A phreatic line is defined as the line within a dam section below which there is/are-
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A.
Negative equipotential lines
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B.
Positive hydrostatic pressure
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C.
Positive equipotential lines
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D.
Negative hydrostatic pressure
Correct Answer:
B. Positive hydrostatic pressure
Explanation:
In an earthen dam, the phreatic line (also known as the seepage or saturation line) represents the upper boundary of the water flowing through the dam's body. It is defined by several key characteristics: - Hydrostatic Pressure: By definition, the phreatic line is the locus of points where the water pressure is equal to atmospheric pressure (zero gauge pressure). Consequently, any point located below this line within the dam section experiences positive hydrostatic pressure.
- Saturation Levels: This line serves as a divider between the saturated soil below it and the unsaturated or partially saturated soil above it.
- Flow Properties: The phreatic line is considered the topmost flow line. While hydrostatic pressure is positive below the line, it is negative (suction) above it due to capillary action.
- Shape: In a homogeneous earth dam, the phreatic line typically follows the path of a base parabola.Therefore, the correct answer is C: Positive hydrostatic pressure.
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