Seepage pressure

Seepage pressure is the pressure (force per unit area) exerted by water flowing through the soil on the soil grains in the direction of flow. For a hydraulic head loss $h$ over a length $L$, the seepage force per unit volume of soil is $j = i\gamma_w$, and the seepage pressure over the length $L$ is

$$p_s = h\,\gamma_w = i\,\gamma_w L$$

It acts upward in upward flow and reduces effective stress; it acts downward in downward flow and increases it.