Behavior of an Earth Dam during Rapid Drawdown of Water in Reservoir – Case Study

In this study, the finite element method is used to study seepage through the body of an earthfill dam. For this purpose, the software Geostudio 2007 is used through its subprograms SEEP/W and SLOPE/W. The water levels on the upstream and downstream sides, the properties of materials and boundary conditions of the dam were input variables and the water flux and pore water pressure were the target outputs. The Dau Tieng reservoir, which is located in Tay Ninh province in Southern Vietnam was analyzed as a case study. The rapid drawdown is simulated by means of the staged construction mode. It was concluded that the factor of safety against sliding of the dam slopes decreases slightly within the short period after the start of rapid draw down of water in the reservoir, then starts to increase. This is caused by dissipation of excess pore water pressure with time which leads to increase the effective stresses in the soil and hence increase its shear strength. The saturated weight of the slope produces the shearing stresses while the shearing resistance is decreased considerably because of the development of the pore water pressures which do not dissipate rapidly. When the stability of a zoned earth dam is examined during rapid draw down under different water levels in the reservoir, the insufficient stability may occur in the upstream slope as soon as the water is lower than the drawdown level of 1/3 of the dam height. It can be explained that the water load has disappeared during rapid drawdown and the hydrodynamic pressure creates the tensile-downward forces, resulting in a decrease of the shear resistance of the upstream slope. Besides, there is no supporting pressure to resist against mobilizing of the upstream slope.