eli5 Why do the seismic forces on a building depend on the soil type?


there are obviously different soil types – from hard rock to loose clay. Why do the forces, an engineer has to consider for his buildings, depend on it?

I understood that there is a response spectrum and e.g tall buildings got a lower eigenfrequency than lower. But I saw, that there are different response spectra for each soil type.

Why does it make a difference?

In: 2

Different soil types will move different amounts when you apply the same energy to it. Solid rock will move very little even if you apply a lot of forces to it while soft sand will move a lot even if there is little forces. This means that the amount of movement in a building during an earthquake depends a lot on the soil type. Buildings in more solid soil moves less then those in soft soil. On the other hand there is a lot more forces involved in solid soil types, the bedrock might actually end up tearing the foundations apart if it moves.

Different soil provides different amounts of dampening of the vibrations. Imagine standing in the back of a pickup truck as it goes over a bumpy road. Now imagine standing on 2 mattresses in the back of the truck as it goes over the same bumps. Would it feel the same to you, or would it feel at least somewhat different because you’re now standing on something that can compress and absorb vibrations?

Loose soil can also move, compress, and absorb vibrations more than compact or rocky soil can. That changes the vibrations felt by the building, and therefore the stresses on the structure.

Except for rock, the ground is usually not solid. In fact, at the right period of shaking, most ground types assume at least some qualities of a liquid and tend to resonate.

Rock resonates at much less destructive frequencies.

Gravel tends to dampen the effect. Clay and wet sand magnify it. The degree to which you expect surrounding ground to resonate determines how deep your foundation must be.