5.1.2 Pore pressure ratio

The ratio, at any given point, of the pore water pressure to the weight of the material acting on unit area above it is known as the pore pressure ratio and is given the symbol ru. (See also Section 5.5.1.)

Flow parallel to the surface and at the surface

The flow net for these special conditions is illustrated in Fig. 5.3.

If we consider the same element as before, the excess pore water head, at the centre of the base of the element, is represented by the height hw in Fig. 5.3. In the figure, AB = zcos/3 and hw — AB cos ß. Hence, hw = zcos2/3, so that excess pore water pressure at the base of the element = 7wz cos2 ß.


7Z 7Z 7

The equation for F becomes:

7Z 7Z 7

Equipotentials z


Fig. 5.3 Flow net when flow is parallel and at the surface.

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