The uplift pressure on the face of a drainage gallery in a dam is taken as A. hydrostatic pressure at toe B. average of hydrostatic pressure at toe and heel C. two-third of hydrostatic pressure at toe plus one-third of hydrostatic pressure at heel D. none of the above

The correct answer is: B. average of hydrostatic pressure at toe and heel

The uplift pressure on the face of a drainage gallery in a dam is taken as the average of the hydrostatic pressure at the toe and heel of the dam. This is because the uplift pressure is caused by the water pressure acting on the face of the dam, and the water pressure is greatest at the toe and heel of the dam.

The hydrostatic pressure at the toe of the dam is equal to the weight of the water above the toe of the dam divided by the area of the toe of the dam. The hydrostatic pressure at the heel of the dam is equal to the weight of the water above the heel of the dam divided by the area of the heel of the dam.

The average of the hydrostatic pressure at the toe and heel of the dam is equal to the sum of the hydrostatic pressure at the toe and heel of the dam divided by 2.

The uplift pressure on the face of a drainage gallery in a dam is a significant factor in the design of the dam. The uplift pressure must be taken into account in order to ensure that the dam is safe.

The other options are incorrect because they do not take into account the fact that the uplift pressure is caused by the water pressure acting on the face of the dam.