo's Principle of Natural Proportionality establishes a relationship between natural strain and deviator stress, however in this work that principle is applied in a slightly different way, since strain is associated with the secant modulus in such a way that, by solving the corresponding differential equation, it is possible to unify the two branches of pre-peak and post-peak stress-strain curves in a single equation which depends on three parameters; the initial tangent modulus, the deviator stress failure in post-peak branch and the third, called by the authors “ductility exponent,” which controls the “form” and allows the description of the “decay” of the post-peak stress-strain curve. The proposed model allows the fitting the experimental stress-strain curves and secant modulus against axial deformation of different rockfills that were employed in the construction of the rockfill dam named “La Yesca”. Taking into account the low strain level of experimental tests due to devices limitations, in all tested materials was adopted a value of ψ = 2. The model was applied to settlements calculations of “La Yesca” dam using a commercial finite element program with light modifications in order to accommodate the proposed behavior model, and thus obtaining good results.