To remain stable, a dam must be strong enough in compression to resist the horizontal pressing force of millions, even trillions of gallons of water that would otherwise topple the structure or send it sliding down the river valley. To counter the force of the water, which presses horizontally against the dam, the dam itself and the rock either side of it and beneath it must provide an equal force from the opposite direction.
Some dams, known as gravity dams, rely on their mass, or quantity of matter, to manage the load of the water. For others, known as arch dams, shape is more important. An arch directs the load along its curve to where it meets resistance from support structures on either side, often from solid canyon walls. The narrower shape of an arched dam requires considerably less building material than a typical gravity dam, so engineers often choose this design to save on construction costs.
The maximum force of the water pressing against the bottom of Hoover Dam, a concrete arch-gravity dam, is about 45,000 pounds per square foot. Moving upward, this force decreases linearly, and the average force exerted on the dam is about half this figure. The dam's solidity and weight -- it is composed of 6.6 million tons of solid concrete -- provide all that's needed to withstand this load. However, the dam's engineers added other design features, just for good measure.
Straight walls will readily topple against immense water pressure, so the downstream side of Hoover Dam is sloped. In profile, the dam looks like a right triangle, with the perpendicular side toward the water. Hoover Dam is also arched, projecting outward into Lake Mead.
How does a curved dam resist the force of the water pushing behind it? Draw a diagram to show the different forces at work. Now draw a straight dam. How are the forces different? Why is this design "weaker?" How does a very heavy object resist sliding sideways when you push hard on it? Which heavy things are easy to push sideways and which are difficult? Can we make things harder and easier to push sideways? Explain. Some very heavy things, such as cars and trains, can move sideways fairly easily. Why would just being heavy be enough to prevent a dam like the Hoover Dam from moving sideways?