Seepage endangers the stability of earth dam built on pervious soil foundation because of piping, which depends upon:-

Seepage endangers the stability of earth dam built on pervious soil foundation because of piping, which depends upon:- Correct Answer Valve of exit gradient

Concepts:

When seepage water flows from a soil with relatively fine grains into a coarser material, there is a danger that the fine soil particles may wash away into the coarse material.  If the seepage that discharges at the downstream side of the dam carries particles of soil, an elongated cavity or “pipe” is formed toward the reservoir through the embankment, foundation or abutment. This is called internal erosion in earth dams or piping.

It can be prevented by the use of a filter or protective filter between the two soils.

The seepage water exerts a force at each point in the direction of flow and tangential to the stream line. For stabilization, the upward component of this force should be counterbalance by the weight of  earth dam. The force has maximum disturbing tendency at the exit end because the direction of this force is vertically upward at the exit end. The disturbing force at any point is directly proportional to the gradient of pressure of water at that point. This gradient pressure of water at the exit end is called as exit gradient.