It has been known that soil shear strength parameters are stress level dependent. On the other hand, foundation size has a significant effect on the level of imposed stress on subsoil elements. In this study, the Zero Extension Lines (ZEL) method, which has wide applications in determination of bearing capacity and load-displacement behavior of foundations and retaining walls, is employed to consider the stress level dependent nature of soil shear strength parameters to predict the actual bearing capacity of foundations. The ZEL equations which are capable of considering variations in soil shear strength parameters have been employed to consider their dependency on stress level and solved numerically by a computer code developed for this study. The presented approach has been compared with experimental data showing reasonable predictions when the effect of stress level is taken into account. It is then utilized to develop some design charts showing modified values of N (gamma) , as a function of foundation size and soil properties based on Bolton (1986) equation for stress level effect in cases of smooth and rough base foundations. The charts represent the decreasing tendency in N (gamma) with an increase in foundation size and it shows the decreasing tendency in the reduction rate when the foundation size increases.
