Abstract: Analyzing piles subjected to lateral loads significantly depends on soil resistance at any point along the pile as a function of pile deflection, known as the p-y curve. On the other hand, the deformation characteristics of soil defined as “the soil strain at 50% of maximum deviatoric stress (ε50)” has a considerable effect on the generated p-y curve. In this research, several models are proposed to predict ε50 specifically for designing very long pile foundations of offshore oil and gas platforms in the South Pars field, Persian Gulf, Iran. Herein, ε50 is evaluated from extensive soil data of marine clays including in-situ and laboratory test results using evolutionary polynomial regression (EPR). It is demonstrated that the normalized cone tip resistance, which is an indication of soil undrained shear strength, leads to more realistic ε50 values compared with the laboratory-derived undrained shear strength parameter. Furthermore, the results of a numerical study on the lateral pile-soil systems are used in order to show the efficiency of the proposed model in predicting lateral pile response.