Why dams are made stronger and thicker at the bottom that at the top?

The dams of water reservoir are made thick near the bottom. Why?

A cylindrical gas container is closed at the top and open at the bottom. If the iron plate of the top is 5/4 times as thick as the plate forming the cylindrical sides, the ratio of the radius to the height of the cylinder using minimum material for the same capacity is (A) 3:4 (b) 5:6 (c) 4:5 (d) none of these

Why two holes are made to empty an oil tin ?

In Regnault's apparatus for measuring specific beat capcity of a solied, there is an intel and an inlet and an outlet in the sterm chamber .The intel is near the top and the outlet is near the bottom .Why is it better than the opposite choice where the inlet is near bottom and the outlet is near the top ?

Why are bells made of metals?