Seepage Analysis of Earth Dam Under Drawdown Condition

Abstract

Seepage analysis of earth dam is one of the major interesting points in water resource engineering. The amount of water seeping through and under an earth dam together with the distribution of the water pressure can be estimated by using the theory of flow through porous media. This theory is one of the most valuable analytical tools available to the engineer. In this research, the finite element method was used for solving the governing equation of flow within and under earth dam. The program Geo-Studio is used in this analysis via its sub-programs named SEEP/W. It is used to determine the phreatic line, amount of seepage within the dam and its foundation, the distribution of pore water pressure, the distribution of total head, and the exit gradient of homogenous earth dam. Al-Shahabi dam is taken as a case study. It is a homogenous earth dam located in Wasit, Iraq. The dam at actual design was analyzed.  When the reservoir is rapidly drawdown, pore water pressures within dam body are reduced in two ways: first way is a slower dissipation of pore water pressure due to drainage and second way is an immediate elastic effect due to the removal of the partial or total water head. During rapid drawdown, the pore water pressure at all points within the dam body decreases. Some points in the dam may be affected by negative pore water pressure during the period of water drawdown, this indicates that the water level becomes below these points. The exit gradient at the downstream of the dam almost reduces during the period of water drawdown, which means that the safety factor versus boiling rises with time. The rate of flow in the dam reduces with time; such decrease is caused by the rapid flow of water caused by emptying the reservoir in a short period.