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Short Summary
Main recommendation: Build underwater structures by first removing the water from the work area, a process called dewatering, using temporary barriers such as cofferdams, or by avoiding dewatering entirely with methods like drilled shafts and underwater concrete placement through a tremie. The choice depends on site conditions, safety, and cost.
Key Points
- Divers can perform welding, cutting, and formwork underwater with SCUBA equipment, but the work is dangerous and the usable tools are limited, which is why remote vehicles and dewatering are often preferred.
- A cofferdam holds water back from a site, and it must be buildable in wet conditions; a simple version is an earthen berm, while others use interlocking steel sheet piles, cellular soil-filled enclosures, steel frames with membranes, or water-filled rubber bags.
- Dams are often built in phases, with most of the structure built away from the main channel and the river diverted through the new intake or spillway, as with Hoover Dam's diversion tunnels, which later became spillways.
- Caissons are hollow boxes or cylinders floated in and sunk to rock or strong soil; pneumatic caissons inject compressed air to balance water pressure, which caused "Caisson Disease" (decompression sickness) in workers.
- Modern practice has shifted toward avoiding dewatering, with drilled shafts filled with reinforced concrete and underwater concrete placement that works well if turbulence is minimized.
Details Worth Noting
- Concrete instructions: For underwater concrete placement, use a tremie, a tube that pumps or gravity-feeds concrete to the bottom of the form. Keep the end of the tube below the top of the concrete as it fills, which prevents water from washing away aggregate and diluting the cement paste. Avoid turbulence, since it weakens the final product. For drilled shafts, excavate a hole, install steel reinforcement, and fill it with concrete without dewatering.
- Earthen cofferdams need constant pumping because soil is permeable and seepage builds up; saturated, uncompacted soil is weak, so an engineer should design them.
- Sheet piles are driven into subsurface soils with a large hammer; if soils are too rocky, cellular cofferdams with soil-filled enclosures can be used instead.
- Cofferdams are almost always built to be dismantled and removed after construction.
- Dewatering a caisson creates a pressure difference that causes seepage and possible soil erosion and flooding, which is why pneumatic caissons keep air pressure equal to or higher than water pressure at the bottom.
- Pneumatic caissons are now fairly rare because of safety issues.
Bottom Line
Underwater construction is mostly a question of whether to remove the water with temporary structures and pumping or to build directly in it using specialized methods, with safety and site conditions driving the decision.
