Unsaturated Drainage Layers for Diversion of Infiltrating Water

An approach for draining water from unsaturated soil is presented that utilizes a sloping system of a fine sand overlying a coarser sand or gravel. In this configuration, referred to as an unsaturated drainage layer, the sand laterally drains water that accumulates above the capillary break provided by the coarse material. Drainage capacity is maximized when the sand layer has sufficient moisture to be relatively conductive, yet remains unsaturated so as to prevent failure of the capillary break. Results from field tests indicate substantial downward infiltrating water can be laterally diverted with an unsaturated drainage layer. Numerical simulations are presented that illustrate the potential effectiveness of unsaturated drainage layers to divert sufficient infiltrating water and prevent the development of positive pore water pressures.