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?
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Architect here! Very over-simplified but easy visual for you:
Imagine you are at the beach and you set up your lawn chair. The front and back bars of the chair will sink into the sand quite a bit. That is your building walls sinking into the earth.
Now, imagine that you placed two strips of plywood, each 3 inches wide, on the sand, and THEN you place the bars of your lawn chair in those strips of plywood. Your chair would not sink into the sand very much. That is your building foundation (footing, really).
The foundation footing spreads out the load so that your building’s walls and columns can “float” on the earth.
Extra credit concepts – foundations also:
– Provide anchorage against lateral forces (wind, earthquake)
– Resist hydrostatic pressure (water in the soil)
– In cold climates, prevent heaving by being below the frost line
Nothing stops the weight of the house from sinking the foundation.
But the foundation is designed so it only sinks or tilts a little bit.
There are a few schemes and the type of soil matters a lot. The simplest is a raft foundation. You pour a big mat of concrete and then everything sits on that. The concrete pushes down on a very large area of soil, so it only moves a little bit.
A pile foundation doesn’t sink very much because it has friction along the entire length where the soil and the pile meet.
But if you want to build a really really heavy building, you want to build it over soil – or even better bedrock – that can support that weight. Shifting sand would be bad.
A cartoon of the two most common types of foundations. [https://www.mdpi.com/applsci/applsci-11-03099/article_deploy/html/images/applsci-11-03099-g002-550.jpg](https://www.mdpi.com/applsci/applsci-11-03099/article_deploy/html/images/applsci-11-03099-g002-550.jpg)
As very simple way to see this is by comparing it to a boat. The water push on the boat’s hull like the soil push on the fondation. The viscosity is different, but the principle of floatability is the same.
The foundation spreads out the weight of the house over a much wider area than the (relatively) narrow footprint of each of the walls. Soil has a certain strength to it (that’s expressed as a force per unit area, i.e. pressure). By spreading out the force over a wide area we keep that pressure low.
It’s much like pushing one finger into sand (fairly easily) vs your whole hand (takes much more force). Or compare a road bike vs mountain bike tire in mud.
They are like snowshoes. If you stick your foot in the snow, it sinks. If you stick your foot in the snow while wearing snowshoes or skis, they do not sink!
Here’s a worse sounding example. If I were to take a stick and poke you in the chest with it, it will hurt you and possibly even stab into you. If you put a book between the stick and your chest and I poke the stick with the same force…you will not be hurt by it.
This is because the book (foundation) is spreading out the force of the stick(the weight of the house along the load bearing walls).
also ill note that most garages have a 4 inch fall that most people do not notice. this prevent water from entering the garage on a flat grade
It’s like how a boat floats because the weight is put out over a wider area. If a house was just sitting on the ground there would be alot of weight in certain parts which makes it easier to sink. The foundation spreads it out.
There are some solid answers, so I won’t restate them.
I *will* give an example of building a big heavy building on an *insufficient* foundation.
[At this point, the Millennium Tower in San Francisco has sunk more than 18 inches, as well as leaning over since one side is sinking faster than the other.](https://www.theguardian.com/us-news/2022/jan/10/san-francisco-millennium-tower-sinking)
There’s plenty of great descriptions in here of standard foundation walls and retaining walls. I’ll just add that some houses are actually *screwed into the ground*. Recently I designed a small cottage that sits on a series of ‘helical piles’ (others are called micropiles). The contractor brings in a machine that torques the piles, which look like big dumb screws (like big big, 20′ long), and keeps screwing them down into the ground to the specified torque/depth. Then, large wood beams bolt into the exposed top of the piles. Conceivably, it’s possible to deconstruct the house in the future and ‘unscrew’ the piles again, making it a more sustainable foundation option for soils/climates that can bear it. [Here’s an explainer video](https://www.youtube.com/watch?v=ImW4htCUcwk).
Not a lot of ELI5 responses in here. Think snow shoe. Without a snow shoe, your foot focuses the downward force of your body to a smaller area. The snow shoe takes that force and spreads the love over a larger area.
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