Have you ever looked at a large bridge or other structure whose foundation was rooted underwater and wondered how engineers ever went about constructing it or will ever fix it? When construction needs to take place somewhere that is submerged underwater, engineers use a series of large driven piles into the waterbed called cofferdams to create a dry workplace.
The piles of a cofferdam are driven into the earth in whatever formation necessary to a specific depth. When water is on one side of a wall and water is pumped out of the other side of the wall, this creates a hydraulically unstable system which can cause water to seep up through the ground. Without getting too complex into the geotechnical engineering of this hydraulic phenomenon, there is a depth at which a wall can be driven into the ground that will keep water from seeping to the other side of the wall – typically defined by soil type and water table. The piles used in cofferdams are usually driven into the surface at a minimum of this calculated height in order to keep water out.
Once the entire cofferdam is in place, pumps are used to extract the water interior to the dam structure, ultimately creating a dry workspace. Sometimes, getting the piles that make up the cofferdam to a necessary depth on the lake/ocean/river floor is simply too expensive or impractical. In cases like this, a series of pumps are set in place to constantly pump out excess water as it seeps into the cofferdam structure.
These structures are used very commonly when constructing dams, piers for bridges or other forms of aquatic engineering. While it may seem that having such a large work area under the surrounding water level may be dangerous, and it is, it’s not as dangerous as you may think. Work inside of cofferdams is usually only allowed under the most pristine conditions when the water is generally static. In these states, failure modes of the pile dam are slow and predictable in nature. To help fight against these slow failures as well, a series of primary or backup pumps can kick into overdrive to help keep the inside of the cofferdam dry until crews can evacuate.
It also may seem like creating these large cofferdams are very expensive – they are. Engineers avoid using any forms of underwater construction at all costs, but when it is needed, cofferdams are much safer than other methods of underwater construction like using divers. They are also a more permanent solution when continued projects need to take place on the edge of lakes or oceans like pictured above.
As soon as a project is completed, water is pumped back inside the cofferdam and the piles are removed. In terms of temporary construction workspaces, giant cofferdams may be one of the coolest and most impressive.