The provision of a drainage gallery in a gravity dam help in reducing? 1- hedrostatic pressure 2- silt pressure 3- seepage pressure

Thanks for the invitation

I would say  1- hedrostatic pressure

The optimization of foundation drainage systems of concrete gravity dams consists in determining the number and position of drainage galleries and the positions,inclinations,lengths,diameters and spacings for the drains in order to reduce the uplift pressure to adequate values.

Reductions to the uplift force will permit the structure's weight (concrete volume) to be also reduced without changes to the safety factor value.