DAM WALLS ARE THICKER AT THE BOTTOM BECAUSE THE WATER PRESSURE IS GREATEST THERE
WRONG!!
It is true that water pressure IS greater at the bottom than the top, but it's not the reason for the wall's shape. The wall is thicker at the bottom for structural reasons: it needs most strength at the bottom because the bending effect (moment) is greatest there so it needs to be strong. (compare it to a diving board with someone on it - it is most likely to break where it is held!)
thanks to James Brookman.

Also many thanks to Greg for his contribution:
In ‘gravity dams’, the walls are thicker at the bottom because of the: 1) bending (moment) effect and 2) to create a large surface area in contact with the underlying rock resistant to sliding. The large surface area on the base of the dam is like that on a large sliding block*if the area of the base is greater then the weight of material above it can be greater thus for a given coefficient of sliding friction (between the concrete dam and the rock), the greater the resistance against sliding at the base of the dam. The bending moment and dam strength becomes a more important issue in ‘arch dams’, where the dam is lighter weight and thinner that a massive gravity dam. Gravity dams rely less on bending strength and less on being socketed into the rock.
Greg Farrand Engineering Geologist

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