Abstract: The precipitation is one of the important factors that triggering the slope failure. Therefore, a welldesigned drainage system is needed to manage the seepage flow and to stabilize the slope. The seepage characteristics and stability aspects of a typical residuum slope of granite origin under different drainage patterns are analyzed by using the saturated and unsaturated seepage theories. The results indicate that a critical precipitation intensity exists in the seepage drainage patterns. If the rainfall intensity is within this value, the maximum pore pressure at the shallow depth and the groundwater table are increasing with the duration and the precipitation intensity. The seepages are concentrated at the toe of the slope. The location of the horizontal drainage pipes has significant impact of the global stability of the slope. The optimum length of the horizontal drainage pipe should be the horizontal projected length with a 10 degrees inclination angle. The double layered drainage system will result in a better solution.

Key words: Unsaturated Residual Soil, Precipitation, Seepage, Horizontal Drains