Exit gradients into subsurface drains

The exit gradients into drain pipes of various diameters and at various depths below the ponded soil water surface and with an impermeable barrier at varying depths below the drain were investigated by programming Kirkham’s equation for flow into drains under ponded conditions. It is shown that a quick condition or a weightless condition of the soil can result only under the drain lines. It is only at this point that the upward hydrodynamic seepage force is greater than the downward gravity force of the soil particle. The seepage forces result in an exit gradient that is higher than the usual critical exit gradient of one for a fine sand material without surcharge load. In almost every case, the critical exit gradient is exceeded with drain lines located 2, 3, and 5 feet below the water table and for drain diameters of 0.2-1.2 feet. These conditions will occur under irrigated agriculture during leaching operations and also may occur during the irrigation season. The effect of a surcharge load and the effect of the cohesive forces among the soil particles is discussed. As a result of the analysis, it is tentatively recommended that the gravel envelope be located beneath the drain rather than all around the drain as is currently practiced. In addition, the top of the drain can be covered with an impermeable material to prevent piping of soil into the drain during the settling of the soil in the trench. These findings are confirmed by the field observations of Pillsbury, 1967.