Building of underwater structures | By Interesting Engineering

 🎧 Listening comprehension: Source 👉 Building of underwater structures

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Bridges, tunnels, dams, these monoliths of infrastructure are all around us. However, if you take a moment to really think about these structures, you might start to wonder how they were even built in the first place; how do engineers construct the foundation of bridges; when they're fully underwater; does it require a skilled team of divers working with concrete; or can the foundation simply be dropped into place; The main ways engineers build structures underwater are through cofferdams, caissons, or newer methods like drilled shaft foundations. All three of these construction techniques involve some manner of construction underwater or in the water. And they also require a bit more information to fully understand.

Let's start with cofferdams. Cofferdams are temporary structures, usually made out of welded steel sheet piles or other rigid interlocking materials which can be arranged in the riverbed or seabed around a construction site. Once a fully enclosed space has been created, the water inside is pumped out, leaving a dry workspace. Early cofferdams were made out of simple dirt mounds in the water, but these were dangerous and prone to failure. Later cofferdams then shifted to wooden piles and structures. With today's custom steel framework or piles being the safest type ever made, cofferdams are commonly used for bridge foundation construction or for pier construction closer to shore. Cofferdams are also used to put ships into dry dock to allow crews to do maintenance. For dam construction, engineers will often construct cofferdams first to keep the dam dry while they finish construction. The temporary cofferdams will then be removed and the dam structure itself allowed to serve its intended purpose. In this way, one builds a dam by first building several temporary dams, close in function to cofferdams but slightly different is a caisson. 

Caissons are essentially large concrete or steel boxes that are pushed into the seafloor, and then similar to cofferdams, the water is pumped out of the inside; however, unlike cofferdams, caissons are one solid structure. These structures will usually have crew access ports to allow workers to access the inside. Caissons and cofferdams are some of the main methods for underwater construction. They both create a dry workspace allowing work to proceed; However, they are also very expensive and are still relatively dangerous. Anytime workers are inside one of these structures, they are at risk of catastrophic failure and collapse of the dams. It's for this reason that engineers are constantly trying to find new safer underwater construction methods. 

One of the newest methods is that of drilled shafts. Drilled shafts are exactly what they sound like. Massive shafts that are drilled into the river or seabed. These shafts can be drilled with equipment mounted on barges or other structures, meaning that caissons or cofferdams are not needed for these constructions. Once the shaft is drilled, they can be backfilled with concrete or other reinforcing structures. This entire process allows engineers to avoid the dewatering process altogether. When it comes to how engineers build structures underwater, the real answer is that they try to avoid it when they can. Even for structures like bridges and tunnels, engineers will work to build as much of the structure on the surface as possible. Dewatering and underwater construction are highly expensive and dangerous endeavors but clever engineers have developed innovative engineering methods over the years to make the process as efficient as possible or even eliminate the need for it altogether.

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