Disastrous failures of shallow residual soil slopes

which result in great loss of lives and properties

are mostly attributed to heavy rainfall. To evaluate the stability of colluvial landslide during rainfall

a new analytical infiltration model for slope is derived from the traditional Green-Ampt (GA) model. The new model assumes the pore water flow parallel to slope surface in saturated layer above the wetting front

and overcomes the deficiency of GA model which is only applicable to the infinite slopes. In addition

a new method is proposed to calculate the safety factor for soil slice based on the shear strength criterion related to the depth of saturated layer

and in this method the safety factor is deduced by taking the seepage force into account. The results show that the size effect of rainfall infiltration is significant

and the movement rate of wetting front increases with the increase of slope length. The proposed infiltration model is identical with GA model in analyzing infinite slope

demonstrating that the GA model is a special case of the proposed model. It is shown that the stability of slope decreases sharply at the initial time of rainfall

but the rate of decrease slows as rainfall continues. The predicted results of the new infiltration model show a high consistency with that of slope model test

showing the reliability of the proposed method.