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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“The foundation” is both the concrete footings, and the ground underneath the footings.
The load is transferred through the ground underneath the footings. You couldn’t just place the footings on mud or clay, because the house would sink.
Before you pour the footings, you have to make sure you have good bearing soil underneath the footings. Sometimes that means you have to dig out what’s there and replace it with sand or gravel.
A foundation spreads weight out more evenly… think of difference between poking a pin through a piece of paper vs. a brick through a piece of paper.
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.
How much do you think a house will sink without a foundation?
The purpose of the foundation isn’t to prevent sinking. It creates a rigid and level structure to build the house on. Additionally a proper foundation needs to move water away from the structure. Placing wood directly on soil will cause it to rot as it absorbs moisture.
A newly built house will settle (sink) over the first year it is built, so having a foundation doesn’t prevent the house from sinking.
For the same reason that hitting a nail with a hammer will put a hole into wood, but hitting the wood directly with the hammer with the same amount of force won’t do anything. The force is concentrated into a much smaller area, which creates a much higher pressure.
Pressure = Force / Area
Say the tip of a nail 300 times smaller than the face of the hammer. Holding force constant (by swinging the hammer with the same force), decreasing the contact area by 300 times results in an increase in pressure by 300 times.
House foundations work the same way. Most of the load of the house is concentrated into the load-bearing walls (like the nails). By supporting those concentrated loads with a stiff, large foundation (like the hammer), the load gets spread out so that the area of the foundation that makes contact with the ground (like the wood) becomes much larger, in turn drastically decreasing the pressure on the ground.
A house “sinking” doesn’t mean the same thing as like what happens when you drop a marble in a puddle. It means that the different materials expand and shift and sag. If you’ve ever had a wooden door that’s harder to open when it’s humid, you’ve experienced this. A foundation gives it a hard, rigid platform to keep it from shifting too much. And depending on where you live it’s also a major support for keeping your house from completely collapsing in an earthquake or being tossed by a tornado with your inside. If a house were going to just sink into the mud or sand, than that is not a suitable place to build a house.
Not all houses have foundations. Sometimes you’ll see a manufactured home, or some cabin that’s propped up in a platform or some cinder blocks. Those houses don’t just sink into the ground, but they also won’t last as long or be as safe in.
Usually compacted soil. You can get some serious problems if you haven’t first properly prepared the ground. Google “foundation failure” for details.
Soil or substrate that is known to be unstable or shifting is tested by soil engineers and a remediation plan is made by which a certain amount of virgin soil is dug up and ‘better’ soil is brought in and compressed a bit at a time with testing done between layers by soil engineers. We live in an area that has patchy areas of unstable soil (river delta) and build houses. Stable soil obviously doesn’t need this remediation. The foundation, whether concrete monolith or pier and beam is constructed to carry the weight well and evenly when building codes are adhered to.
Foundation is to distribute the load of the structure in such a way that the soil/material beneath the foundation disperse it. The whole surface of the foundation first takes the load from the structure above it and then uniformly carries the load underneath. Thus converting high point loads into uniformly distributed loads. Even if in some cases the structure sinks, the foundation settles uniformly and thus keeping the integrity of the structure intact. Yes there are cases where the foundation sinks or settle from one side but that is tackled by the help of grouting to support the soil underneath the foundation to stabilize. In case of high rise building, usually the reliance is on deep columns that are extended to solid base (usually a stable rock) and thus even if the soil sinks /settle the building stays intact. Besides you can build directly without foundation if you think the soil is enough compact (even during earthquakes) or would not sink . And if you find a solid rock you just build over it without worrying of the structure sinking
Another thing to consider, if the build involves any excavation.
A house weighs a LOT less than the dirt taken out for its basement, and sits in the same place.
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