Describe the method for determining Phreatic line for earthen Dam with horizontal filter.
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The phreatic Line or seepage line is defined as the line within a dam section flow which they are positive hydrostatic pressure in the dam the hydrostag phreatic line graphically:

  1. AB is upstream face let it horizontal projection be L. On The Water surface measure a distance BC = 0.32. Then the point C is the starting point of the base parabola.
  2. The directrix of the parabola is located by utilising the principle that any point on the parabola is equidistant from the focus as well as from the directrix
  3. Hence with C as the centre and CF radius is drawn an arc to cut the horizontal line through CB in D. Draw a vertical tangent to the curve FD at D. Since CD = CF the vertical line BH is Directrix.
  4. Between F and H the last point G of the parabola will lie.
  5. To locate the intermediate point on the parabola the principal that its distance from the focus and directrix must be equal will be used. For example to locate any point P drawn vertical line QP at any distance x from F
  6. Measure QH with F as the centre and QH as the radius draw an Arc to cut vertical line through Q at point P
  7. Join all these points (C, P, G) to get the box parabola.
  8. Entry point correction the phreatic line is flow line it must start from B and not from C it should be perpendicular to the upstream face AB which is 100% equal potential line.
  9. Therefore portion of the phreatic line at B is sketched freehand in such a way that it starts perpendicularly to AB. The base parabola should also meet the downstream.

    dicularly to AB. The base parabola should also meet the downstream.

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