There are numerous slope failures caused by periods of heavy and intense rainfall in Singapore, Malaysia, China and other countries in the world. During rainfall, not only will the water content in the unsaturated vadose zone increase, but the ground water table in the saturated vadose zone may also rise as rain water infiltrates into the slopes. One of the methods to lower the ground water table and to maintain the matric suction in the slopes is by horizontal drains. To study the flow mechanism of water in the slopes with horizontal drains and to examine the effectiveness of the horizontal drains, a three-dimensional finite element computer programme for seepage analysis in unsaturated soils has been developed. The horizontal drains in the slopes are modelled as internal flux boundary conditions. The flow mechanism of water in the slopes with horizontal drains and the effectiveness of the drainage are discussed in detail using an idealised model slope in this paper.
