The conventional earth pressure analysis method only considers the shear strength of saturated soil

which ignores the influence of matrix suction as well as its change. Based on the principle of effective stress of unsaturated soil and Rankine's earth pressure formula of saturated soil

the Rankine's earth pressure formula of unsaturated soil is deduced. The earth pressure formula of unsaturated soil under the condition of rainfall is established by the combination of the formula derived above and Iverson’s rainfall infiltration analytical solution. In the formula

earth pressure of unsaturated soil under rainfall infiltration is expressed as a function of time and depth. The study result shows that the earth pressure value calculated by the improved method is relatively larger than the conventional calculation and its action point is higher. With the occurrence

infiltration and stop of rainfall

the value of active earth pressure is at a state of “decreasing

increasing

decreasing and lastly stable”; but the value of passive earth pressure presents a tendency of “increasing

decreasing

increasing and stable”. The phenomenon is due to the change of matrix suction during rainfall. The distribution and change rule of earth pressure calculated by the proposed formula can be applied to design of retaining engineering structure.