The theory of Zero Extension Lines (ZEL), based on the solution of soil plasticity equations along ZEL directions, has wide applications in determination of the bearing capacity and load–displacement behavior of foundations and retaining walls. It is known that soil behavior and shear strength parameters are stress level dependent. In fact, a dense soil presenting a dilative behavior under low stress levels may show a contractive behavior under higher levels of stress. On the other hand, foundation size has a significant effect on the level of imposed stress on subsoil elements. In this work, the ZEL method is employed to consider the stress level dependency of soil strength in the bearing capacity computation and load–displacement behavior of foundations. A computer code is developed to solve ZEL equations in MATLAB. The results obtained by this numerical model have been then compared with experimental tests and those obtained by other methods.