The whole house rests on the foundation or it will sink. What does the foundation rest on? Why doesn’t the foundation sink just as easily?

Question

I get that some foundations do have sinking issues, but how does having a foundation help? If the foundation is poured and solidifies, what stops the weight of the house from immediately sinking/tilting the foundation?

In: 2809

Answer 1

The house *doesn’t* rest on the foundation – not directly, anyway. It rests on the support structure (e.g. load-bearing walls), which is much smaller than the house in terms of area. If you let those support structures sit directly on the ground, the weight of the building gets concentrated under their small surface area, which applies much more pressure than the ground can support.

Instead, you create a foundation of very tough materials (that can bear the higher pressure), then make the foundation wide so that it can spread the load onto the ground over a wide enough area that the ground can support it. [Here’s a diagram](https://i.imgur.com/VpbIj7q.png). It’s the same principle as a snowshoe: a person’s weight applied over the area of their normal shoe exceeds the yield strength of snow, but that same weight distributed over a snowshoe does not.

(Deep foundations have an extra component: they bypass the weak layer of surface sediment and spread the load out over a range of layers or, in the strongest foundations, a tougher bedrock layer below.)

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Obligatory edit since this is starting to get non-ELI5 regulars:

> LI5 means friendly, simplified and layperson-accessible explanations – not responses aimed at literal five-year-olds.

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EDIT2: To address some commenters below: yes, there are other reasons to have a foundation (frost heaving, anchoring to bedrock, even settling, etc), but OP asked specifically why simple post construction sinks when a foundation doesn’t, and the answer is that the posts overwhelm ground yield strength and foundations, by distributing the weight across a wider area, don’t.

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EDIT3: Some commenters suggest [this video](https://www.youtube.com/watch?v=0_KhihMIOG8) from the excellent youtube channel Practical Engineering for a longer discussion of the topic.