ELI5: How do they prepare sand dunes to build roads, houses, and other projects on top of?

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Piles and different foundation setups. There are many kinds you can use actually and correct one is down to what the engineers are experienced in and what the project needs. But imagine you put two phonebooks together, so that the leafs are neatly laid from both sides evenly. The friction between two sheets of phone book paper is basically nothing, but if you multiply it by 1000, you get lots of friction. And this is how piles work. They are long and have lots of surface area against anything you push them in to, even soft sand. These prevent the foundations on top from moving sideways, or up or down. After this you can connect these to make a raft foundation. This setup is so solid and permanent after it’s natural settling that it can be considered to not move at all. Often you can find a sort of a strange hump near old building development, these were the foundations of the tower crane used for them. They don’t move, but the ground around it will move, so they can become quite pronounced. After the you lay a cast foundation on this all. But now… How do you do this for something that doesn’t have “foundations” in the same traditional sense. Like a road; no one is driving piles for a road. Well here we start to utilise the natural properties of the ground, or sand in this case. First you dig ditch, basically with 45 degree slopes on the sides (this is important). Then you place some special fabric at the bottom, fill a layer of sand, then another layer of fabric, another layer of sand… so on and so forth. You are basically creating sand and fabric sandwich composite. Why fabric? It tends to be soft and fragile, what good does it do? Well consider something like a cotton blanket. Yeah it doesn’t really have shape it’ll compress if you put something on top of it, you can’t build up with it, if you tear it from the side it’ll rip apart. But here is the funky engineering, it doesn’t need to. That an average cotton dish towel, hold it perfectly so it is a plane, and now pull on it as hard as you can. You can’t rip it apart by evenly pulling it. The threads it is made of all share the load equally and therefor they all take very little of it. Anyway back to the sand layers. As we know from playing in a sandbox, loose sand is soft and it flows out under you if compressed. it is like liquid in a way. Right now, remember the properties of the dish rag? How you couldn’t pull it apart? Now lets combine these two. The fabric prevent the whole layered foundation from spreading, the sand prevents it all from compressing. So now you have a foundation that doesn’t want to really move made entirely from fabric and sand. On top of this you build your road. Last question remains: Why 45 degree slopes? Because triangle is the strongest shape, and 45 degrees has the properties of using least surface area (volume of sand or dirt) while having the greatest tringle properties. If you take a perfect square and cut diagonally from point to point, you get a triangle with 90 degree and 2 45 degree angles. With these methods of piles, fabric, dirt and foundations, you can build anything on just about anything. You could basically even build on water in theory, in an ideal setup. But as we know cows in a frictionless vacuum don’t live for long.

SinisterCheese

Depends what’s under them. I worked building roads through the desert. Mostly the dune was scraped flat then some form of grave base was laid down, more layers of gravel then tarmac. You needed a lot! Then you find the dunes move / get blown and you’re forever clearing sand off the road 🙄

mcPetersonUK

Millions of dollars in foundation materials The burj khalifa used multiple, thick concrete pillars going 50+ metres down into the dunes, with a solid 15m thick slab of thick concrete to act as a foundation. The weight of all the sand around the pillars keeps them straight, while the final slab is thick enough to support things This wasn’t your papi’s house by a long shot, it was really a case of “I’m gonna build this here, and if mother nature wants me to fail, she can suck my magnum dong”. Not practical, but the only real option.

DTux5249

If it’s like other beach construction… they go through the sand to bedrock. Usually the sand layer isn’t so thick and it’s easy to move.

dang_dude_dont

One of the key ideas missing from the discussions below is the concept of “buoyancy”. For sand, the concrete pilings are driven into the (dense) sand, so much of the downward force of the pilings + roadway, the weight, is offset by the upward force of the displaced sand. Also, there is also a component of the ‘movement resistance’ of sand that also counters the weight of the pilings and roadway, which can unfortunately vanish with the precise ground vibrations, like what occurs during an earthquake, i.e., soil liquefaction. I don’t know the frequency of earthquakes in the desert, but they make life in California a bit *too* interesting.

MathPerson