The Bizarre Paths of Groundwater Around Structures

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Love to see an engineering channel getting so many views

👍︎︎ 63 👤︎︎ u/crazylsufan 📅︎︎ Jun 08 2022 🗫︎ replies

This was incredibly timely for me. We just performed a dam inspection, and found a surface void next to a concrete splitter box that extends below grade. It's possible the void is a sinkhole similar to the one issue with the casson, but let's hope not! That would mean we've got big issue with the splitter box.

👍︎︎ 28 👤︎︎ u/submarine_sam 📅︎︎ Jun 08 2022 🗫︎ replies

The YouTube link, so you can like and monetize the effort

https://youtu.be/bY1E2IkvQ3k

👍︎︎ 14 👤︎︎ u/Engine_engineer 📅︎︎ Jun 08 2022 🗫︎ replies

Very interesting an intuituve seeing the pressure gradients. I'm not civil but it's incredible to think that even a megastructure like a dam is ultimately just placed loosely on the ground

👍︎︎ 2 👤︎︎ u/Miniman125 📅︎︎ Jun 09 2022 🗫︎ replies

Cool

👍︎︎ 1 👤︎︎ u/EatTheVegetables 📅︎︎ Jun 09 2022 🗫︎ replies

Yes, seepage do be a bitch.

The 3D homogenous anisotropic steady state flow equation is the scariest shit I ever saw during my degree.

👍︎︎ 1 👤︎︎ u/[deleted] 📅︎︎ Jun 09 2022 🗫︎ replies
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In 2015, an unusual incident happened on the  construction site for a sewage lift station   in British Columbia, Canada. WorksafeBC, the  provincial health and safety agency, posted a   summary of the event on YouTube. A steel caisson  had been installed to hold back soil while the   lift station could be constructed. One worker on  the site was suddenly pulled into a sinkhole when   the bottom of the caisson blew out. The cause of  the incident was related to groundwater within the   soils below the site. We don’t all have to live in  fear of the ground opening up below our feet, but   engineers who design subsurface structures do have  to consider the impact that groundwater can have.   The solutions to subsurface problems are almost  always hidden from public view, so you might never   even know they’re there. This video is intended  to shed some light on those invisible solutions   (including what could have been done to prevent  that incident in BC). I’m Grady and this is   Practical Engineering. In today's episode, we’re  talking about how groundwater affects structures.  This video is sponsored by HelloFresh,  America’s Number 1 meal kit. More on that later.  Groundwater has always been a little mysterious  to humanity since it can’t easily be observed.   It also behaves much differently than surface  waters like rivers and oceans, sometimes   defying expectations, as I’ve shown in a few of my  previous videos. One of the most important places   where groundwater shows up in civil engineering  is at a dam. That’s because groundwater flows   from high pressure to low pressure, and a  dam, at its simplest, is just a structure   that divides those two conditions. And what do  you know, I’ve got an acrylic box in my garage   full of sand to show these concepts in real life. You can imagine this soil sits below the base of   a dam, and I can adjust the water levels on either  side of the structure to simulate how groundwater   will flow. Blue dye placed in the sand helps show  the direction and speed of water movement below   the surface. A higher level on the upstream side  creates pressure, driving water in the subsurface   below the dam to the opposite end of the model.  I’ll be the first to say it: this is not the most   mind-blowing revelation. You probably could have  predicted it without the fancy model. But to a   civil engineer, this is not an inconsequential  phenomenon, and for a couple of reasons.  First, water seeping below a dam  can erode soil particles away,   a phenomenon called piping. Obviously, you don’t  want part of your structure’s foundation to be   stolen from underneath it, and piping can create  a positive feedback loop where failure progresses   rapidly. I have a whole video on piping that  you can check out after this one. The second   negative effect of groundwater is less obvious.  In fact, until around the 1920s, dam engineers   didn’t even take it into account (leading to  the demise of many early structures in history).  The engineering of a dam is largely an  exercise in resisting hydrostatic pressure.   Water in the reservoir applies an enormous force  to the upstream face of a dam, and if not designed   properly, that force can cause the dam to slide  downstream or overturn. The hydrostatic force is   actually pretty simple to approximate. Pressure  in a fluid increases with depth, so you get a   triangular distributed load. Once you know that  load, you can design a structure to resist it,   and there are a lot of ways to do that. One of  the most common types of dam just uses its own   weight for stability. Gravity dams are designed  to be heavy enough that hydrostatic forces   can’t slide them backwards or turn them over.  But, to the dismay of those early engineers,   pressure from the reservoir is not  the only destabilizing force on a dam.  Take a look at this pipe I’ve included in the  model that shows the water level between the two   boundaries. If the base of a structure was below  the water level shown here, the groundwater would   be applying pressure to the bottom, counteracting  its weight. We call this uplift pressure. Remember   that the only reason gravity dams stay put is  because of their weight, so you can see how having   an unanticipated force effectively subtracting  some of that weight would be a bad thing. Many   concrete gravity dams have failed because  this uplift force was neglected by engineers,   including the St. Francis Dam in California that  killed more than 400 people when it collapsed in   1928. Many consider this to be the worst American  civil engineering disaster of the 20th century.  Unlike the hydrostatic force of a reservoir,  uplift pressure from groundwater is a much more   complicated force to characterize. It exists  in the interface between the structure and   its foundation, in the cracks and pores of the  underlying soil, and even within the joints of   the concrete structure itself. The flow of  groundwater is affected by soil properties,   the geometry of the dam, the water  levels upstream and downstream,   and even the subsurface features. How these  factors affect the uplift pressure can be pretty   challenging to predict. But engineers do have  to predict it. After all, we can’t build a dam,   measure the actual uplift force, and add weight  if necessary. It’s gotta work the first time.  One way to characterize groundwater flow around  structures is the flow net. This is a graphical   tool used by engineers to estimate the volume  and pressure of seepage in the subsurface.   In simple terms, you divide the flow area into  a curvilinear grid, where one axis represents   pressure and the other represents flow. If this  looks familiar, you might notice that a flow   net is essentially a 2D solution to the Laplace  equation, which also applies to other areas of   physics including heat flow and magnetic fields.  Developing flow nets is almost an art as much as   a science, so it’s probably a good thing that  groundwater problems are mostly solved using   software these days. But, we can still use flow  nets to demonstrate a few of the ways engineers   combat this nefarious uplift force on gravity  dams. And one common idea is a cutoff wall.  If water flowing below a dam causes so many  problems, why not just create a vertical wall   to cut it off? We do it all the time. But, how  deep does it need to be? Some dams might have a   convenient geological layer into which a cutoff  can be terminated, creating an impenetrable   envelope to keep seepage out. But, many don’t.  Cutoff walls can still reduce the volume of   flow and the pressure, even if seepage can still  make its way underneath. Let’s take a look at the   model to see why. I’ve added a vertical wall  of acrylic below the upstream face of my dam,   and we’ll see how it affects the flow. The  groundwater flow lines adjust to go under the   wall and back up to the other side of the model.  If you look closely you’ll see a slight decrease   in the uplift measurement pipe below the dam. The  only thing I changed between this model and the   last one was adding the cutoff wall. So why would  the pressure decrease on the downstream side?  The flow of groundwater is described with a  fairly simple formula known as Darcy’s law.   Besides the permeability of the soil, the only  other factor controlling the speed water flows   is the hydraulic gradient, which consists of the  difference in pressure over the length of a flow   path. By adding a cutoff wall, I didn’t change  the difference in pressure between one side of the   model and the other, but I did increase the length  of the flow path water had to take below the dam,   reducing the hydraulic gradient. I can sketch  a flow net over the model to make this clearer.   The black lines are equipotentials; they connect  areas of equal pressure. The blue lines show the   directions of flow. Without a cutoff, the flow  paths are shorter, and thus the equipotential   lines are closer together. With the cutoff wall,  the equipotential lines are spread out. That means   both the volume of seepage and the uplift pressure  at the base of the structure have been reduced.  Cutoff walls on dams have a long history of  use, and nearly all large gravity dams have   at least some kind of cutoff. It can be as simple  as excavating a wide area of the dam’s foundation   before starting on construction, and that’s  a popular choice because it gives engineers   a chance to observe the subsurface conditions  and make sure there are no faults or problems   before the dam gets built. Another option is to  excavate a deep trench and fill it with grout,   concrete, or a slurry of impermeable clay. For  smaller or temporary structures, sheet piles can   be driven into the subsurface to create a cutoff.  One final option is to inject high pressure grout   to create an impenetrable curtain below the dam. The other way to deal with seepage and uplift   pressure are drains. Drains installed below a dam  do two important jobs. First, they filter seepage   using sand and gravel so that soil particles can’t  be piped out from the foundation. Second, they   relieve uplift pressure by removing the water.  Let’s see how this works in my model. Upstream   of my uplift monitor, I’ve added a hole through  the back of the model with a tube to drain seepage   out. Instead of flowing all the way downstream,  now some of the seepage flows up to and through   the drain, and you can see this in the streamlines  of dye flowing in the subsurface. Again, the   effect is subtle, but the uplift pressure monitor  is showing a slight decrease in pressure compared   to the original configuration. There is less  pressure on the base of the dam than there would   be without the drain. Plotting a flow net over the  model, you can see why it behaves this way. The   drain relieves the uplift on the base by creating  an area of low pressure below the dam. You can   also note that the drain actually increases the  hydraulic gradient by shortening the flow paths,   so there’s actually more seepage happening  than there would be without the drain. However,   because the drains are installed with  filters to reduce the chance of piping,   that additional seepage is often  worth the decrease in uplift pressure.  Many concrete dams include a row of vertical  drains into the foundation, and some even use   pumps to depress the groundwater level further,  minimizing the uplift. I can simulate this by   lowering the downstream level as if a pump was  removing the water. Watch how the flow lines   adjust when I make this change in the model. Like  drains, these relief wells create more seepage   below a dam because of the greater difference  in pressure between the two sides, but they   can significantly reduce the uplift pressure  and thus increase a structure’s stability. I’ve been using dams as the main  example of managing groundwater flow,   but lots of other structures have similar  issues. Retaining walls and temporary shoring   have to contend with groundwater challenges,  including caissons, which are watertight chambers   sunk into the earth to hold back soil during  construction. Remember the worker I mentioned   in the intro? He was on a site near a caisson.  It’s typical to dewater a structure like this,   meaning the water is pumped out, creating a  dry area for construction crews to work. Let’s   take a look at how this works in the model. I’m  simulating the act of pumping water out of the   caisson by draining out of the model at the bottom  of the structure. When a caisson is dewatered,   it is essentially working like a dam, separating  an area of high pressure from low pressure within   only a short distance between them. And, as  you know, distance matters when it comes to   groundwater, because the shorter the flow paths,  the greater the hydraulic gradient, and thus the   higher the volume and velocity of seepage. If you look closely, you can see the sand   boiling up as the seepage exits the soil into the  bottom of the caisson. This elevated pressure in   the subsurface and high velocity of flow means  that the soil particles themselves aren’t being   strongly held together. All it takes is a little  agitation for the soil to liquefy and flow into   the bottom of the caisson, creating a sinkhole  that can easily swallow anything at the surface.   One way of mitigating this hazard is dewatering  the soil outside the caisson. Construction crews   use well points, small evenly spaced wells  and pumps, to draw water out of the soil so it   can’t seep to areas of lower pressure. Caissons  can also be driven deeper into the subsurface,   creating a condition similar to a cutoff wall  on a dam. They can even go deep enough to reach   an impermeable layer, creating a better seal that  prevents water from flowing in through the bottom.  Thankfully for the worker in BC, his  colleagues were able to rescue him before   he was consumed by the earth. Next time  you see a dam, retaining wall, caisson,   or any other subsurface construction, there’s a  good chance that engineers have had to consider   how groundwater will affect the stability.  Even though you’d never know they’re there,   some combination of drains and cutoffs were  probably installed to keep the structure   (and the people around it) safe and sound.  Speaking of sounds, here is the peaceful  sound of a napping toddler while my wife   and I prepare a home-cooked meal from  the sponsor of this video, Hello Fresh.  “What do you call that?” “Hot raisin water”  HelloFresh has kid-friendly recipes for  picky eaters but this one is just for   us. The meals take about 30 minutes to  prepare which is perfect for nap time   because it’s really hard to film yourself  preparing lunch while chasing a toddler at   the same time, especially when the  photographer is 9 months pregnant.   But… our little helper just couldn’t stand to be  left out of the fun, so he woke up early anyway.  They also have different plans to meet your  various nutritional or dietary goals, like   pescatarian, Fit and Wholesome, or vegetarian  which is what we usually get. It’s a perfect way   to make a change or try something new. And this  is not us being mean, he really loves lemons.  And here’s the most important thing: Hello  Fresh meals are delicious. We’ve done this   for quite a while now, and there isn’t a single  recipe that we haven’t enjoyed. Actually we got   invited to a party at the last minute, so we  ended up combining the portions of this meal   and bringing it to share, and it was a huge hit. “This might be the best one you’ve made yet”  Go try it yourself at HelloFresh.com, and if you  use code PRACTICAL16, you’ll get up to 16 free   meals plus 3 surprise gifts. I wouldn’t recommend  it if I didn’t think you would love it. That’s   HelloFresh.com and use my code PRACTICAL16. Thank  you for watching and let me know what you think.
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Channel: Practical Engineering
Views: 11,002,396
Rating: undefined out of 5
Keywords: caisson, groundwater, subsurface, structures, Groundwater, dam, piping, hydrostatic pressure, Gravity dam, uplift pressure, Cutoff wall, hydraulic gradient
Id: bY1E2IkvQ3k
Channel Id: undefined
Length: 14min 1sec (841 seconds)
Published: Tue Jun 07 2022
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