Lesson 7 - nptel

Knowing the focus, the directrix and the point P0, a parabola can be drawn, which gives
the Base Parabola shape. It may be recalled that point B is mid way of points A and C.
At its upstream point, however, the parabola has to be modified such that it takes a
curve upwards and meets the point P with the gradient of the phreatic line being
perpendicular to the dams upstream face.
Homogeneous dam with rock toe
An embankment having a rock toe is shown in Figure 51. The Base Parabola may be
drawn in much the same way by considering point A as the focus. The upstream face of
the rock toe may be at any angle, say α, to the horizontal. Accordingly, an appropriate
⎛ Δa
⎞
value of ⎜ ⎟ has to be read from the curve shown in Figure 52, which is used to
⎝ a + Δa
⎠
determine the point of attachment of the actual seepage line with upstream face of rock
toe at point D, as shown in the figure. For α less than 30 0 , the above method has to be
determined by a slightly modified technique. However, since rock toes with such angles
are not practically provided, they are not considered here.
Version 2 CE IIT, Kharagpur