Explain with neat sketch Phreatic line in earthen dam with pressure head at different point and show construction points of this line.

Explain with neat sketch Phreatic line in earthen dam with pressure head at different point and show construction points of this line. 
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1 Answer

Answer :

Procedure for locating the Phreatic line

i) Draw an earthen dam having upstream face AB with water surface.

ii) On water surface measure distance BC = 0.3 L C is starting pt of base parabola. Let ‘F’is focus.

iii) Working principle: Any point on parabola is equidistant from focus and directrix.

iv) Locate directrix using pt ç’as centre and CF as redius to draw an arc and cutting line passing through CB at pt ‘D’, so that CF = CD . DH is directrix

v) Find midway between ‘F’& ‘H’.

vi) Repeat the above step by selecting points at base AE measures their distance from H pt. With ‘F’as centre and respective distance as radius, draw an arc cutting that vertical line passing through selected point.

vii) Join all such points to get base parabola i.e. Phreatic line.

viii) Here G,Q1,Q2,Q3,Q4 and B represent Phreatic line 

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