The uplift pressure at the face of a drainage gallery in a gravity dam as per recommendations (U.S.B.R) is taken as equal to
1
the hydrostatic pressure at the toe, plus one-third of the difference of the hydrostatic pressure at the heel and toe
2
the hydrostatic pressure at the heel, plus one-fifth of the difference of the hydrostatic pressure at the heel and toe
3
average of hydrostatic pressures at the toe and heel
4
two-fifth of the hydrostatic pressure at toe