LOADING: CONCEPTS AND CRITERIA 125
customary to assume a linear or, if pressure relief drains are installed, a
bilinear pressure distribution envelope. Pressures are assumed to diminish
from the external hydrostatic value (^) wz 1 at the upstream face to tailwater
pressure or to zero, as appropriate, downstream. The assumed linear pres-
sure envelopes are shown schematically in Fig. 3.2(c).
Uplift load, Pu, is represented by the resultant effective vertical com-
ponents of interstitial water pressure uw. It is referred to as internal uplift
if determined with respect to a horizontal plane through the dam. Deter-
mined for an external plane it may be identified as base uplift if at the
dam–rock interface, or as foundation uplift if within the underlying rock
horizons.Puis a function of the mean pressure (uw avg) across a plane and
of plane effective area. The latter is defined as the relative proportion of
horizontal plane area, Ah, over which interstitial pressure can operate,
allowing for the ‘solid’ mineral skeleton, i.e. effective area AhAh,
whereis an area reduction coefficient; thus
PuAh(uw avg) (kN m^1 ). (3.5)
If no pressure relief drains are provided or if they cease to function
owing to leaching and blockage, then
PuAh (^) w. (3.6)
z 1 z 2
2
Fig. 3.2 Internal uplift and pressure envelopes