Document Type : Original Article

Authors

• Atefeh Arab ¹
• Hojjat Ahmadi ²
• NourAli Haghdoust ²
• Bahram Nourani ³

¹ Phd Candidate, Department of Water Engineering, Urmia University, Iran

² Department of Water Engineering, Urmia University, Iran.

³ Department of Water Engineering, University of Tabriz, Iran

Abstract

Extended Abstract
Introduction
       Earth dams are the most common type of dams due to their cost-effectiveness, and they make up the majority of the world's dams. Water seepage is one of the most important issues in the design and safety of earth dams, as it can lead to increased pore water pressure, reduced shear strength, the occurrence of piping phenomena, and ultimately dam failure. Statistics show that approximately 35% of dam failures are directly or indirectly related to seepage. This risk is particularly exacerbated in dams built on permeable alluvial foundations.
To control seepage, various measures are employed, including drainage systems in the dam body and foundation. Blanket drains, toe drain, and chimney drain, whose main roles are to collect leaked water, reduce pore water pressure, and lower the water table. Although the use of chimney drains has become more common in recent years, new studies show that horizontal drains, especially their length, can have a significant impact on reducing pore water pressure and improving dam stability, while parameters such as drain thickness, height, and angle play a lesser role.
Seepage in earth dams is examined using numerical and experimental methods. Numerical methods are fast and cost-effective, but laboratory studies are of high importance as a basis for validating numerical results and for a detailed examination of the dam's actual behavior. Given that the impact of different drainage geometries with the same length has not been fully investigated to date, this study designs a laboratory model of a homogeneous earth dam to examine the effect of various drainage types with equal lengths and different geometries on seepage discharge and flow patterns in an earth dam located on an alluvial bed.
Methodology
In this study, a homogeneous earth dam was constructed using glass bead material, which contains glass particles with sizes ranging from 0.7 to 3.5 millimeters. The dam model was designed in a drainage box with dimensions of 150 cm length, 10 cm width, and 60 cm height, and included 5 piezometers to monitor changes in pore water pressure under the dam structure. The purpose of this experiment was to investigate the effect of the downstream drain's geometric shape on seepage from the dam. To observe the water seepage, artificial sunset yellow dye and potassium permanganate were used to visualize the flow lines. An air tank was also placed above the laboratory box to keep the water level in the dam constant.
The hydraulic conductivity of the material was estimated to be 0.128 cm/s using the constant head permeability test. The height of the constructed dam was 25 cm above the level of the permeable layer, which, considering the location of the overflow weirs, resulted in a reservoir water level of approximately 22 cm. The crest width was 10 centimeters, and the upstream and downstream body slopes were equal to the dam width at the foundation, which was 130 centimeters. Figure 1 illustrate the details of the constructed dam.
To study the effects of different drain system and their geometry, three different drain system involving blanket, toe and chimney drain were studied and compared with the basic state of an earth dam with no drain system. The length of all implemented drains was almost identical equal to 21 cm. The dimensions of the drains were presented in table1. The coarse gravel was used to create the employed drains. To avoid any mixing of the fine glass beads within the coarse materials of drains, a fine mesh fabric was placed around the drain system. The thickness of the gravels in the drains was 3 cm. 
Results and Discussion
The summery of the achieved results were presented in figure 2 in terms of pore pressure in the base of the dam. In this figure each curve belongs to the specific drain. As seen, the toe and blanket drains have similar effect in removing of pore water pressure, while the chimney drain is very effective, comparing to other drain.
Additionally, the seepage line was investigated in the toe and blanket types to evaluate the seepage velocity and their lengths. The results are shown in figure 3. As seen in this figure, the seepage path through the earth dam from the identical inlet point has different outlet point. The length of the seepage path in toe drain and blanket drain significantly is shorter than that in no-drain. Among toe drain, blanket drain, and chimney drain , the toe drain has the shortest seepage path.
Conclusions
The objective of this study is to evaluate the performance of various drainage systems in controlling seepage and mitigating pore water pressure within a homogeneous earthen dam constructed on an alluvial foundation under laboratory conditions. To quantitatively assess the influence of drainage systems on the seepage rate and the dam's hydraulic behavior, three drain configurations: horizontal or blanket drains, toe drains, and chimney drains, were investigated. Their performance was subsequently compared to a control scenario with no drainage system. The findings of this investigation demonstrate that chimney drains are superior comparing to other drain types in reducing pore water pressure; however, they concurrently lead to a higher rate of seepage. When drain length is held constant, horizontal drains are preferable to toe drains. Nevertheless, the implementation of toe drains can enhance the structural stability of the dam body due to the incorporation of a substantial volume of coarse-grained material. This particular aspect, however, warrants further investigation to enable a quantitative comparison of its outcomes with those of other drainage configurations. Based on a value analysis conducted by considering three parameters—pore water pressure reduction, seepage discharge, and the volume of material utilized as a cost factor—the chimney drain yields the highest value, whereas the toe drain yields the lowest. It is noteworthy that the differential in value between the chimney drain and the toe drain is marginal.

Keywords

• Blanket Drain
• Chimney Drain
• Permeable base
• Seepage Rate
• Toe Drain

Main Subjects

• Geotechnic