FILTRATION PROCESSES IN EARTHEN DAMS INVESTIGATED

ABSTRACT

Today, there are more than 70 reservoirs in our Republic, 95% of which are earthen dam reservoirs. The main part of these reservoirs was built at the end of the 19th century, and one of the urgent issues is the identification, analysis and assessment of factors affecting the reliability of reservoir dams. The study was conducted to assess the changes in the filtration flow gradients from the earthen dam body of the Chimkurgan reservoir over the years. During the study, changes in the hydraulic gradients between the piezometers located in the dam body from 1992 to 2020 were determined, and the impact of the dams on the dam was assessed. The need to develop measures to reduce the filtration rate from the dam body was identified.

АННОТАЦИЯ

В настоящее время в нашей Республике насчитывается более 70 водохранилищ, из которых 95% составляют водохранилища с грунтовыми плотинами. Основная часть этих водохранилищ была построена в конце XIX века, и одной из актуальных задач является выявление, анализ и оценка факторов, влияющих на надежность плотин водохранилищ. В данном исследовании проведена оценка изменений градиентов фильтрационного потока через тело грунтовой плотины Чимкурганского водохранилища за ряд лет. В ходе работы определены изменения гидравлических градиентов между пьезометрами, расположенными в теле плотины, за период с 1992 по 2020 год, а также оценено воздействие плотины на её состояние. Выявлена необходимость разработки мероприятий по снижению фильтрационных расходов из тела плотины.

Keywords: Reservoir, earth dam, filtration, hydrodynamic process,hydraulic gradient.

Ключевые слова: водохранилище, грунтовая плотина, фильтрация, гидродинамический процесс, гидравлический градиент.

1. Introduction. Climate change, economic growth and population growth are leading to an increase in the need for water. Reservoirs are being built to provide consumers with water in a timely manner during the growing season. It is necessary to develop a work plan for the reliable and efficient use of reservoirs, and to accurately know the volume of water stored in the reservoir for the safe operation of structures. At the same time, it is necessary to increase the safety and service life of reservoir dams, pay special attention to the reliable operation of dams, and assess the impact of ensuring the timely supply of water during the growing season and maintaining a proper water balance on the safe operation of reservoirs [1]. Researchers have evaluated the impact of other factors on the water balance in reservoirs in their studies and have reached the following results: knowing the level of waterlogging in the reservoir from the upper water level, correctly assessing the losses from the reservoir basin and the groundwater dam through filtration, and obtaining high-precision results, as factors that are important for the proper management of the water balance in reservoirs [2]. The filtration process through an earth dam body depends on various factors, including the slope of the dam, the composition of the soil, the adopted construction method, the upper slope cover, the upper water level, rodent nests in the dam body, tree roots, cracks in the dam body, drains that carry away filtration water, earthquakes, and other factors. Water leakage through an earth dam body occurs due to the difference in water levels between the upper and lower banks of the earth dam. Darcy experimentally studied the flow of water through a porous medium. It was found that the flow rate is directly proportional to the change in pressure and inversely proportional to the length of the flow path [3,6]. One of the main elements affecting the stability of hydraulic structures is the filtration process [4,5]. Failure of earth dams through the filtration process can occur due to changes in hydraulic elements or structural failures [7]. Most dam failures are associated with the leakage of filtration water through the dam body or foundation [8]. The core is a zone of low permeability within an earth dam, where seepage significantly reduces the flow rate [9]. The core should be thick enough to prevent water from flowing through the dam body [10,11]. The core is usually constructed using clayey soils with low permeability. In core dams, the core reduces the seepage rate and reduces the depression curve [12]. It also provides safety against suffocation and slope failure in the dam body[11]. Similarly, a diaphragm is a thin impermeable wall constructed in the central part of the dam used to prevent seepage of water. A thin impermeable diaphragm wall is made of concrete or other impermeable material. The main difference between core and diaphragm earth dam types is the thickness of the impermeable part. The thickness of the diaphragm should be less than the height of the dam [13,17]. On the other hand, a diaphragm is a barrier that can be built on the upper or lower sides of a dam and can extend from the core to the impermeable layer to reduce the infiltration water flow [14,15]. Concrete is usually used to construct the inner diaphragm wall in earthen dams. This is due to its unique properties, including durability, low permeability, and structural stability [16,18]. Therefore, it is very important to control the infiltration through earthen dams. This is done by using drains, cores, and diaphragm walls. To assess the aggressiveness resulting from the movement of filtration waters from the body of the reservoir dam, it is necessary to know the laws of the movement of filtration waters along the dam body and its impact on the dam structures. The results of the assessment are considered important in ensuring the stability of the reservoir dam and its structures. Therefore, it is important to analyze the changes in the filtration process from the body of the Chimkurgan reservoir earth dam built on the Kashkadarya River bed over the years and to study its impact on the reliability of the reservoir.

2. Materials and methods.

The Chimkurgan Reservoir is built on the Kashkadarya River in the Kamash district of the Kashkadarya region. It is a core dam with a maximum height of 33 m and a length of 7,500 m. There are 116 piezometers in 21 sections along the dam body, which are located at different depths.

Figure1. Cross-section of the Chimkurgan reservoir dam PK 50+00

The filtration water along the body of the reservoir dam usually moves in a random flow (linear), that is, the filtration water is classified as non-pressure water. It is known that non-pressure water moves from a place of high hydraulic pressure (level) to a place of low pressure. In the case of a reservoir dam, water moves from the reservoir basin above the dam, that is, from the upper side of the dam to the lower side. In this case, the difference in hydraulic pressures is ΔH = H1- H2.

2.1. Theoretical Background

The rate of water flow through a dam depends on the pressure difference (ΔH = H1- H2) and the length of the filtration path L.

The ratio of the pressure difference (ΔH) to the length of the filtration path is called the hydraulic gradient and is usually denoted by J:

                                                                   (1)

The movement of filtration water in the dam body is chaotic and obeys Darcy's law. Such movement is more often observed in sand, loam, and sandy rocks that form the basis of the dam body.

The theory of the movement of filtration water in the dam body is expressed as follows, based on the law created by the French scientist Darcy [6].

                                             (2)

here:  Q – flow rate, that is, the amount of water that has filtered and flowed through per unit of time, m³/day;

k_f - filtration coefficient,  the ability of the soil forming the dam to pass water through itself, m/day;

w - cross-sectional area of ​​the filtration flow, m²;

L- filtration path length, m;

ΔH - difference in water pressure at the upper and lower levels, m;

If we divide the two parts of the equation by F and define Q/F in terms of the filtration rate ν,  ν=K_f J   

Therefore, according to Darcy's law, the filtration or movement rate of water in the soils forming the dam body (ν) is directly proportional to the pressure gradient or flow slope (J).

                                                               (3)

Pressure gradient       under the circumstances ν=K_f J equation ν=K_f   takes the form, that is, the filtration coefficient is numerically equal to the filtration rate.

2.2. Field Investigation

During the study, during the period from 1992 to 2024 for 32 years, the reservoir was filled to the NDS in 1992, 1993, 2002, 2004, 2017, 2019 and 2020  for 7 years. Taking this into account, the data obtained from the dispatchers were mathematically and statistically analyzed (Figure 1).

Figure 2. Natural field research at the Chimkurgan reservoir

The change in gradients between piezometers 1a, 1ya, 2ya, 2a, 2, 3, 4 and 5 located at PK 50+00 of the dam over the years was calculated when the reservoir was at its highest water level at NDS 488.20. There are 8 piezometers located at PK 50+00 of the reservoir dam, located at 18 stories. The data obtained from field surveys were analyzed and some results were obtained.

3. Results and discussion

Table 1.

The change in filtration gradient between each piezometer at PK 50+00 located in the dam body is presented in

In the study of the filtration process in the dam body of reservoirs, the change in the hydraulic gradient is expressed as a change in the filtration rate in the dam body. The study site consisted of 8 piezometers placed at different depths. When analyzing the data obtained during the field study, we obtained results that observed a change in the filtration gradient in the dam body. In this case, the depth of the piezometers P1ya – P2ya is ▼ 470, located in the core of the dam. Therefore, the filtration gradient between the piezometers located in the core is observed to increase from 1992 to 2020 (Figure 3).

The depth of the piezometers (P1a – P2a, P3–P4) in this layer is ▼ 458, and the hydraulic gradient between them has decreased between 1992 and 2020 (Figures 4 and 7).

The depth of the piezometers (P2–P3) is ▼ 458, and the hydraulic gradient between them has not changed between 1992 and 2020 (Figure 6).

The depth of the piezometers (P2a–P2, P4–P5,) located in the dam body is ▼ 458, and the hydraulic gradient between them has not changed between 1992 and 2020 (Figures 5 and 8). According to the calculation results, in the section between the piezometers P1ya–P2ya, P2a–P2 and P4–P5, the velocity of the filtration water and the pressure difference are large, while in the section between the piezometers P1a–P2a, and P3–P4, the opposite is true, that is, the velocity of the filtration water and the pressure difference are small, and in the section between the piezometers P2–P3, the velocity of the filtration water and the pressure difference do not change.

4. Conclusions

During the study, the filtration process from the dam body of the Chimkurgan reservoir was assessed. According to it, the sections wфhere the hydraulic gradient between the piezometers located at PK 50+00 has changed over the years were identified. An increase in the hydraulic gradient is expressed in an increase in the filtration rate in this section. This has a significant impact on the reliability of the dam. The use of methods that reduce the filtration rate in sections with an increased filtration rate from the dam body will increase the priority of the reservoir dam.

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