Influence of speed of filling and draw-off to the filtration regime of Earth-fill dam

To use and manage water resources and carry out protection measures from the destructive effect of water flow, water reservoir hydrosystem construction has greatly developed. The article presents the results of field studies to determine unsteady filtration in the Earth-fill dam core. In the research process, static data from literature review, field study data, and theoretical processing of research results were used. Numerical data processing was carried out with methods of mathematical statistics, and the graphical part was done using Microsoft Excel. Calculations were carried out for phreatic line curve in the core of Earth-fill dam under unsteady filtration for various velocities of reservoir draw-off and water yield coefficient. At the same time, it has been established that the increase of velocity and time of water reservoir draw-off leads to the increase of filtration pressure, and the decrease of water yield factor leads to the decrease of filtration pressure. The increase of filtration pressure, in turn, contributes to the increase of the intensity of unsteady filtration of the Earth-fill dam core.


Introduction
To use and manage water resources and carry out protection measures from the destructive effect of water flow, water reservoir hydrosystem construction has greatly developed. Following the development of water reservoirs, dangerous phenomena began to occur, which are related to shortcomings in the design, construction, operation, and imperfection of structural elements that make up hydrosystems. Construction of water reservoir hydrosystems has increased in the past century, and the potential threat from them has increased respectively. When designing and constructing hydrosystems, one of the most important tasks is to provide their safety. In this regard, increasing the operational reliability of hydrotechnical structures, providing their strength and stability, are of particular importance. Great attention is being paid to purposeful research works devoted to theoretical and experimental studies of unsteady filtration in earthfill dams and their influence on the stability of the earthfill dam upper slope. In this regard, one of the important tasks is to improve methods of unsteady filtration design in the body and bank *Corresponding author: shdjabbarova1985@mail.ru massifs of earthfill dams, determining the influence of unsteady filtration on the carrying capacity of water passage structures [2][3], [5][6], [21][22].

Methods
In the research process, static data from literature review, field study data, and theoretical processing of research results were used. Besides, numerical processing of data was carried out with methods of mathematical statistics, and the graphical part was done using Microsoft Excel.

Results and Discussion
Design velocity of water reservoir filling must be about 0.3-0.5 m/day, and reservoir water level draw-off velocity must be 0.3 m/day. The analysis of field observations gave the following:  violations in filling and draw-off regime of the water reservoir (in 2009 it took place about 10 times);  during the year 2010 at a mean velocity of water reservoir filling velocity of 0.13 m/day, the maximal velocity of water reservoir filling velocity was 1.73 m/day, while allowable velocity was 0.5 m/day;  mean velocity of water reservoir draw-off was 0.42 m/day, and the maximal velocity of water reservoir draw-off was 0.73-1.5 m/day in 2010; at this intolerable velocity, water reservoir draw-off lasted 10 days. That mostly happened due to the violation of the regime of water reservoir operation without accounting for the carrying capacity of spillway structures. Calculations were carried out using V.M.Shestakov's method [1] with the following boundary conditions and assumptions: the core is considered to be a homogeneous Earthfill dam erected of loamy clay soil with the coefficient of filtration k ya =0.001 m/day.  water level at the head race is taken equal to 960.0 m;  it is assumed that the water level at the upper prism and transition zones will decrease simultaneously with the water level at the head race;  phreatic line position was determined for steady filtration at water level decrease time at the head race of t=0. Phreatic line position after time t is determined using the following relationship: where v is mean velocity of water level decrease at head race;    Based on data in Tables 1-4, phreatic lines in the core were constructed under conditions of unsteady filtration for various water yield factors and draw-off velocities of the water reservoir, which are partially given in 'figures 1-4'. As seen from Table 1 and Fig. 1, the changes in the phreatic line after 5 days and 10 days are small. Further drainage of the water reservoir with velocity 2.5 m/dah increase filtration pressure, when t=15 days ∆Н=17 m, when t=20 days ∆Н=28 m, when t=25 days ∆Н=53 m, when t=30 days ∆Н=100 m, i.e., there is a significant delay of the phreatic line from water level in the water reservoir.
The increase of water yield coefficient from µ=0.028 (Fig. 1) to µ=0.03 (Fig. 2) at similar draw-off velocity v=2.5 m/day results in the decrease of filtration pressure from ∆Н=100 m to ∆Н=90 m at t=30 days.
The increase of draw-off velocity from 3.5 m/day to 5 m/day increases the filtration pressure ∆Н (Figure 3 and 4).
All this proves the increase of the intensity of unsteady filtration depending on the increase of water reservoir draw-off velocity increase, which may result in the damage to the stability of the upper slope and its lining

Conclusions
Calculation results on determining phreatic line position in the core of design dam of Tupolang water reservoir shows the following: the height of seepage into the lower edge is at 850.50 m. According to filtration study data, given by institute "Uzgiprovodhoz" in 1981 (using EGDA device), it is at 851 m. The difference is 0.5 m, which proves the accuracy of the chosen design method. The increase of the velocity and time of water reservoir draw-off results in the increase of water yield factor, and its decrease result in the increase of filtration pressure. The increase of filtration pressure, in turn, facilitates the increase of the intensity of unsteady filtration in the core of the Tupolang dam.