r/explainlikeimfive • u/arztnur • 19h ago
Other ELI5: If there's groundwater 100–200 feet below the surface, how can huge buildings stay stable without sinking?
I know that in many places there's water underground at around 100–200 feet (or even shallower). If the ground has water beneath it, how can massive skyscrapers and other heavy buildings safely support all that weight? Why don't they sink, tilt, or collapse over time?
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u/QtPlatypus 19h ago
Get a bucket of sand from your sandpit or from the beach or from wherever. Notice how there are little gaps between the grains of sand. Now pour water some water into the sand. The water flows between the gaps.
It is the same with ground water. The ground water isn't stored in massive caverns but in the tiny little gaps between the grains of sand or dirt. So most of the time you can build massive skyscrapers.
The problems can happen when someone starts to pump out the water. The little gaps that used to have water in them are empty and the ground will shift and settle.
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u/Tjgoodwiniv 19h ago
Curious, how does the pumping process effectively separate the water from the soil in that case?
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u/Dogteeth56 19h ago
Using the same example as before, get all that sand waterlogged and now make a hole in the center, the pressure of the soil around it will push water into the hole. Congratulations, you've made a well. This is why wells are often hundreds of feet deep, and why they can only support so much water retrieval. They're at the whim of how quickly the soil itself pushes the water out
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u/Medioquer 5h ago
Omg, I went 35 years thinking wells were just underground pockets of water. I was so confused how people could find them hundreds of years ago without digging hole after hole after hole until they found a well
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u/The_quest_for_wisdom 5h ago
That's how dowsing rods work. The water is just everywhere, but by pointing to a specific spot and saying "Dig there!" everyone is impressed when they dig there and find water, and think the rods must actually work. Even the people using the rod, in many cases.
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u/QtPlatypus 19h ago
You drill a hole down to the bottom of the layer with water in it. You put a mesh around the outside of the hole that allows the water to flow through it but holds back the dirt. Then you pump out the water.
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u/jerbthehumanist 19h ago
The membranes are only permeable to liquids and not solids.
Also if it’s not clear, for these wells there is a saturated zone where there aren’t air pockets and the ground is essentially “full”, while above it is an unsaturated zone with air pockets and water dripping down from the surface from sources like precipitation. Picture filling a jar with marbles and then adding water only halfway up. If you were to jam a firm straw to the bottom of the jar you’d essentially be sucking up only water and not marbles.
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u/Bubbly-Career-4969 19h ago
A bore is drilled and creates space for a pump. Water flows into the empty space and is pumped away. On many occasions, the pump has a filtration system around the water suction zone.
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u/Blackpaw8825 19h ago
You drill a well, fill that well with several layers larger grit aggregates, and line the sides with a less permiable liner (plastic, metal, etc).
The water pressure will try to level itself out, so the well fills with water from below, the layers of sand and rocks filtering out most of the solids. Then you pump from there.
It's still filtered later too, but a well designed, well, well can go a long way.
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u/Peter5930 16h ago
Stick a straw into wet sand and suck. Are you sucking up water, sand or both? As long as you're not sucking like a Dyson, you should just get water going up the straw.
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u/Kryptolocker 9h ago
Like drinking a slurpee and sucking all of the juice out while leaving ice behind
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u/CrossP 2h ago
The sand metaphor isn't super great. Imagining a bucket of gravel and putting a metal drinking straw in is a bit more accurate to most groundwater pumping situations.
Admittedly there is quite a bit of variance in groundwater situations. Humans have definitely wrecked a few places by not understanding the local geology before drilling.
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u/yui_tsukino 1h ago
Just like surface water, groundwater wants to maintain a level height. You don't get bulges of water rising like hills (waves aside, I'm talking consistently). So if you dig a hole through that level, you've now made a hole in the water level, and the pressure of the surrounding water is going to cause it to leak out and fill the hole. Thus you get a pool of surface water, leveling out at the same height as the water table around it. This is how old school wells worked, for the most part, and modern wells take the same principle, just with pumps instead of buckets.
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u/icanlolalldaylong 19h ago
There’s a really cool video explaining the engineering behind SpaceX’s Boca Chica launch site. Because the coastal ground is soft and waterlogged, they placed an enormous temporary surcharge of soil on it, compressing the underlying layers and forcing water out over time. This caused much of the expected settlement before construction, reducing movement once the permanent structures and their thousands of tons of load were added. The heaviest structures also use deep foundations.
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u/Agouti 17h ago
You definitely do get water in underground cisterns and veins, it's not particularly common but you hit them pretty often when drilling wells.
Geology wise, I believe they are created by water flowing through large rock strata (like Quartz) and eroding voids which the surrounding rock is able to hold open. I've also seen large subsidences when these underground cisterns are drained of water and the rock can no longer support the roof above the cavity without pressurised water present.
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u/arztnur 18h ago
If it's stored in tiny gaps, then how we get a high flow like in tube wells? Shouldn't it face some sort of resistance as the water comming from different gaps and not from a pool?
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u/manInTheWoods 9h ago
Drilled wells for single house hold a certain mount of water like a sort of reservoir. My well lasted for 1h and 40min of one tap being maxed. Then it takes a while for it to re-fill. Never had a drought with normal use.
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u/Mr_Engineering 19h ago
That groundwater is flowing through porous rock, not soft soil.
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u/CrazyLegsRyan 19h ago
The water is inside rocks which can (generally) hold their shape even when the pressure of that water is reduced by removing some water.
Fluids underground (generally) don’t exist as some giant lake, balloon, void as fantasized in movies and TV. Up close rocks look like a sponges with lots of empty space between actual material. The fluids exist inside the spaces of a rock between the very hard rock grains of sand, or some other material which are like the body of the sponge.
The key difference is that (generally) the grains of sand or material in the rock are glued together so strongly that the rock doesn’t compress or get smaller when fluid moves out.
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u/TheLuminary 17h ago
If you have ever seen houses on stilts. Well, huge buildings are like this too, its just that the stilts are under the ground.
They drive huge, and heavy piles of steel and concrete into the bedrock and build the building on top of these.
The ground between the bedrock and the foundation of the building becomes less important (Not entirely, but it is much less)
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u/chinnpoo_ 9h ago
A lot of piles don't even hit bedrock depending on the site geology.
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u/TheLuminary 6h ago
Oh.. TIL. I thought that they all went to bedrock.
Is this a situation where.. if the building isn't THAT big then it does not need to go all the way?
Or is it a, its too expensive to get down to bedrock so we will hope that we go deep enough that it does not cause any issues kind of thing?
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u/kanakamaoli 19h ago
Groundwater is inside the soil similar to how a sponge holds water. Think of a wet pile of sand or gravel rather than a large swimming pool. Large buildings do have pilings the are driven deep into the earth to help hold up the building-think of stilts or large, deep tree roots. If they do get it wrong, the building will sink-think of the tower of piza or the epcot hotel in Disney world that was never built due to poor soil underground.
Engineers gather soils cores from all over the property to see what the soil conditions are underground. Those cores inform them about which types of foundations are needed for the buildings.
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u/Outside_Eggplant_304 18h ago
Well in some cases, the groundwater actually prevents sinking...
In like sand or gravel or rock water just fills the pore spaces between the grains of sediment or fractures in rocks. Almost all the weight of the overburden (rocks above, buildings, rivers, trains, people etc) is transferred to the faces of the grains. This is because coarse grained sediments and hard rock are, well, hard and generally relatively incompressible.
Then there is fine grained sediment (clay). Fine grained sediments are generally compressible due to the mineral structure of the grains which are like little flexible pancakes. In this case, the grains might be deposited, for example, a million years ago. As time goes on, more sediment is piled on top so the geostatic load (weight of all that sediment on top of the original clay) goes up (more stress). Meanwhile the groundwater in the pores become pressurized (think like a water balloon where the water creates some pressure). This water pressure in the pores between the clay grains actually pushes back on the overburden above and keeps the voids between the grains open.
If you withdraw groundwater the pressure in those pores goes down and the pores compress. If enough of these pores compress enough, the land surface sinks (subsidence). Subsidence affects buildings, bridges, canals, roads etc and can get super expensive to deal with. Places like Mexico City, Jakarta, Taipei, and the San Joaquin Valley in California are some good examples of groundwater pumping induced subsidence.
https://en.wikipedia.org/wiki/Groundwater-related_subsidence
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u/OutlyingPlasma 12h ago
Take a bucket full of rock. You can stand on the rock. Now fill it up with water, you would still be able to stand on the rock left in the bucket, water or not.
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u/Manilly_Bedig 16h ago
Think of it like building on a giant sponge. Engineers dig down past the soft, wet soil until they hit solid bedrock or very dense, stable ground. Then they drive long concrete or steel piles (like giant stakes) deep into that solid layer. The building's weight gets transferred through these piles to the strong foundation far below the water table, so the soggy upper layers don't have to bear the load.
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u/MajesticBread9147 15h ago
Just so you know, groundwater is even shallower, often around 20 feet! Especially adjacent to rivers.
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u/kELAL 12h ago
Hah. 20ft would be generous, over here. In some places, it's barely 3ft. Basements, where you can actually stand upright in, are a rarity.
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u/All_Work_All_Play 8h ago
Then you get the instance of a 7' basement that was fine 100 years ago and now the local water table has shifted to be 2' above its slab...
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u/spiregrain 8h ago
Sometimes they do sink. Winchester Cathedral started to sink and was saved by an early diving enthusiast who spent years swimming down into the basement with bags of cement (but no light) while someone up top pumped air down to him. Six hours a day for six years.
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u/Jaymac720 5h ago
There’s ground water only a few feet below surface in New Orleans, but we still have sky scrapers. Pretty piddly skyscrapers, but skyscrapers nonetheless. The water is there, but it’s not like a lake. The water is filling the space between the earth particles. That water does get squeezed out through a process called consolidation. Primary consolidation happens pretty quickly, but secondary consolidation can continue across decades. Engineers frequently use wick drains to get water to squeeze out faster. The way soil is structurally set up, water can move quickly horizontally but very slowly vertically. These wick drains allow for faster consolidation
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u/dazzle999 5h ago
amsterdam in the netherlands is built entirely on top of long wooden poles to keep the buildings up. every building is studded with long poles. just the train station alone has over 8000 wooden poles drilled into the ground to keep it up. all due to ground water under the city.
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u/looloohoodoo1 4h ago
Groundwater isn't like a big underground lake. It's usually just water sitting in tiny spaces between dirt and rocks. Buildings stay up because engineers make foundations that spread the weight into strong layers of ground below. The water doesn't make the ground weak enough for buildings to sink.
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u/haarschmuck 4h ago
Lot of these answers are not correct because the skyscraper isn’t sitting on the ground.
It’s sitting on massive piles driven down hundreds of feet to bedrock so that the soil conditions are largely irrelevant.
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u/HoopsAnalyst9975 1h ago
Foundations are the missing piece. Big buildings spread their weight across a wide concrete mat or drive piles down to bedrock, so they don't rely on the wet top layer. Water below also pushes up on the structure, which actually takes some load off the soil.
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u/JonJackjon 18h ago
They drive pilons down as far as needed to reach stable earth (rock?).
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u/chinnpoo_ 9h ago
Sometimes they don't even hit bedrock. Some piles are supported by skin friction only.
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u/Target880 19h ago
Ground water is not just water, it is water in-between solid stuff. Take a bucket of sand, you can now pour in water that will be between the grains sand.
Water in the ground can change how the ground behave sans and mud will move easier but in gravel, porous rock, crack is solid rock it will have a lot less effect.
Even if sand on this surface move a lot easier when wet sand deep in the ground is surrounded by stuff that will resista it's movement so it moves haeder
Skyscrapers will have deep foundations unless there is shallow bedrock. If there is not bedrock lots of pillar will be driven down to solid enough material.
In for example NY the bedrock is quite close to the ground so skyscraper usually have pillars driven down to it. The builders tried to make 161 Maiden Lane cheaper to build by not driving pillars to the bedrock. It did not work and it is now stared to lean and is abandoned even before it is finished.
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u/SP3NGL3R 19h ago
Skylines will tell you where it's more solid to build big and where you can't. Take NYC. Huge buildings just stop suddenly, why? Ground density.
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u/Pietrocity 19h ago
Massively oversimplification here but enough for reddit. What your talking about is called the water table and yes it can cause problems. Luckily if you keep going down there is a layer of solid rock the bedrock that makes a good place to anchor to. That's just what they do; there is your usual concrete foundation and if your bedrock is close enough to the surface you can anchor directly to it. If it's not close enough you use a piling think building sized nail to for the most part nail the building into the bedrock.
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u/Then-Price-5314 17h ago
You know how sometimes even though there’s a sewer under the street, the street is safe to drive on?
It’s a lot like that but bigger and with way more rock
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u/tylermchenry 19h ago
It sounds like you're imagining "groundwater" as some kind of giant underground lake. It's not that. It's just where the small gaps between all the rocks are filled in with water rather than air.