Introduction
Earth dams are the most common type of small dams. Local materials can usually be used to build these dams. For earth dams, the ratio of material cost to labor cost is very low and light equipment with minimal cost can be used to build them. The biggest disadvantage of earth dams is their destruction when the flow overflows over the dam crest. In addition, holes created by animals or tree roots in the body may lead to dam destruction. Earth dams are usually subject to the effects of runoff and internal erosion, and if left untreated, the effects of runoff can lead to dam failure. Among the things that must be considered in the design and construction of earth dams are the selection of the correct and appropriate location from a geotechnical point of view, the selection of appropriate bonding materials in dam construction, the investigation of loading conditions, dynamic analysis, seepage, leakage, and the stability of the dam structure. Today, dam evaluation and control during operation and throughout its life has become of particular importance, because leakage and seepage are considered the main factors responsible for the destruction of earthen dams.
Methodology
Based on research and studies that have been conducted so far on the problem of seepage in earthen dams, it is indicated that one of the most important factors of destruction in earthen dams is the evaluation of seepage. Considering that the dry flood dam is a dam that has surface cracks after impoundment, it has been evaluated as the subject of this research. In order to simulate the instability of the shell of a heterogeneous earthen dam with a core and a clay seal under the effect of seepage in the dry flood dam, the Plaxis finite element software was used. In the Plaxis software, the analysis process is based on the advanced finite element method for analyzing deformations and stability and is used in geotechnical engineering projects. This software is presented in four extensions: input, output, calculation and graph. An advanced behavioral model is required to model the nonlinear and time-dependent behavior of soils depending on the intended purpose. With this software, staged excavation and embankment with different loading conditions and boundary conditions can be modeled using 6-node and 15-node triangular elements.
Results and Discussion
The results showed that the maximum hydraulic gradient of the outlet was 0.22, which was much lower than the critical hydraulic gradient of soil particles, and there was no possibility of scouring. The upstream stability safety factor in this case was 1.925, which was higher than the proposed value of 1.25. The safety factor in the case of rapid decline of the reservoir was 2, which was higher than the minimum value of 1.25. In the case of rapid decline, a larger volume of water was discharged from the reservoir, but water was still present at lower levels. Therefore, the calculated safety factor for the stability of the body increased compared to the previous situation. The presence of water in the reservoir indicated a decrease in the pressure on the particles, which led to a relative improvement in the conditions. The evaluation of the results obtained from the modeling indicated that there were other reasons that led to the problem in the dam body. The main cause of the slope slide was not the stability problem of the earthen dam.
Conclusion
The evaluation of the results obtained from the modeling indicates that there are other reasons that led to the problem in the dam body. Therefore, a comprehensive field survey was conducted and, by adapting the findings to the existing conditions, the following conclusions were reached.
As it was determined, the main cause of the slope landslide was not the stability problem of the earthen dam. However, the following points can be considered the main reasons for this landslide:
- The flood of the region entered the Namin Dam reservoir before the completion of the dam construction and water intake from this dam. The aforementioned flood caused a strong driving force to be created on the upstream slope before the pore water pressure at the end of the construction operation disappeared, causing small cracks to appear, and over time, as the dam was refilled, the width of the cracks increased.
- Complete filling of the reservoir. Some free height should always be considered below the dam crest so that the dam does not fill completely. However, due to incorrect estimates of the spillway and dam, the dam has always been filling. It is possible that the lack of connection of the core crest to the dam crest in this case is very problematic.
- Lack of general use of the embankment as a protection of the upstream slope against the flow
- Untimely exploitation before the completion of the dam construction according to the announcement of the project stakeholders
- Wrong results of grading and laboratory tests on which all designs and implementation were based.
- Inadequate density of the dam shell. Of course, considering the results of the dam soil density control and the appropriateness of the density in all controls, the possibility of landslides caused by this factor is negligible
- Complete filling of the reservoir. Some free height must always be considered below the dam crest so that the dam does not fill completely. However, due to incorrect estimates of the spillway and dam, the dam has always been filling. It is possible that the lack of connection of the core crest to the dam crest is very problematic in this case.
- Lack of general use of the embankment as a protection against the upstream slope from the flow
- Untimely exploitation before the completion of the dam construction according to the announcement of the project stakeholders
- Incorrect results of grading and laboratory tests on which all designs and implementation were based.
- Inadequate density of the dam shell. Of course, considering the results of the dam soil density control and the appropriateness of the density in all controls, the possibility of landslides caused by this factor is negligible