A Brief Guide On How A Cofferdam Is Built When Constructing A New Bridge Pier Underwater

Excavating in and around water and loose soil can be very dangerous. Cofferdams are necessary when building a new bridge because the piers of the bridge will be sunk into the ground below the surface of the water. A cofferdam keeps water and dirt out of the hole being dug, and they help to keep the excavated hole dry so workers can construct the pier in relative comfort and safety. Here is a brief guide on how a contractor will build a cofferdam in an excavated hole in the middle of a river so workers can build the piers for the bridge.

Digging the Hole

The first step is to remove the soft sediment and soil on the floor of the river. The area where the cofferdam will be installed will be flattened so the floor of the ground underneath the water is even. The ground will be sloped away from the excavated hole to prevent it from quickly filling back up with dirt and sediments before the contractor has time to build the cofferdam.

Install Support Piers

Cofferdams for bridge piers are typically square-like structures. Support piers will be piled-driven into each corner of the cofferdam. Pile-driving consists of using a heavy mechanical device on the end of a crane to pound the support pier into the ground. Bracing will be installed between the support piers to keep the force of the water from moving or tipping over the piers.

Install Sheet Metal Walls

Large and thick metal sheets are driven into the ground on the outside of the support piers for the cofferdam. The metal sheets are built up on top of each other until the sides of the cofferdam is above the surface of the water far enough so waves and water movement won't flow over the top of the dam walls and into the hole.

The pieces of sheet metal are tied together and to the bracing frame using metal ties.

Put in Concrete Floor

The water in the hole will be pumped out once the walls are in place (the pumps will be running almost non-stop to remove water that will constantly leak in during the building and use of the cofferdam). Once most of the water is gone, concrete will be poured into the bottom of the cofferdam to create a work floor and to provide support against the pressure of the water and ground pressing against the outside of the bottom of the cofferdam.

The outside of the cofferdam will then be back-filled with dirt. The back-fill presses the exterior bottom of walls against the concrete slab inside the cofferdam. This action helps keep the base of the walls firmly in place.

Build Bridge Pier

Work can now begin on building the base for the bridge pier.

For more information about bridge piers, contact a company such as Haas Construction.   

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