Caissons and cofferdams are both used to help pump water away from structures so that they can be built or repaired. To the layperson, it might seem like caissons and cofferdams can be used interchangeably, but there are some differences.
What is the difference between a caisson and a cofferdam? A caisson is built as a part of the permanent structure while a cofferdam usually is not. Because of this, caissons are typically used during the construction phase rather than for repairs. Cofferdams can also be used during the construction phase, but are more often used during repairs, upgrades, and other temporary situations that call for a dry work area.
A Further Look at Caissons
A caisson creates a completely dry work area by pumping out the water around the structure and retaining it after it has been pumped out. They are installed through the mud and on top of the hard surface underneath of it. This could be hard clay or any other hard surface that can be used as a foundation.
Caissons are made from many different materials. This will depend on the type of caisson being used. This being said, you’ll find that concrete is the primary building material used in the construction of a caisson. Many different types of caissons are used. Although other types exist, these are the four basic types of caissons that are commonly used:
- Box Caisson: These are prefabricated concrete boxes that come in various shapes and may form the foundation for some other structures. They must be anchored to remain in place and are filled with concrete once in place.
- Open Caisson: These types don’t have a bottom, so they have to be pushed into the ground to create a base. This makes them unsuitable for areas underwater that have excess debris or where the ground isn’t soft enough to allow penetration.
- Pneumatic Caisson: These caissons comprise a closed top and an open bottom. Compressed air is utilized to keep water out of the caissons chamber. This type of caisson is constructed in the same way as others, except that the shaft and working chamber have been made airtight.
- Monolithic Caisson: This is an improved version of the open-air caisson type. Monolithic caissons are primarily used because they have the strength to withstand the possible impact from ships.
The type of caisson used in a project will vary based on the nature of the project, the scope of the project, as well as the local environmental conditions.
Advantages of Caissons
- Long-term costs
Many applications will call for permanent foundations to support a structure. This is where you would want to use a caisson versus a cofferdam. In this case, the caissons permanency is its advantage. The caisson will be a part of the structure and will work to retain or move water indefinitely.
A caisson is often more durable than a cofferdam as well. This is because caissons are often built off-site where they can undergo strict quality control guidelines before on-location installation. Not only this, but their permanency often requires that they be constructed from lasting materials. This is especially true in foundational applications.
Because of a caisson’s strength and build-quality, it can be used at depths that are much greater than a typical cofferdam can operate in.
Caissons initially have a higher cost than cofferdams. However, because of the permanent nature of a caisson, their use becomes less expensive over time.
Disadvantages of Caissons
There are some drawbacks to using caissons, and you wouldn’t want to use them in all situations. Here are some of the disadvantages of using caissons:
- Initial cost
- Availability of laborers
- Deployment Speed
Building, installing, and maintaining a caisson is expensive. It requires the use of highly skilled laborers and the use of divers is often needed. The working environment can also be quite dangerous. Workers can be subjected to working environments that can’t be found in most other industries. Caissons are not built for rapid deployment. This means that you would not be able to use a caisson to help with flooding caused by natural disasters.
Examples of Caissons Being Used
Caissons are often used to support underwater bridge posts. These caissons are driven deep into the ground until they reach a foundation which is solid enough to support the bridge without any worry of settlement. It is critical that these caissons be placed far enough down so that they will not shift under the weight of the bridge or the surrounding water pressure.
A caisson is often used in the creation and operation of a canal. They are used to create the locks within the channel as well as to pump water in and out of these locks. Water is pumped into the canal to lift the boat to the elevation it needs to allow for movement between each channel. Without the use of caissons, canals would not be possible.
Concrete caissons can be created to control the flow in and out of dams. These usually consist of concrete walls and steel gates which can be opened and closed when needed. These dams can be used to create electricity, control flood waters, and to help provide clean drinking water to nearby towns.
Caissons can be used to create the foundation of a boat pier. This is done by pushing the caisson through the mud and into the solid part of the ground to form the basis of the boat pier’s foundation. Without caissons, you would not be able to create permanent boat piers.
A Further Look at Cofferdams
Cofferdams work in the same way that caissons work. They pump water and they retain water. The main difference is that these structures are typically only used temporarily. They are great for short-term projects like repairs and salvage operations.
A cofferdam can help to quickly move water away from an area so that workers can repair structures or make salvage efforts within that space.
Cofferdams are made using a wide range of materials. For rapid deployment situations, you’ll find that inflatable tubing is often used. This tubing is stretched across or around the area that needs to be contained and then inflated to create a temporary water barrier.
Other materials used are wood, steel, and even fabric. Watertight fabrics and inflatable tubing are used when materials need to be lightweight so that they can quickly be transported to various job locations. More durable materials like steel and wood are used when a stronger, more semi-permanent water barrier needs to be put in place.
The following are some of the different types of cofferdams that are commonly used:
- Earthen: This is a simple cofferdam whereby a sloped earthen embankment is constructed around the space to be enclosed. It is mostly used in places where the water is relatively shallow, and the current has a low velocity. A combination of sand and clay, or clay and gravel is typically used for the foundation.
- Rock-fill: In cases where the water is too deep for earthen cofferdams to be used, stone or rubble substitutes earth as the construction material. Stones are constructed in the needed shape of the cofferdam, and the spaces are filled up with gravel, stone-chips, or earth.
- Single-filled: This kind of cofferdam is mostly used in cases where the region to be enclosed is small with a deep-water level, such as four to six meters. Timber guide piles are then driven through the ground and braces are constructed before sheet piles are pushed into place. These are secured to the braces using bolts.
- Double-walled: When the area to be enclosed is large and the water deep, single-walled cofferdams may not be feasible. In this case, two walls are constructed with some space in between. The thickness of the gap will be determined by the depth of the water. The general rule here is that the thickness of the wall should equal the water depth up to three meters. The wall faces are linked at the top with the help of steel rods positioned at close intervals.
Cofferdams are not a substitute for permanent caissons, but they do have many applications in which they excel. Here are some advantages of cofferdams over caissons.
Advantages of Cofferdams
- Rapid deployment
Any experienced cofferdam technician will tell you that creating and operating a cofferdam is no easy task. This being said, cofferdams are usually easier to install than their more permanent counterpart.
Cofferdams are also much faster to deploy. This is because the cofferdam will not be providing a permanent foundation and only needs to last as long as the job entails. Because of a cofferdam’s ability to be deployed quickly, it can do jobs that caissons simply cannot.
A cofferdam is also less expensive than a caisson. The materials used do not have to be as durable as the material used in the caisson, and they can often be re-used. For example, the inflatable tubing used to stop one flood can be deflated, transported to a new location, and then used again to stop another flood.
Disadvantages of Cofferdams
- Depth of use
Cofferdams are not meant to be permanent. For this reason, they should not be used for retaining water on a continuous basis. Cofferdams are often not as durable and cannot be used in applications that require greater depths and greater structural loads. You wouldn’t use a cofferdam to support a suspension bridge or a permanent boat pier.
Examples of Cofferdams Being Used
A cofferdam could be used by the military to salvage the wreckage of a naval vessel. For example, a cofferdam was once deployed in Havana Harbor to salvage the wreckage of the USS Maine.
Flood control is an excellent example of a cofferdam being rapidly deployed to an area. A cofferdam can quickly be moved to an area to keep water away from flood-stricken homes and businesses. The swift use of a cofferdam can save an entire town from being flooded.
Boat Ramp Repairs
Cofferdams are often used around boat ramps to create a safe and dry working environment. A cofferdam is erected around the boat ramp, and the water inside of it is pumped out. Workers can then access the boat ramp and repairs can be made before the cofferdam is removed and the water is allowed back into the area.
Even solidly constructed caisson bridges will eventually need to be repaired. Sometimes this will involve draining the water around the bridge so that construction workers can move their equipment into place. In this case, a cofferdam will be deployed and the water inside the cofferdam will be drained so that the workers have a chance to access areas that would normally be underwater.
Caissons and cofferdams both play critical roles in construction, repair, safety, and salvage operations. They can even be used to prevent flooding. To define the difference between a caisson and a cofferdam, a general rule of thumb is that caissons are used for a more permanent solution and cofferdams for provisional applications as well as applications that must be deployed quickly.
Which type you use for your project will depend on the nature of your particular project. We at Milestone are experienced at defining the right product for the job. We invite you to take a peek at our various civil construction services, and give us a call so we can talk about your next construction project.