phreatic line in earthen dam without filter
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`T z}J"31^? In case of discharge face being a rock toe this angle is greater than 900. The favorable factors for the slope stability are the decrease in , directly related to the decreasing rate of the water level , the permeability coefficient , the specific yield , and the water thickness . Equation of Continuity Flow Nets Hydraulic Uplift Force under a Structure Flow Nets in Anisotropic Material Construction of Flow Nets for Hydraulic Structures on Nonhomogeneous Subsoils, Experimental conceptualisation of the Flow Net system construction inside the body of homogeneous Earth Embankment Dams, Modification of The Casagrande's Equation of Phreatic Line. In contrast, the safety factor does not change substantially with large changes in the water level. The third problem is related to the velocity of the decreasing water level, which is again not clearly defined in the slope stability calculation. A Simplified Solution for Calculating the Phreatic Line and Slope Stability during a Sudden Drawdown of the Reservoir Water Level, State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China, University of Chinese Academy of Sciences, Beijing 100049, China, College of Civil Engineering & Architecture, China Three Gorges University, Yichang 443002, China. A typical landslide in the Three Georges Reservoir is used as a case study to analyze the influential factors for slope stability during conditions of decreasing water level. By applying variational inequalities, Zheng et al. The force diagram of a single soil slice is presented in Figure 4. So he gave a chart for the exit correction a the distance by which the basic parabola is shifted to locate the point where the actual seepage line cuts the discharge face. And the distance JF is written as a + a. Additionally, from (28) and (29), the slip surface normal stress at both ends may not be zero either.
So hydraulic gradient causing the flow at this section can be written as change in head over the distance over which the change occurred. The parameters and are the interslice force increments in the horizontal and vertical slice, respectively; and are the bar weight and seismic force, respectively; and and are the components in the horizontal and vertical directions on the outer contour line , respectively. let's assume an earth dam that retains the water up to the height h. We also assume that foundation of the dam is impermeable and water will only seep through the body of the dam. Also we name few points in the diagram. It is defined as the amount of water released as a result of gravity in a unit volume of saturated rock or soil and can be expressed as the ratio between the water volume released by the gravity and the rock or soil volume in saturated conditions. The distance to the Three Gorges dam site is 50 km. By assuming that the sliding surface meets the Mohr-Coulomb criterion and the landslide body is in the limit equilibrium state, the equation can be written aswhere is the safety factor and and are the effective shear strength parameters. Therefore, when , and the speed of the free surface water on the slope and the water level become equal. Calculation sketch of differential soil slice. 106 0 obj <>
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Well, Casagrande observed this a varies with the downstream slope angle . The geographical location map of Woshaxi Slope in the Three Gorges Reservoir. As a result, this type of problem is straightforward to solve in practice. -Cr8. Similarly, the larger values cause unfavorable conditions for the slope stability. In this context, the expression of normal stress on the slip surface under seepage forces is deduced, and a procedure for obtaining the safety factors under hydrodynamic forces is proposed.
When the reservoir water level decreases, the overflow points are usually located on the steep slope surface and the flow follows the slope gradient.
We should keep in mind that this chart is applicable only when the downstream slope angle is greater than or equal to 300 ( >= 300). However, with increasing , the change in the phreatic line is gradual because more time is available for the water to be transported down the slope (Figures 13(b)-13(c)). Overall, as the decreasing water level rate increases, it causes a decrease in the safety factor. This study is supported by the National Natural Science Foundation of China [Grant no. 11572009] and the CRSRI Open Research Program [Program SN: CKWV2015212/KY] and the Youth Innovation Promotion Association CAS. The force is the hydrostatic force at the border; is the hydrostatic force at the border; is the hydrostatic force at the border; is the normal force at the base, which is also known as the effective force; is the angle between the bottom surface and the horizontal direction; and is the angle between the phreatic line and the horizontal direction. 2018, Article ID 1859285, 14 pages, 2018. https://doi.org/10.1155/2018/1859285, 1State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, China, 2University of Chinese Academy of Sciences, Beijing 100049, China, 3College of Civil Engineering & Architecture, China Three Gorges University, Yichang 443002, China. 0000037222 00000 n
It is further observed that at a given decreasing water level rate, the safety factor tends to be more stable with an increase in water level. Figure 11(c) shows the phreatic line for different water level conditions and a decreasing water level rate of m/d. This inclination of the discharge face is described relative to the bottom boundary flow line as angle and is measured clockwise. The reason for introducing two undetermined parameters is that three equilibrium equations are used to solve for three unknowns, in which the safety factor is one of the unknowns. To clarify these issues, the calculation of the seepage forces can be studied in terms of the water forces using the boundary of each differential slice; this is the most direct way to analyze the seepage forces [18]. Overall, the slope permeability coefficient is inversely proportional to the safety factor, which indicates that an increasing permeability coefficient can cause a decrease in the safety factor. Shape Of Phreatic Line In Earth Dam Without Filter, Unit Weight Determination - Core Cutter Method, Effect Of Capillary Rise Of Water On Effective Stress, quantity of seepage through the body of an earth dam with a horizontal filter at its downstream, 1. The reservoir water level decreases at a speed . The water that the dam holds flows from upstream through the voids present in the soil of the body of the dam to downstream by the action of hydraulic head difference. Theoretically, the distribution of the slip surface normal stress in global analysis methods should be easier to understand than the distribution of interslice force in the Morgenstern-Price method. You can help Wikipedia by expanding it. Y. Zhao, P. Cui, L. Hu, and T. Hueckel, Multi-scale chemo-mechanical analysis of the slip surface of landslides in the Three Gorges, China,, Y.-P. Yin, B. Huang, X. Chen, G. Liu, and S. Wang, Numerical analysis on wave generated by the Qianjiangping landslide in Three Gorges Reservoir, China,, G. Sun, H. Zheng, Y. Huang, and C. Li, Parameter inversion and deformation mechanism of Sanmendong landslide in the Three Gorges Reservoir region under the combined effect of reservoir water level fluctuation and rainfall,, G. Sun, S. Cheng, W. Jiang, and H. Zheng, A global procedure for stability analysis of slopes based on the Morgenstern-Price assumption and its applications,, G. Sun, Y. Yang, S. Cheng, and H. Zheng, Phreatic line calculation and stability analysis of slopes under the combined effect of reservoir water level fluctuations and rainfall,, Q. Jiang, Z. Zhang, and W. Wei, Research on triggering mechanism and kinematic process of Qianjiangping landslide,, Q. Jiang, C. Yao, Z. Ye, and C. Zhou, Seepage flow with free surface in fracture networks,, Q. Jiang, Z. Ye, and C. Zhou, A numerical procedure for transient free surface seepage through fracture networks,, Q. Jiang and C. Zhou, A rigorous solution for the stability of polyhedral rock blocks,, C. W. Cryer, On the Approximate Solution of Free Boundary Problems Using Finite Differences,, R. L. Taylor and C. B. 0000000708 00000 n
In the absence of experimental data, (12) can be used to calculate the specific yield. In hydrology or groundwater dynamics, the effective porosity is the ratio between the pore volume filled by water and the water released from gravitational pull and the total pore volume of these pores. Seepage force calculation is the key factor for evaluating the slope stability under seepage. aT(6W/KyB{(zvR@0jgevz~N[Cb~E](+j>Nf% +N^F$`WW"Sl-
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S*\.u#0D{p."Pw|K^751cP=@oA!D ]cwU\(%p05w"B^-4d+Xlcz ]1'&o9fD;g7t|"Xr;g According to (6), integral calculation is required, which is difficult to perform and not convenient for engineering applications. Figures 13(a)13(c) and 14 show the phreatic line and safety factors, respectively, under different water levels when the permeability coefficient is 0.01m/d, 0.10m/d, and 1.00m/d, respectively. All the water loads below the phreatic line in the soil slice are shown in Figure 6. %PDF-1.5
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The geological section is often drawn using CAD software [5, 18]. The geographical position of the landslide is shown in Figure 9. What is the value of a. We join all those points to find our base parabola. Effect of reservoir drawdown height on landslide stability with different permeability coefficient. The Qinggan River flows from west to east and passes in front of the landslide site. 1957 0 obj
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In such a case, the key calculation is to determine the free surface that delimits the flow boundaries. Such action leads to a rapid decrease in the water level and in some cases triggers landslides. [29] and Zhu and Lee [28] used quadratic interpolation to solve the slip surface normal stress distribution and derive one variable cubic equation with an unknown safety factor. Thus, the accuracy of the seepage force calculation strongly influences the results. 1903 0 obj
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Alternatively, other limit equilibrium methods can be categorized as global analysis methods, including the graphic method [25], the variational method [26], and Bells global analysis method [24]. Such geological conditions lead to gravelly soil forming in the upper part and a geological rock structure in the lower part. By integrating and applying simple mathematics we arrive at a very simple equation which gives the value of a. endstream
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Many slice methods have demonstrated that the interslice force is important in calculating the slope stability in statically indeterminate problems [24]. %%EOF
The geometrical interpretation of the full water immersion saturated area in the soil slice is that the water gravity and hydraulic gradient are equal to the seepage force and hydrodynamic force, respectively. In contrast, the phreatic line on the slope is relatively flat. Phreatic line meets differently to the different types of discharge faces. Such pores are generally termed as effective pores in hydrogeology. To arrive at these equations we use some approximations and for discharge angles greater than 600 the error because of those approximations becomes large. It can be observed that at the same water level decreasing rate, the variation in causes differences in phreatic line behavior. So by this triangle we can determine the value of dy and dx as zsin and zcos , So discharge though this section can be written as. (3)The expression of the slip surface normal stress under the dynamic water forces is derived, and an equation for solving the safety factor under different dynamic water forces is proposed. 1927 0 obj
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The forces along the arc length ds in the antislide direction of the differential slice are shown in Figure 7. Now we can obtain the parabolas directrix using the property of the parabola that states, the distance of any point on the parabola from the directrix and its focus is equal. When , and the free surface water on the slope and the water level are unchanged. a can be find out by equating the discharge through any section of the dam and discharge through the section RS of the dam. In the formula, will take the value of 1, 2, and 3 and provide three moment centers (, ). To improve the accuracy and computational efficiency of the EP method, an iterative error analysis is used with the finite difference equations [17]. The -direction component of all the water loads isThe -direction component of all the water loads isFrom the geometrical relationships, as shown in Figure 6, the equation can be represented asTherefore, the forces from all water loads become.
For example, in Bells derivation process, Zhu et al. L 04h(b5=rI%4y&j-,Lr->c
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6A!yMlwIu5(#pK`bya]6- 6k\!k@)}@sJ*E5i}wTS"]s_b_C!cC#Q(]?MB+#0;novJ+^_%aC@.h,OAdtFm(f9[{y?[p Sorry, preview is currently unavailable. Until 2002, researchers in many studies [4, 2830] have used similar methods. When the decreasing water level rate is relatively small, there is sufficient time for the water in the slope to discharge from the slope, which causes a slow change in the phreatic line, as shown in Figure 11(a). Lets say a slight distance dx away from this section we observe a head difference of dy. First, there is no scientific basis for the determination of phreatic lines. k is the permeability of the dam material. We have already determined the quantity of seepage through the body of an earth dam with a horizontal filter at its downstream. In this study, the following condition can be used to terminate iteration during numerical simulations and can be expressed aswhere is the difference between the two consecutive iteration steps and is the selected tolerance level. No we will attempt no such dangerous task. Soil Mechanics And Foundation Engineering by K R Arora, 2. 0000000456 00000 n
The Woshaxi landslide occurred on 13 July 2003 at the right bank of Qinggan River, a tributary of the Yangtze River. @JGeY)PI#zB?uR,G,Jb 4Cmz1qd nu JO~~/W Forces distributing diagrammatic sketch of differential soil slice. The free surface can be found using nonlinear numerical techniques, such as the finite difference method with adaptive mesh [10] and the finite element method with adaptive mesh [11] or fixed mesh [12]. Many studies have confirmed that changing water levels are the main cause of landslide events [29]. A decrease in water level is the most unfavorable situation in terms of slope stability and usually leads to landslides. Afterwards, the simplified formula of the phreatic line under a constant declining speed can be obtained. In this study, is defined as a linear function [30]:where and are the coordinates of two endpoints on the slip surface in this two-dimensional problem. Most engineers generally analyze the effects of decreasing rate of water level and permeability coefficient on the slope stability. If we remember we also did not have to apply the exit correction in the case of horizontal filter. Usually the interslice forces and at both ends of the sliding surface are zero; however, their derivatives and may not be zero. In rock and soil, only some pores are filled by water and movement is possible through connected pores. You can download the paper by clicking the button above. Phreatic lines of reservoir drawdown height with different permeability coefficient.
