The capillary barrier concept, using fine and coarse soil layers to reduce water infiltration into piles of hazardous wastes, is investigated theoretically. A detailed account of the hydrological and physico-chemical basis for the phenomenon is given. It is established that the capillary barrier will, in practice, only function if the fine layer remains somewhat unsaturated, i.e. the upper menisci exist and no ponding over the fine layer occurs. Accordingly, water reaching the fine layer must be transported laterally within this layer. The pressure conditions are dependent on the length of the interface, the slope of the interface, the thickness of the fine layer, the type of soil in the fine layer, the water influx at the surface, and the total volume of water infiltrated during an infiltration event. A simple estimate shows that the capillary barrier concept is feasible only for small heaps with steep interfacial slopes.