Numerical investigation of seepage in tailings earthen dam with clay core

Seepage is one of the most important dam stability problems that could not be explained fully in detail. Seepage is identified as the natural way of water from upstream to downstream of a dam along the dam body. It has significant impacts on dam life and dam stability in both water retention dams (WRD) and tailings dam directly, e.g., internal erosion and piping are observed through the dam body. "Tailings" or "tails" is the name given to fine-grained industrial wastes commonly, though not exclusively, produced as a waste product from mining and quarrying activities. The particles range from sand to clay-size fraction. The technology of tailings dam design is based on the same geotechnical principles as water dams, however the presence of saturated tailings solids as the stored medium, versus water only, presents unique challenges and design benefits. The tailings dam is an embankment designed to enable the deposited tailings to settle, and to retain tailings and process water. The gradation of mine tailings typically varies from silty medium fine sand to clayey silt. Tailings dam design sections vary considerably. For example, the dam can be made entirely out of unprocessed tailings with upstream construction, or the dam may be made out of borrow material with little or no reliance on the tailings. Impounded tailing solids have hydraulic conductivity and shear strength properties that can be used to the advantage of the designer. On the other hand, sulphide rich mine tailings have a potential to oxidize and leach metals through acid rock drainage. Seepage control, for environmental, becomes a critical design parameter which can lead to much lower tolerances for seepage losses from the impoundment, compared to water dams. Tailings dam differs from WRD in many cases. Design, construction, operation, and closure of tailing dams have some fundamental differences when compared to conventional water storage dams. One of the significant differences is life time of the dam. WRD's design life is finite, on the contrary a tailings dam needs to be stable over long time periods even after the closure of mining activities. Furthermore, leakage in tailings dam would be hazardous for environment because of deposition of mining waste material. Therefore, seepage is a very crucial design criteria for tailings dam. However, the number of studies aiming at understanding the seepage mechanism of tailings dams is limited in literature. In this work, seepage through a typical tailings earthen dam was investigated using a mathematical model-spreadsheet analysis- based on the finite difference solution method. The spreadsheet analysis is a very powerful tool with a wide range of mathematical related capabilities. It provide a comprehensive and practical tool for organizing numbers and performing numerical calculations. For the finite difference application a template is created to include the problem domain and all applicable boundaries. As the boundaries, the base of the dam and the impervious boundary with limitation to only straight horizontal and vertical lines can be included in the template. The finite difference grid with the value of potential or head at each node can also be combined in the template. Finally, the template become a ready to analysis and solve the system. These steps make twice to calculate streamlines and potential lines values. After these calculations, the streamlines and potential lines graphics are drawn according to the number of iterations. In this study, seepage problem analyzed by making 100 iterations. Furthermore, a couple of different iterations were made and these solutions were shown in appendix part. The reason of choosing 100 iterations is that all iterations after 100 are very close each other. This enable study to be had reasonable and applicable solutions. In addition to the suggested mathematical model, the analyses were performed using finite elements based software Geostudio SEEP/W. This software shows how easy it is to find the downstream seepage face and amount of seepage. The upstream boundary nodes are designated as head boundaries with total head equal to the water level in the reservoir. The downstream toe is assigned a total head of zero m (H = elevation). The downstream slope is assigned a potential seepage face type of boundary condition. A potential seepage face review boundary is a Q (seepage discharge) = zero boundary with the potential seepage face review option checked. After tailings dam section draws, materials are identified and materials properties like hydraulic conductivity also enter to the system software. Finally, the amount of seepage flows through dam body and phreatic line figure obtain at the end of the successful seepage analysis. The seepage values were observed with both the finite difference method and SEEP/W. In the spreadsheet analysis, nine streamlines and twelve potential lines are obtained. By using these solutions in graphical method, seepage flow net, phreatic line and amount of seepage are also found. Comparison between finite difference results and Geostudio results determines the main goal of the study. The results showed that differences between the seepage values obtained by two different models were acceptable and satisfying. In addition, the results bring a new and feasible perspective to complex seepage problems in terms of application. Keywords: Seepage, Tailings Dam, Finite Difference Method