Seepage in Earth Dam
Seepage in Earth Dams and seepage control are important aspects of dam design and construction. Seepage flow in the dam is defined as the movement of water from the upstream side of the dam to the downstream side through embankment of below the foundation or base.
The seepage problem is most commonly found in earthen embankment dams. This will directly affect the stability of embankments, sloughing of slopes due to the rising of pore water pressure, and also causes internal erosion which may further lead to piping. Failure to prevent seepage in earthen dam could lead to Slide Failure of Earth Dams.
Causes of Seepage in Earth Dam
The seepage in dams primarily depends on the properties of soil such as the plasticity of soil, the gradation, the degree of compaction, etc. Plastic clay with a plasticity index less than 15 mostly (for well compacted too) venerable to seepage.
Also, poorly graded soil most often shows less resistance to seepage. It is also considered that the poor design practices will permit the seep of water through the embankment and then leads to the discharge to the downstream side.
Seepage Analysis in Earthen Dam
The seepage analysis in the earth dam is carried out to analyze the phreatic line, the pore pressure within the dam or in its foundation, the exit gradient at the downstream face of the dam, and the amount of seepage flow that may pass through the dam’s cross-sections.
Seepage analysis is also used to evaluate the efficiency of various seepage prevention methods and to select suitable seepage prevention methods for the particular dam. In most of the cases, the seepage analysis is done by various analytical methods (Dupuit’s method, Casagrand’s method, and conformal mapping method), Experimental method (Permeability tank), Numerical methods (Finite element analysis, Finite-difference models).
Numerical methods are now commonly adopted in seepage analysis instead of an experimental method for deciding control methods, because of its ease of analysis, and its takes only less time.
Flow nets are the graphical representation of water movement through the soil and it is used to analyze the uplift water pressure and seepage amount. The flow nets are comprised of flow lines and equipotential lines, which is a graphical representation of Laplace’s equation.
From the above figure, the Phreatic Line (top parabolic line) meets at point A which is at the downstream slope of the dam. Below the line, the slope is saturated and water flowing. In this condition the following problems can happen:
- The high piezometric pressure can develop I the downstream slope which later leads to slope instability.
- It causes high seepage flows and later causes internal erosion and piping.
To solve these problems, drainages are introduced in the bottom of the dam.
It is the top flow line of saturated soil mass is called the phreatic line. Below the phreatic line, the seepage takes place. The hydrostatic pressure is acts below the phreatic line. This is not an equipotential line. In the earth dam, the phreatic line shapes like a parabola.
Problems due to Seepage in Earth Dams
The stored water in the dam always seeks a way out. So there are higher chances to seep the water through small pores, previous strata, joints, fissures, cracks, etc. These risks are mostly found in earthen dams. Considering all these factors, it is recommended to include instrumentation to measure piezometric pressure and seepage flows in the design stage. The three basic problems due to seepage are given below:
Piping happens when the seepage water carries out soil particles while moving out through embankment or foundation. When it continuously happens, pipes or channels will form inside the soil mass and initiates the seepage flows at a higher rate. Later on, this may lead to the failure of the dam.
Heaving and slope instability
The excess seepage causes instability of slopes in the foundation and sometimes the seepage forces to develop in the foundation soils immediately downstream of the dam, may cause heaving
Excess water loss
It is obvious that the seepage is a loss of water. And it happens when the soil strata or foundations are pervious. But proper stability management against piping and heaving water loss will not pose problems to the dam.
Seepage control measures in Earth Dams
Providing drainage filters is the best method to prevent seepage. Filters are provided for the free discharge of water and prevent piping and heaving. Drainages are used to reduce the pore water pressure inside the embankment and foundation soil.
A multi-layered drain system that consists of graded of both fine and coarse materials is adopted in the earth dam in order to prevent the seepage of fine materials from the embankment. Various kinds of drains which commonly used in earth dams are given below:
In this arrangement stone size which varying from 15 cm to 20 cm is arranged in the downstream toe end of the dam. It is arranged graded in layers which consist of fine sand, coarse sand, and gravel as shown in the fig below. The height of the rock toe usually kept between 15 to 30% of the reservoir height.
Horizontal filter extends from downstream side of the dam to inside at a distance of 25% to 100% from toe to center line of the dam. In common case height equal to three times the height of the dam is sufficient.
The horizontal drainage is better in bringing down the phreatic line down and also causes stratification in a horizontal direction. When large-scale stratification occurs horizontal filters are efficient. To solve this issue a vertical filter is placed along with the horizontal filter, which carries out the seeping water effectively as shown in the figure. This arrangement is termed as chimney drain.
Also the seepage through the foundation can be controlled by the following methods,
Impervious cut off
A vertical cut off which made of concrete or sheet pile extended up to the depth of impervious foundation. If the depth is large partial cutoff is provided (up to lesser depth).
Relief wells to prevent Seepage in Earth Dams
When large scale seepage happens through pervious foundation relief wells are adopted. The relief wells are used for relieving uplift pressure in the previous foundation. It is usually located at the downstream end of the dam.
Design criteria for Drainage filters
The drainage filters are to be designed in such a way that neither the foundation nor embankment material should not enter into the drain to form a clog. The permeability or size of the filter material should sufficient to carry water freely and without any blockage. According to the Terzaghi equation, the filter materials should satisfy the following equations,
The embankment soil or foundation soil which is surrounded by the filter materials is called base materials. When the ratio of D15 of filter to D85 of base materials is less than 4 to 5, the base material is prevented from passing through the filters.
Similarly, when the ratio of D15 of filter materials to D85 of the base material is greater than 5, the seepage forces in the filters are controlled up to the permissible magnitude.