The phreatic surface in earth dams should be kept at or below the downstream toe. Thephreatic surface within a dam can be controlled by properly designed drain. This paperpresents a numerical evaluation of phreatic line location within an earth dam withdifferent drainage systems in both steady-state and rapid drawdown condition of reservoirwater table. Toe drain, horizontal drainage blanket and chimney drain effects on phreaticline have been studied. The results have been compared with the case of no drainagesystem. It is found that, in steady-state condition, the toe drain installation is just in aneffort to prevent softening and erosion of the downstream toe and its efficiency attenuateas the dam height increase. By the use of horizontal drainage blanket, the phreatic linerecedes from the downstream slope and when the chimney drain is installed the phreaticline tends to remain mainly in upstream side so the seepage will not continue throughoutthe embankment. In rapid drawdown condition, in which the important matter is porepressure dissipation within the embankment, it was observed that the phreatic line heightand its corresponding pore pressure are decreased faster in the dam with chimney drainthan the dam with toe or blanket drain.
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