Document Type : Research Paper
Authors
Department of Civil Engineering, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran
Abstract
Keywords
Numerical investigation of the influence of the combined seepage reduction scenarios on the hydraulic performance of the Alborz dam body
Extended Abstract
The value of hydrostatic pressure from the reservoir to the dam body and foundation leads to a seepage phenomenon, which is one of the significant issues in dam design. By analyzing the seepage of an earthen dam, the values of seepage flow, pore water pressure, and hydraulic gradient are determined at any point of the body and foundation of the dam. Due to the flow created under the effect of hydraulic pressure in the body of earthen dams, the internal flows increase over time and lead to erosion its downstream and ultimately the instability of the dam. Therefore, it is necessary to investigate various factors to reduce these effects. For this purpose, the solutions that can be effective in reducing the scouring phenomenon and flow seepage are the use of supplementary systems such as shear walls, clay blankets, and drainage. In the present research, the most optimal conditions for the combination of sealing elements have been used to reduce seepage.
SEEP/W model is based on the finite element method, which is prepared for modeling seepage and pore water pressure distribution in porous media such as soil and rock. SEEP/W software can be used for simple and complex seepage problems in saturated and unsaturated environments. In this software, the permeability of materials and the volume of water in the soil is a function of the pressure. In the analysis of models with a free flow surface and output-type boundary conditions, since both the location of the water outlet and the permeability coefficients at different points are unknown, so the program uses both trial and error inside. Thus, it first solves the problem with an assumed boundary condition until the permeability coefficients do not change at different points of the model. In this case, the first trial and error converged, and the program controls the boundary conditions with these assumptions. If the boundary condition is fulfilled at the water outlet from the model, the second trial and error is completed.
The clay blanket increases the length of the stream lines and as a result, reduces the hydraulic gradient as well as increases the potential drop and decreases the water energy. The length and thickness of the impervious surface depends on the depth of the water behind the dam. The results showed that the diversity in the type of flow seepage reduction systems has a significant effect on reducing the flow inside the dam body and foundation. So, increasing the length and dimensions of the seepage reduction parameters does not have much effect compared to the combined scenarios. Also, in two analytical areas for the hydraulic gradient of the clay blanket and drain and shield wall scenario, it has the highest reduction rate with an effect of 85%.
Although the use of one type of sealing system in earthen dams can play a role in reducing the flow, the main reduction of seepage depends on the combined use of other sealing systems. The presence of a drain alone is the main factor in reducing flow seepage, but to reduce the hydraulic gradient, it is necessary to use a shield wall along with a clay blanket. The length of the clay blanket is considered very short in the combined use of sealing systems; however, the results show that the combined issue of sealing systems has a more decisive role outside of the discussion of their size. While in the scenario of only clay blanket with a length of 385 m, it was the lowest type of system in reducing flow seepage.
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