Background and Objectives

Earth dams are among the oldest engineering structures built by human civilization. The stability of their slopes, particularly under various conditions, such as rapid drawdown, is important. In this study, one of the most critical parameters in the design of earth dams, slope stability, has been investigated. Rapid drawdown is the main factor contributing to the instability of upstream slopes in earth dams. This research presents an innovative method to improve the safety factor and enhance the slope stability of old and operational earth dams. For this purpose, numerical modeling was carried out using GeoStudio/W software. The Alavian earth dam was modeled numerically in this software. The safety factor of the upstream slope was evaluated before and after the nailing under different conditions. The results indicate a significant increase in slope stability after nailing. The safety factor increased by 18.554% under static conditions after construction, by 17.746% under steady seepage conditions, and by 22.164% under rapid drawdown conditions.



Methodology

To obtain realistic results, the cross-section of the Alavian earth dam, located in East Azerbaijan Province, on the slopes of Mount Sahand, approximately 120 kilometers southwest of Tabriz and 3.5 kilometers north of Maragheh, was used. The Alavian dam was selected due to the relatively comprehensive studies previously conducted on it and the availability of its mechanical and strength parameters. The analyses in this study were carried out using the GeoStudio software package. The Seep/W module of this package is used for analyzing soil seepage, while the Slope/W module is employed for slope stability analysis and determining the factor of safety (FOS) of dam slopes. To evaluate the stability of sloped surfaces and calculate the factor of safety in slope design, the Slope/W component of the software was used, applying the limit equilibrium method based on the Morgenstern–Price approach. It is important to note that the finite element method is not used in this part of the analysis.

In the modeling conducted in this study, only one parameter was altered at each stage to eliminate the influence of other factors on the results. These are different conditions that were studied in this paper:

- Examination of the upstream slope stability under static conditions at the end of construction with and without nailing.

- Examination of the upstream slope stability under static conditions after steady seepage with and without nailing.

- Examination of the upstream slope stability under static conditions after steady seepage with and without nailing during rapid drawdown.

- Examination of the upstream slope stability under dynamic conditions at the end of construction with and without nailing.



Findings

Some of the flow lines are located above the zero-pressure water level, which is because the Seep/W software does not precisely draw the flow net but only provides a path composed of connected vectors. In this study, the output of the Seep/W software was used as the input for the Slope/W software, and the flow lines and seepage discharge were not examined.

Results of the static stability factor of safety for the upstream slope at the end of construction indicate that in the static model after construction, without considering seepage, the dam’s factor of safety is 1.757. By adding nailing to the same model, the factor of safety increases to 2.083.

In the presence of seepage, due to seepage forces and reservoir loading, the upstream safety factor generally increases. The results indicate that in the static model with steady seepage, the dam’s factor of safety is 1.775. By adding nailing to the same model, the factor of safety increases to 2.090.

The results indicate that in the static model under rapid drawdown conditions, the dam’s factor of safety is 0.961. By adding nailing to the same model, the factor of safety increases to 1.174. Under rapid drawdown conditions, it was observed that the dam’s factor of safety reached a critical level. However, after adding nailing and reevaluating the results, it was found that the factor of safety increased significantly.



Conclusion

Results showed that under static conditions after construction and during steady seepage, the safety factors of the dam slopes were automatically calculated using the Morgenstern–Price method. Analysis of the calculated values showed that after the establishment of steady seepage, the downstream safety factor decreased due to seepage pressure, while the upstream safety factor increased as a result of reservoir loading. In addition, the percentage increase in the dam’s stability factor under all three examined conditions was substantial. This improvement was particularly remarkable in the rapid drawdown condition, where the safety factor increased by more than 22%, effectively removing the dam from the critical state.

The increase in the safety factor under all three analyzed conditions was considerable, resulting in an 18.554% increase under static conditions after construction, a 17.746% increase under steady seepage conditions, and a 22.164% increase under rapid drawdown conditions.