Abstract: Rainfall and evaporation give rise to the change of moisture in unsaturated soil, hence resulting in the change of matrix suction. In view of this, we applied the water flow control equation to the calculation of Rankine’s earth pressure of unsaturated soil at steady state. Meanwhile, we overcome the shortcomings of Rankine’s earth pressure by deriving the formulas of active earth pressure and passive earth pressure under rainfall and evaporation conditions in consideration of the influence of intermediate earth pressure. Results showed that under the condition of rainfall, with the increase of parameter b and the decrease of rainfall intensity, the active earth pressure decreased gradually and the passive earth pressure increased gradually; while under the condition of evaporation, with a gradual increase of parameter b and evaporation intensity, the active earth pressure decreased gradually and the passive earth pressure increased gradually. Taking the influences of rainfall, evaporation and intermediate principal stress into account in calculating the earth pressure is more consistent with the actual situation, and could also give more play to unsaturated soil’s properties.

Key words: unsaturated soil, rainfall, evaporation, intermediate principal stress, soil pressure