Summary of a Powerpoint on Underwater

¶ … Construction

Bridges, dams, and other structures with underwater foundations require special construction techniques. Especially in areas of great water depth, underwater construction poses several risks and challenges. One of the most important challenges is to create a dry, water-free environment during the construction process, without compromising the integrity and stability of the structure.

Underwater Construction Techniques: Caissons and Cofferdams

Caissons and cofferdams are the two most significant underwater construction techniques. Caissons are permanent watertight retaining structures that are constructed to keep the working environment dry by allowing water to be pumped out. Caissons are generally used for bridges, piers, dams, and ship construction. There are several types of caissons including open caissons, box caissons, pneumatic caissons, and suction caissons. Box caissons are prefabricated concrete boxes that are placed on concrete bases underwater. They become permanent parts of the structure. Open caissons are like boxes but are open at the bottom. They work best in areas with soft clay or similar substrates that eliminate obstructions, but they can fill with water if they sink. Pneumatic caissons are inverted boxes, used commonly in riverbeds and in areas where there may be quicksand. Compressed air must be used to keep water and mud out. Suction caissons are used more often in off-shore situations. Pushing down and creating negative pressure allows the caisson to be stable in marine sediment. The advantages to all caissons are their adaptability to various environments and situations, and also their cost-effectiveness. However, caissons do present several drawbacks to workers including caisson disease.

Cofferdams are temporary structures that create dry working environments. They are best used in shallow (less than 10m of depth) situations. Cofferdams consist of sheet piling, brace piling, and concrete seals. They provide the advantage of a safe working environment, and the materials used to construct them can be reused on other projects. The sheet piles are also relatively easy to install and remove, and they can be used in otherwise difficult environments.

Underwater Concreting

The four underwater concrete placement methods include the Tremie method, the pump method, toggle bags, and bag work. The Tremie method uses the Tremie, a watertight pipe around 250mm in diameter with a funnel at the upper end and a plug at the bottom. The Tremie method permits the highest quality concrete to be poured. The pump method delivers concrete horizontally and vertically directly into its final position, but can only be used for small-scale projects. Likewise, toggle bags are used for small-scale light projects, including repairs. Toggle bags are actually squeezed out by specialized divers. Bag work also requires divers using bags made of open weave material.