How are Underwater Structures Built?

Water bodies are a major part of the landscape. Oceans, rivers, and lakes are often beautiful, but they’re not necessarily.


Water bodies are a major part of the landscape. Oceans, rivers, and lakes are often beautiful,
but they’re not necessarily convenient places to build things. Most of our tools and construction materials
– not to mention our labor force – work better in the dry. And yet, many types of the infrastructure
we depend on every day, including wharves, bridges, and dams, are founded below the water. How do they do it? Hey, I’m Grady and this is Practical Engineering. On today’s episode, we’re talking about
different types of underwater construction. This video is sponsored by Hello Fresh, America’s
most popular meal kit. More on that later. One of the major costs of large construction
projects on or near bodies of water is how to manage that water. The first thing you might think of when you
consider underwater construction is divers. Humans can survive and even accomplish work
underwater with the help of SCUBA equipment that allows them to breathe and stay warm. Professional divers can accomplish a wide
variety of tasks like welding, cutting, and erecting formwork and other structures. But, professional diving is dangerous, and
the types of tools and equipment that both function underwater and can be used safely
by a diver are fairly limited. Remote vehicles and submersibles can take
some of the risk away, but they’re also limited in the types of tasks they can accomplish. So, a lot of underwater construction actually
involves getting rid of the water so you’re not building under it at all. In the industry, this is called dewatering,
or sometimes just “care of water,” and it includes a wide variety of construction
equipment and techniques that all have a single goal in mind: to allow construction to happen
safely and soundly in areas that would otherwise be infeasible or impossible to build because
of water. The heart of many site dewatering plans is
the cofferdam, a usually temporary structure built to hold water back from a construction
site. Obviously, a cofferdam needs to be a structure
you can build in the wet, otherwise, you’d need another cofferdam to construct it. In many cases, this is simply an earthen berm. You can just dump soil into the water until
it creates an embankment tall enough to serve as an impoundment. Once your construction area is enclosed, you
simply pump the water out. Of course, soil is somewhat permeable, so
you need to constantly pump out the water that manages to seep through. And, uncompacted and saturated soil is not
very strong. A collapse could be extremely dangerous for
any workers below the cofferdam, so these types of structures require careful design
by an engineer to make sure they’re safe. Besides earthen embankments, there are a huge
variety of cofferdam designs used on construction sites. Some use sheet piles, thick steel plates that
interlock together. These are driven into the subsurface soils
using a huge hammer to create a watertight barrier. If the soils are too rocky to drive sheet
piles, or the depth is very high, sometimes sheet piles are used to create small individual
enclosures filled with soil called a cellular cofferdam. There are also cofferdams built of steel frames
with a membrane and even water-filled rubber bags. No matter what they’re made from, cofferdams
are almost always built to be dismantled and removed after construction. One important use of cofferdams is to build
actual dams. In this case, often two cofferdams are necessary
to block off the river on the upstream and downstream ends. But, when performing construction across a
river, obviously impoundment isn’t the only necessary activity for dewatering. You also need a way to divert the normal river
flows around the construction site. This is why many dams are constructed in phases. You can build most of the structure away from
the main channel, then divert the river through the recently-constructed intake or spillway,
and finally construct the closure section of the dam. The Hoover Dam was built in a narrow canyon
and needed massive diversion tunnels through the rock on either side. The lower entrances to these tunnels were
sealed off after construction, but the rest of the tunnels became part of the spillways. For certain types of structures, the foundation
can be constructed off-site and floated in by barge, commonly called a caisson. A hollow box or cylinder is lowered into its
proper location, and then the soil is excavated and removed from within until a sound layer
of rock or strong soil is reached. Finally, the caisson is filled with concrete. Of course, that part about excavating down
to a sound rock layer isn’t as easy at it sounds. In the past, this was often done by workers,
which meant that the inside of the caisson needed to be dewatered. And when you dewater a caisson, you create
a difference in pressure between outside and inside, with only soil in between. This means that a constant battle of seepage
flowing in from the bottom of the caisson, or much worse, unstable soils that can rapidly
erode and allow the caisson to flood. These problems are what led to the pneumatic
caisson, a variation on the original design where compressed air is injected into the
structure to balance pressure from the water below. In a pneumatic caisson, the air pressure is
maintained equal to or higher than the water pressure at the bottom of the structure so
that seepage and soil instability can be avoided. It works just like when you turn a cup upside
down before putting it under water, except in this case the cup is much bigger, and the
pressures inside can be much higher than normal atmospheric air pressure. This is why workers on underwater foundations
often got “Caisson Disease,” the same decompression sickness that divers get if
they surface too quickly. The use of pneumatic caissons is fairly rare
today because of all the safety issues. In fact, most advances in construction technology
with regard to water have been not toward better dewatering methods, but in how to avoid
dewatering altogether. One of those advancements is the use of drilled
shafts. With special equipment and construction techniques,
you can excavate hole, install steel reinforcement, and fill it with concrete to create extremely
strong foundation system without any dewatering required. Concrete is a lot denser than water, so if
you can do it without much turbulence which can dilute the cement paste and weaken the
final product, concrete placed underwater will cure and harden just as well, if not
better, than if it were conventionally placed. The main way we accomplish this is through
the use of a tremie, a tube through which the concrete is pumped or gravity-fed to the
bottom of the form. The end of the tube stays below the top of
the concrete as it fills the excavation, preventing the water from washing away the aggregate
and diluting the cement. Whether the construction site is on the bottom
of a lake or river, or simply located in the floodplain and only at risk during extreme
weather, engineers and construction contractors put a significant amount of thought and consideration
into the feasibility and costs of managing this water. Of course, this video can only scratch the
surface of a topic that is as varied as the types and locations of construction sites
around the world, but I hope it helps you understand a bit more about how we build infrastructure
around water. Thanks to Hello Fresh for sponsoring this
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100 thoughts on “How are Underwater Structures Built?”

  1. You give a good summary of how without complicating it with too many details. That must be the constant battle in your editing.

  2. if you don't have sponges yet, just drop sand in the shape you want then once it all falls down to the seafloor, cover it in glass, then remove the sand. Done! i use this strategy to build my underwater houses cuz i dont wanna kill an elder guardian to get wet sponge then dry it

  3. Seeing your face interspersed throughout the video takes away from the continuity of the topic at hand. It would be better if you popped up at the beginning and end only.

  4. Is there a way where you can take massive flood waters and get them to go into the groundwater faster challenge

  5. So just like that divers swimming to underwater cunstruction iww here in the philippines so many dirty water just like doing now for mrt in the mandaluyong area😂

  6. What the construction companies should do is form an alliance with Posideon, he can easily move the water for them

  7. Very interesting. But, I wish you would do how skyscrapers are made, even to how the steel maker knows what pieces to make for girders, all their various shapes and how to be put together. Thanks!

  8. Really well-presented, great volume, great understandability. I am from the UK so HelloFresh is not an option for me, but it all looks great and American content is fine by me.

  9. Can steel structure be erected as seaport. Is steel structure rated to hold that much weight from the crane and The shipping containers. Is there anything that stops steel from getting rust. Would steel beams coast less than concrete..? Please it would be so kind of you to answer this. But if you do a vedio about it… it would be amazing

  10. Easy.
    1:Get sponge blocks. (Can be difficult due to guardians at ocean momuments.
    3:dry sponges
    3: build structure under water
    4: wall off areas leaving 5-10 blocks between each wall.
    5:confirm sealage
    6: place sponges from top to bottom in each layer until water is gone.
    7: remove interior walls
    8:enjoy underwater building.

  11. Interesting video. Battling and triumphing against the assortment of challenges that exist working sub surface is always interesting and fulfilling.

  12. Everybody knows all you have to do is sump out whatever body of water your in. Atlantic ocean? Sump it out bruh. Easy peasy

  13. You must be able to build a cofferdam in the wet otherwise you'd have to first build another cofferdam. Yep he said that

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