How Bridges Are Built Over Water
It's both amazing and mind-boggling to think about how bridges are built. After all, these structures can be built on massive scales and be used to transport tens of thousands of vehicles every single day. And when you think about how these marvels of engineering are built over large bodies of water there are even more questions that will come to mind.
So to help answer your questions about how bridges are built over water, or even about how any above-water structure is built for that matter, here are the 4 main methods for creating a structure's submerged underwater foundation.
Table of Contents
- The Theory Behind Building Over Water
- Method 1: Battered Piles
- Method 2: Cofferdams
- Method 3: Caissons
- Method 4: Drill Shafts
The Theory Behind Building Over Water
Naturally, with the process of building over large bodies of water, you will need to build underwater. With smaller bodies of water, construction might actually involve getting rid of the water or avoiding it altogether so that you’re not actually building underwater at all as it can be a dangerous and unsafe task. However, removing water from bridge projects is not possible in a lot of cases and the most you can do is alter and displace the water making it easier to build.
Although professional divers can accomplish a wide variety of tasks like welding, cutting, and erecting formwork and other structures while underwater, this isn't exactly the safest or most effective method for construction. This kind of underwater work is typically achieved with the help of scuba equipment that allows them to breathe and stay warm - 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.
You might also consider using remote underwater vehicles to take some of the risks away, which they technically can. However, they’re very limited in the types of tasks they can accomplish.
So, in order to accomplish the dewatering of a construction site, a wide variety of construction equipment and techniques have been developed and used, all with the goal of allowing the safe and effective construction of structures in areas that would otherwise be almost impossible to build in because of water.
Method 1: Battered Piles
This first method that's used to build bridges over water is one of the most popular ways to build any overwater foundation for a structure. Essentially, battered piles are large foundational poles that are driven into the soil underneath the water in order to create a stand-like base for the structure on top.
Usually made with reinforced concrete or steel, large piles are inserted into the submerged ground using pile drivers, which are large mechanical devices. Piles and pile drivers are usually transported to their intended location on a floating pile driving barge. It may be easier to think of a pile as a nail and the pile driver as a hammer. A pile of appropriate size is set onto the subsurface of the body water and then hammered down by the pile driver.
The piles are battered either outward or inward at an angle, thus allowing them to support the lateral load of the upper bridge structure while also being able to withstand the currents of the water. Piles are usually installed in groups that are further apart at the bottom and closer together at the top, essentially forming the shape of a triangle.
The next step is to install the pile caps on the top of the pile grouping. Pile caps are placed on top of a grouping of multiple piles in order to create a stable foundation and offer a larger area for the distribution of the building load onto the piles Once this is done the upper bridge structure is ready to be built.
Method 2: Cofferdams
When it comes to actually displacing water, the cofferdam is often the method of choice for building a bridge or any other structure over water. Usually built to be a temporary structure, a cofferdam holds water back from a construction site through a process of building an enclosed watertight barrier and draining the water within it by mechanically pumping it out.
In shallow water, you can simply make a cofferdam out of the earth that's around the project site. By dumping soil into the surrounding water and creating an enclosed berm or embankment, you can begin to drain the water that's inside. The only problem with this approach is that collapses are more common due to the permeable nature of the soil that you've created your cofferdam walls with.
But because most large overwater bridges are built in deeper bodies of water, there is a huge variety of other, more appropriately designed cofferdams that are used on these construction sites. For example, most cofferdams are made from sheet piles and thick steel plates that interlock together, creating a watertight structure. Using a huge hammering machine, these are driven into the subsurface soils to create a safer, more sturdy watertight barrier.
Depending on the condition of the subsurface below, there are also situations when sheet piles are used to create small individual enclosures that are filled with soil, a technique known as a cellular cofferdam. In addition to that, there are also cofferdams made from water-filled rubber bags.
No matter the type of cofferdam, these temporary structures are almost always built to be dismantled and removed after the project is completed.
Method 3: Caissons
Commonly made from concrete, a caisson is essentially a prefabricated hollow box or cylinder that's lowered into location in the water. For certain types of structures, this foundation can be constructed off-site and transported in by a barge. These foundations are widely used in the construction of overwater bridges and other types of overwater structures because of their cost and safety convenience.
The first step to installing a caisson is to prepare the subsurface floor by leveling it properly. With a combination of radar technology and barge ships, excavated rocks and soil are transported and dumped underwater at the location of where the caisson will be placed. Once this rock-based bedding layer is completed, heavy-duty tug ships carefully transport the prefabricated caisson from the caisson-building dock, which must be located relatively close to the project site, to its desired location. The caisson is then submerged and anchored to its bedding layer, leaving it stable and ready for building. Now that the caisson is sunk to the desired depth, it's filled with concrete, thus ultimately becoming an integral part of the permanent structure
As mentioned above, one of the biggest reasons why caisson technology is so popular worldwide is because it's much safer for workers than traditional methods. Since the construction of this foundation is done on the floating dock, which acts as a kind of floating factory, there are greater and improved safety measures that are much easier to implement.
Method 4: Drill Shafts
With special equipment and construction techniques, you can excavate holes, install steel reinforcement, and fill them with concrete to create an extremely strong foundation system without any dewatering required. This process is called the drill shaft method.
To begin this process, a drilling rig is required to go out into the body of water to transport the required machines to the required location. In deepwater, the drilling rig may be either a drillship, a semi-submersible vessel, or part of a floating production platform. All drilling rigs are equipped with a hoisting system to raise and lower the drill pipe, as well as tools like a blower or pumping system to help circulate fluids in and out of the hole while drilling.
Once the drilling rig is ready to start work, a large base pipe or casing is assembled on the drilling rig floor. Then, a drill bit connected to the drill pipe is run through the inside of the casing. This is done in order to prevent the hole from caving in. Once assembled, it's lowered to the subsurface floor by the rig's hoist.
On the subsurface floor, the drill begins to break the ground, dig downward, and pump any excess water out. After the hole is properly dug, the drill is removed from the drill casing and a cylindrical steel cage is placed in the hole. The hole, with the steel cage for support, gets filled with concrete through the use of a tremie, a tube through which the concrete is pumped or gravity-fed to the bottom of the form.
In case you were wondering, concrete is a lot denser than water. So when used underwater, it will cure and harden just as well, if not better, than if it were conventionally placed on dry land.
Now that you know the different methods for creating the foundation for overwater bridges, you can start to imagine how the upper lateral part of an overwater bridge is built. After all, the foundation of a structure is often the most important part.
All in all, each of these methods is a true testament to the genius of engineers all over the globe. We encourage you to check out some other fantastic structures that were built over water to test your knowledge and see if you can guess which method they used for their construction.